All PNG images contain a singlestatic image.

Some PNG images — calledAnimated PNG (APNG) — also contain a frame-based animation sequence, theanimated image. The first frame of this may be — but need not be — thestatic image. Non-animation-capable displays (such as printers) will display thestatic image rather than the animation sequence.

Thestatic image, and each individual frame of ananimated image, corresponds to areference image and is stored as aPNG image.

This specification specifies the PNG datastream, and places some requirements on PNG encoders, which generate PNG datastreams, PNG decoders, which interpret PNG datastreams, andPNG editors, which transform one PNG datastream into another. It does not specify the interface between an application and either a PNG encoder, decoder, or editor. The precise form in which an image is presented to an encoder or delivered by a decoder is not specified. Four kinds of image are distinguished.

Source image

Thesource image is the image presented to a PNG encoder.

Reference image

Thereference image, which only exists conceptually, is a rectangular array of rectangularpixels, all having the same width and height, and all containing the same number of unsigned integer samples, either three (red, green, blue) or four (red, green, blue, alpha). The array of all samples of a particular kind (red, green, blue, or alpha) is called achannel. Each channel has a sample depth in the range 1 to 16, which is the number of bits used by every sample in the channel. Different channels may have different sample depths. The red, green, and blue samples determine the intensities of the red, green, and blue components of the pixel's color; if they are all zero, the pixel is black, and if they all have their maximum values (2^sampledepth-1), the pixel is white. Thealpha sample determines a pixel's degree of opacity, where zero means fully transparent and the maximum value means fully opaque. In a three-channel reference image all pixels are fully opaque. (It is also possible for a four-channel reference image to have all pixels fully opaque; the difference is that the latter has a specific alpha sample depth, whereas the former does not.) Each horizontal row of pixels is called a scanline. Pixels are ordered from left to right within each scanline, and scanlines are ordered from top to bottom. Every reference image can be represented exactly by aPNG datastream and everyPNG datastream can be converted into a reference image. A PNG encoder may transform the source image directly into a PNG image, but conceptually it first transforms the source image into a reference image, then transforms the reference image into a PNG image. Depending on the type of source image, the transformation from the source image to a reference image may require the loss of information. That transformation is beyond the scope of this International Standard. The reference image, however, can always be recovered exactly from a PNG datastream.

PNG image

ThePNG image is obtained from the reference image by a series of transformations:alpha separation,indexing,RGB merging,alpha compaction, andsample depth scaling. Five types of PNG image are defined (see6.1Color types and values). (If the PNG encoder actually transforms the source image directly into the PNG image, and the source image format is already similar to the PNG image format, the encoder may be able to avoid doing some of these transformations.) Although not all sample depths in the range 1 to 16 bits are explicitly supported in the PNG image, the number of significant bits in each channel of the reference image may be recorded. All channels in the PNG image have the same sample depth. A PNG encoder generates a PNG datastream from the PNG image. A PNG decoder takes the PNG datastream and recreates the PNG image.

Delivered image

Thedelivered image is constructed from the PNG image obtained by decoding a PNG datastream. No specific format is specified for thedelivered image. A viewer presents an image to the user as close to the appearance of the original source image as it can achieve.

The relationships between the four kinds of image are illustrated inFigure1.

The relationships between samples, channels, pixels, and sample depth are illustrated inFigure2.

The RGB color space in which color samples are situated may be specified in one of four ways:

1. by CICP image format signaling metadata;
2. by an ICC profile;
3. by specifying explicitly that the color space is sRGB when the samples conform to this color space;
4. by specifying agamma value and the 1931 CIEx,y chromaticities of the red, green, and blue primaries used in the image and the referencewhite point.

For high-end applications the first two methods provides the most flexibility and control. The third method enables one particular, but extremely common, color space to be indicated. The fourth method, which was standardized before ICC profiles were widely adopted, enables the exact chromaticities of the RGB data to be specified, along with thegamma correction to be applied (seeC.Gamma and chromaticity). However, color-aware applications will prefer one of the first three methods, while color-unaware applications will typically ignore all four methods.

Table1 is a list of chunk types that provide color space information, each with an associated Priority number. If a single image contains more than one of these chunk types, the chunk with the lowest Priority number should take precedence and any higher-numbered chunk types should be ignored.

Gamma correction is not applied to the alpha channel, if present. Alpha samples are always full-range and represent a linear fraction of full opacity.

Mastering metadata may also be provided.

The transformation of the reference image results in one of five types of PNG image (seeFigure6) :

Truecolor with alpha

Eachpixel consists of foursamples: red, green, blue andalpha.

Greyscale with alpha

Eachpixel consists of twosamples: agrey sample and analpha sample.

Truecolor

Eachpixel consists of a triplet ofsamples: red, green, blue. An optional alpha channel can be specified as a single triplet of red, green, bluesamples:pixels of the image whose red, green, bluesamples are identical to the red, green, bluesamples of the alpha channel are fully transparent; others are fully opaque. If the alpha channel is not present, allpixels are fully opaque.

Greyscale

Each pixel consists of a singlegrey sample, which represents overallluminance (on a scale from black to white). An optional alpha channel can be specified as a singlegrey sample:pixels of the image whosegrey sample is identical to thegrey sample of the alpha channel are fully transparent; others are fully opaque. If the alpha channel is not present, allpixels are fully opaque.

Indexed-color

Each pixel consists of an index into a palette (and into an associated table of alpha values, if present).

The format of each pixel depends on the PNG image type and the bit depth. For PNG image types other than indexed-color, the bit depth specifies the number of bits per sample, not the total number of bits per pixel. Forindexed-color images, the bit depth specifies the number of bits in each palette index, not the sample depth of the colors in the palette or alpha table. Within the pixel the samples appear in the following order, depending on the PNG image type.

1. Truecolor with alpha: red, green, blue, alpha.
2. Greyscale with alpha: grey, alpha.
3. Truecolor: red, green, blue.
4. Greyscale: grey.
5. Indexed-color: palette index.

Ancillary information may be associated with an image. Decoders may ignore all or some of the ancillary information. The types of ancillary information provided are described inTable2.

Errors in a PNG datastream fall into two general classes:

1. transmission errors or damage to a computer file system, which tend to corrupt much or all of the datastream;
2. syntax errors, which appear as invalid values in chunks, or as missing or misplaced chunks. Syntax errors can be caused not only by encoding mistakes, but also by the use of registered or private values, if those values are unknown to the decoder.

PNG decoders should detect errors as early as possible, recover from errors whenever possible, and fail gracefully otherwise. The error handling philosophy is described in detail in13.1Error handling.

This section is non-normative.

The PNG format exposes several extension points:

• chunk type;
• text keyword; and
• private field values.

Some of these extension points are reserved byW3C, while others are available for private use.

ThePNG datastream consists of a PNG signature followed by a sequence of chunks. It is the result of encoding aPNG image.

The first eight bytes of a PNG datastream always contain the following hexadecimal values:

89 50 4E 47 0D 0A 1A 0A

This signature indicates that the remainder of the datastream contains a single PNG image, consisting of a series of chunks beginning with anIHDR chunk and ending with anIEND chunk.

This signature differentiates a PNG datastream from other types ofdatastream and allows early detection of some transmission errors.

Eachchunk consists of three or four fields (seeFigure10). The meaning of the fields is described inTable5. The chunk data field may be empty.

The chunk data length may be any number of bytes up to the maximum; therefore, implementors cannot assume that chunks are aligned on any boundaries larger than bytes.

Chunk types are chosen to be meaningful names when the bytes of the chunk type are interpreted as ISO 646 letters [ISO646]. Chunk types are assigned so that a decoder can determine some properties of a chunk even when the type is not recognized. These rules allow safe, flexible extension of the PNG format, by allowing a PNG decoder to decide what to do when it encounters an unknown chunk.

The naming rules are normally of interest only when the decoder does not recognize the chunk's type, as specified at13.PNG decoders and viewers.

Four bits of the chunk type, the property bits, namely bit 5 (value 32) of each byte, are used to convey chunk properties. This choice means that a human can read off the assigned properties according to whether the letter corresponding to each byte of the chunk type is uppercase (bit 5 is 0) or lowercase (bit 5 is 1).

The property bits are an inherent part of the chunk type, and hence are fixed for any chunk type. Thus,CHNK andcHNk would be unrelated chunk types, not the same chunk with different properties.

The semantics of the property bits are defined inTable6.

The hypothetical chunk type "cHNk" has the property bits:

cHNk  <--32 bit chunk type representedin text form|||||||+-Safe-to-copy bit is1 (lowercase letter; bit5 is1)||+--Reserved bit is0     (uppercase letter; bit5 is0)|+---Private bit is0      (uppercase letter; bit5 is0)+----Ancillary bit is1    (lowercase letter; bit5 is1)

Therefore, this name represents an ancillary, public, safe-to-copy chunk.

CRC fields are calculated using standardizedCRC methods with pre and post conditioning, as defined by [ISO-3309] and [ITU-T-V.42]. TheCRC polynomial employed— which is identical to that used in the GZIP file format specification [RFC1952]— is

x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1

In PNG, the 32-bitCRC is initialized to all 1's, and then the data from each byte is processed from the least significant bit (1) to the most significant bit (128). After all the data bytes are processed, theCRC is inverted (its ones complement is taken). This value is transmitted (stored in the datastream)MSB first. For the purpose of separating into bytes and ordering, the least significant bit of the 32-bitCRC is defined to be the coefficient of thex31 term.

Practical calculation of theCRC often employs a precalculated table to accelerate the computation. SeeD.SampleCRC implementation.

The constraints on the positioning of the individual chunks are listed inTable7 and illustrated diagrammatically for static images inFigure11 andFigure12, for animated images where the static image forms the first frame inFigure13 andFigure14, and for animated images where the static image is not part of the animation inFigure15 andFigure16. These lattice diagrams represent the constraints on positioning imposed by this specification. The lines in the diagrams define partial ordering relationships. Chunks higher up shall appear before chunks lower down. Chunks which are horizontally aligned and appear between two other chunk types (higher and lower than the horizontally aligned chunks) may appear in any order between the two higher and lower chunk types to which they are connected. The superscript associated with the chunk type is defined inTable8. It indicates whether the chunk is mandatory, optional, or may appear more than once. A vertical bar between two chunk types indicates alternatives.

Values greater than or equal to 128 in the following fields areprivate field values:

• bit depth
• color type
• compression method
• interlace method
• filter method

Theseprivate field values are neither defined nor reserved by this specification.

Private field valuesMAY be used for experimental or private semantics.

Private field valuesSHOULD NOT appear in publicly available software or datastreams since they can result in datastreams that are unreadable by PNG decoders as detailed at13.PNG decoders and viewers.

As explained in4.5PNG image there are five types of PNG image. Corresponding to each type is acolor type, which is the sum of the following values: 1 (palette used), 2 (truecolor used) and 4 (alpha used).greyscale andtruecolor images may have an explicit alpha channel. The PNG image types and correspondingcolor types are listed inTable9.

The allowed bit depths and sample depths for each PNG image type are listed inImage header.

Greyscale samples represent luminance if the transfer curve is indicated (bygAMA,sRGB,iCCP) orcICP; or device-dependent greyscale if not. RGB samples represent calibrated color information if the color space is indicated (bygAMA andcHRM,sRGB,iCCP, orcICP; or uncalibrated device-dependent color if not.

Sample values are not necessarily proportional to light intensity; thegAMA chunk specifies the relationship between sample values and display output intensity. Viewers are strongly encouraged to compensate properly. See4.3Color spaces,13.13Decoder gamma handling andC.Gamma and chromaticity.

In a PNG datastream transparency may be represented in one of four ways, depending on the PNG image type (see4.4.1Alpha separation and4.4.4Alpha compaction).

1. Truecolor with alpha,greyscale with alpha: an alpha channel is part of the image array.
2. truecolor,greyscale: AtRNS chunk contains a single pixel value distinguishing the fully transparent pixels from the fully opaque pixels.
3. Indexed-color: AtRNS chunk contains the alpha table that associates an alpha sample with each palette entry.
4. truecolor,greyscale,indexed-color: there is notRNS chunk present and all pixels are fully opaque.

An alpha channel included in the image array has 8-bit or 16-bit samples, the same size as the other samples. The alpha sample for each pixel is stored immediately following the greyscale or RGB samples of the pixel. An alpha value of zero represents full transparency, and a value of 2^sampledepth - 1 represents full opacity. Intermediate values indicate partially transparent pixels that can becomposited against a background image to yield the delivered image.

The color values in a pixel are not premultiplied by the alpha value assigned to the pixel. This rule is sometimes called "unassociated" or "non-premultiplied" alpha. (Another common technique is to store sample values premultiplied by the alpha value; in effect, such an image is alreadycomposited against a black background. PNG doesnot use premultiplied alpha. In consequence an image editor can take a PNG image and easily change its transparency.) See12.3Alpha channel creation and13.16Alpha channel processing.

All integers that require more than one byte shall be innetwork byte order (as illustrated inFigure17 ): the most significant byte comes first, then the less significant bytes in descending order of significance (MSBLSB for two-byte integers,MSB B2 B1LSB for four-byte integers). The highest bit (value 128) of a byte is numbered bit 7; the lowest bit (value 1) is numbered bit 0. Values are unsigned unless otherwise noted. Values explicitly noted as signed are represented in two's complement notation.

PNG four-byte unsigned integers are limited to the range 0 to 2³¹-1 to accommodate languages that have difficulty with unsigned four-byte values.

A PNG image (or pass, see8.Interlacing and pass extraction) is a rectangular pixel array, with pixels appearing left-to-right within each scanline, and scanlines appearing top-to-bottom. The size of each pixel is determined by the number of bits per pixel.

Pixels within a scanline are always packed into a sequence of bytes with no wasted bits between pixels. Scanlines always begin on byte boundaries. Permitted bit depths andcolor types are restricted so that in all cases the packing is simple and efficient.

In PNG images ofcolor type 0 (greyscale) each pixel is a single sample, which may have precision less than a byte (1, 2, or 4 bits). These samples are packed into bytes with the leftmost sample in the high-order bits of a byte followed by the other samples for the scanline.

In PNG images ofcolor type 3 (indexed-color) each pixel is a single palette index. These indices are packed into bytes in the same way as the samples forcolor type 0.

When there are multiple pixels per byte, some low-order bits of the last byte of a scanline may go unused. The contents of these unused bits are not specified.

PNG images that are notindexed-color images may have sample values with a bit depth of 16. Such sample values are innetwork byte order (MSB first,LSB second). PNG permits multi-sample pixels only with 8 and 16-bit samples, so multiple samples of a single pixel are never packed into one byte.

Afilter method is a transformation applied to an array ofscanlines with the aim of improving their compressibility.

PNG standardizes onefilter method and several filter types that may be used to prepareimage data for compression. It transforms the byte sequence into an equal length sequence of bytes preceded by a filter type byte (seeFigure18 for an example).

The encoder shall use only a singlefilter method for an interlaced PNG image, but may use different filter types for each scanline in a reduced image. An intelligent encoder can switch filters from one scanline to the next. The method for choosing which filter to employ is left to the encoder.

The filter type byte is not considered part of theimage data, but it is included in the datastream sent to the compression step. See9.Filtering.

Pass extraction (seeFigure 4.8) splits a PNG image into a sequence of reduced images (the interlaced PNG image) where the first image defines a coarse view and subsequent images enhance this coarse view until the last image completes the PNG image. This allows progressive display of the interlaced PNG image by the decoder and allows images to "fade in" when they are being displayed on-the-fly. On average, interlacing slightly expands the datastream size, but it can give the user a meaningful display much more rapidly.

Two interlace methods are defined in this International Standard, methods 0 and 1. Other values of interlace method are reserved for future standardization.

With interlace method 0, the null method, pixels are extracted sequentially from left to right, and scanlines sequentially from top to bottom. The interlaced PNG image is a single reduced image.

Interlace method 1, known as Adam7, defines seven distinct passes over the image. Each pass transmits a subset of the pixels in the reference image. The pass in which each pixel is transmitted (numbered from 1 to 7) is defined by replicating the following 8-by-8 pattern over the entire image, starting at the upper left corner:

1 6 4 6 2 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 73 6 4 6 3 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 7

Figure 4.8 shows the seven passes of interlace method 1. Within each pass, the selected pixels are transmitted left to right within a scanline, and selected scanlines sequentially from top to bottom. For example, pass 2 contains pixels 4, 12, 20, etc. of scanlines 0, 8, 16, etc. (where scanline 0, pixel 0 is the upper left corner). The last pass contains all of scanlines 1, 3, 5, etc. The transmission order is defined so that all the scanlines transmitted in a pass will have the same number of pixels; this is necessary for proper application of some of the filters. The interlaced PNG image consists of a sequence of seven reduced images. For example, if the PNG image is 16 by 16 pixels, then the third pass will be a reduced image of two scanlines, each containing four pixels (seeFigure 4.8).

Scanlines that do not completely fill an integral number of bytes are padded as defined in7.2Scanlines.

Filtering transforms the PNG image with the goal of improving compression. The overall process is depicted inFigure7 while the specifics of serializing and filtering a scanline are shown inFigure18.

PNG allows for a number offilter methods. All the reduced images in an interlaced image shall use a singlefilter method. Onlyfilter method 0 is defined by this specification. Otherfilter methods are reserved for future standardization.Filter method 0 provides a set of five filter types, and individual scanlines in each reduced image may use different filter types.

PNG imposes no additional restriction on which filter types can be applied to an interlaced PNG image. However, the filter types are not equally effective on all types of data. See12.7Filter selection.

Filtering transforms the byte sequence in a scanline to an equal length sequence of bytes preceded by the filter type. Filter type bytes are associated only with non-empty scanlines. No filter type bytes are present in an empty pass. See13.10Interlacing and progressive display.

Filters are applied tobytes, not to pixels, regardless of the bit depth orcolor type of the image. The filters operate on the byte sequence formed by a scanline that has been represented as described in7.2Scanlines. If the image includes an alpha channel, the alpha data is filtered in the same way as theimage data.

Filters may use the original values of the following bytes to generate the new byte value:

Figure19 shows the relative positions of the bytesx,a,b, andc.

Filter method 0 defines five basic filter types as listed inTable11.Orig(y) denotes the original (unfiltered) value of bytey.Filt(y) denotes the value after a filter type has been applied.Recon(y) denotes the value after the corresponding reconstruction function has been applied. The Paeth filter typePaethPredictor [Paeth] is defined below.

Filter method 0 specifies exactly this set of five filter types and this shall not be extended. This ensures that decoders need not decompress the data to determine whether it contains unsupported filter types: it is sufficient to check thefilter method in11.2.1IHDR Image header.

For all filters, the bytes "to the left of" the first pixel in a scanline shall be treated as being zero. For filters that refer to the prior scanline, the entire prior scanline and bytes "to the left of" the first pixel in the prior scanline shall be treated as being zeroes for the first scanline of a reduced image.

To reverse the effect of a filter requires the decoded values of the prior pixel on the same scanline, the pixel immediately above the current pixel on the prior scanline, and the pixel just to the left of the pixel above.

Unsigned arithmetic modulo 256 is used, so that both the inputs and outputs fit into bytes. Filters are applied to each byte regardless of bit depth. The sequence ofFilt values is transmitted as the filtered scanline.

The sumOrig(a) + Orig(b) shall be performed without overflow (using at least nine-bit arithmetic).floor() indicates that the result of the division is rounded to the next lower integer if fractional; in other words, it is an integer division or right shift operation.

The Paeth filter type computes a simple linear function of the three neighboring pixels (left, above, upper left), then chooses as predictor the neighboring pixel closest to the computed value. The algorithm used in this specification is an adaptation of the technique due to Alan W. Paeth [Paeth].

The PaethPredictor function is defined in the code below. The logic of the function and the locations of the bytesa,b,c, andx are shown inFigure20.Pr is the predictor for bytex.

p = a + b - cpa =abs(p - a)pb =abs(p - b)pc =abs(p - c)if pa <= pb and pa <= pc thenPr = aelseif pb <= pc thenPr = belsePr = creturnPr

The calculations within the PaethPredictor function shall be performed exactly, without overflow.

The order in which the comparisons are performed is critical and shall not be altered. The function tries to establish in which of the three directions (vertical, horizontal, or diagonal) the gradient of the image is smallest.

Exactly the same PaethPredictor function is used by both encoder and decoder.

Only PNG compression method 0 is defined by this International Standard. Other values of compression method are reserved for future standardization. PNG compression method 0 isdeflate compression with a sliding window (which is an upper bound on the distances appearing in thedeflate stream) of at most 32768 bytes.Deflate compression is derived fromLZ77.

Deflate-compressed datastreams within PNG are stored in thezlib format, which has the structure:

zlib is specified at [rfc1950].

For PNG compression method 0, thezlib compression method/flags code shall specify method code 8 (deflate compression) and anLZ77 window size of not more than 32768 bytes. Thezlib compression method number is not the same as the PNG compression method number in theIHDR chunk. The additional flags shall not specify a preset dictionary.

If the data to be compressed contain 16384 bytes or fewer, the PNG encoder may set the window size by rounding up to a power of 2 (256 minimum). This decreases the memory required for both encoding and decoding, without adversely affecting the compression ratio.

The compressed data within thezlib datastream are stored as a series of blocks, each of which can represent raw (uncompressed) data,LZ77-compressed data encoded with fixed Huffman codes, orLZ77-compressed data encoded with custom Huffman codes. A marker bit in the final block identifies it as the last block, allowing the decoder to recognize the end of the compressed datastream. Further details on the compression algorithm and the encoding are given in thedeflate specification [rfc1951].

The check value stored at the end of thezlib datastream is calculated on the uncompressed data represented by the datastream. The algorithm used to calculate this is not the same as theCRC calculation used for PNG chunkCRC field values. Thezlib check value is useful mainly as a cross-check that thedeflate algorithms are implemented correctly. Verifying the individual PNG chunk CRCs provides confidence that the PNG datastream has been transmitted undamaged.

The sequence of filtered scanlines is compressed and the resulting data stream is split intoIDAT chunks. The concatenation of the contents of all theIDAT chunks makes up azlib datastream. This datastream decompresses to filteredimage data.

It is important to emphasize that the boundaries betweenIDAT chunks are arbitrary and can fall anywhere in thezlib datastream. There is not necessarily any correlation betweenIDAT chunk boundaries anddeflate block boundaries or any other feature of thezlib data. For example, it is entirely possible for the terminatingzlib check value to be split acrossIDAT chunks.

Similarly, there is no required correlation between the structure of theimage data (i.e., scanline boundaries) anddeflate block boundaries orIDAT chunk boundaries. The complete filtered PNG image is represented by a singlezlib datastream that is stored in a number ofIDAT chunks.

PNG also uses compression method 0 iniTXt,iCCP, andzTXt chunks. Unlike theimage data, such datastreams are not split across chunks; each such chunk contains an independentzlib datastream (see10.1Compression method 0).

This clause defines chunk used in this specification.

This clause gives requirements and recommendations for encoder behavior. A PNG encoder shall produce a PNG datastream from a PNG image that conforms to the format specified in the preceding clauses. Best results will usually be achieved by following the additional recommendations given here.

SeeC.Gamma and chromaticity for a brief introduction togamma issues.

PNG encoders capable of full color management will perform more sophisticated calculations than those described here and may choose to use theiCCP chunk. If it is known that the image samples conform to the sRGB specification [SRGB], encoders are strongly encouraged to write thesRGB chunk without performing additionalgamma handling. In both cases it is recommended that an appropriategAMA chunk be generated for use by PNG decoders that do not recognize theiCCP orsRGB chunks.

A PNG encoder has to determine:

1. what value to write in thegAMA chunk;
2. how to transform the provided image samples into the values to be written in the PNG datastream.

The value to write in thegAMA chunk is that value which causes a PNG decoder to behave in the desired way. See13.13Decoder gamma handling.

The transform to be applied depends on the nature of the image samples and their precision. If the samples represent light intensity in floating-point or high precision integer form (perhaps from a computer graphics renderer), the encoder may performgamma encoding (applying a power function with exponent less than 1) before quantizing the data to integer values for inclusion in the PNG datastream. This results in fewer banding artifacts at a given sample depth, or allows smaller samples while retaining the same visual quality. An intensity level expressed as a floating-point value in the range 0 to 1 can be converted to a datastream image sample by:

integer_sample = floor((2sampledepth-1) * intensityencoding_exponent + 0.5)

If the intensity in the equation is the desired output intensity, the encoding exponent is thegamma value to be used in thegAMA chunk.

If the intensity available to the PNG encoder is the original scene intensity, another transformation may be needed. There is sometimes a requirement for the displayed image to have higher contrast than the original source image. This corresponds to an end-to-endtransfer function from original scene to display output with an exponent greater than 1. In this case:

gamma= encoding_exponent/end_to_end_exponent

If it is not known whether the conditions under which the original image was captured or calculated warrant such a contrast change, it may be assumed that the display intensities are proportional to original scene intensities, i.e. the end-to-end exponent is 1 and hence:

gamma= encoding_exponent

If the image is being written to a datastream only, the encoder is free to choose the encoding exponent. Choosing a value that causes thegamma value in thegAMA chunk to be 1/2.2 is often a reasonable choice because it minimizes the work for a PNG decoder displaying on a typical video monitor.

Some image renderers may simultaneously write the image to a PNG datastream and display it on-screen. The displayed pixels should begamma corrected for the display system and viewing conditions in use, so that the user sees a proper representation of the intended scene.

If the renderer wants to write the displayed sample values to the PNG datastream, avoiding a separategamma encoding step for the datastream, the renderer should approximate thetransfer function of the display system by a power function, and write the reciprocal of the exponent into thegAMA chunk. This will allow a PNG decoder to reproduce what was displayed on screen for the originator during rendering.

However, it is equally reasonable for a renderer to compute displayed pixels appropriate for the display device, and to perform separategamma encoding for data storage and transmission, arranging to have a value in thegAMA chunk more appropriate to the future use of the image.

Computer graphics renderers often do not performgamma encoding, instead making sample values directly proportional to scene light intensity. If the PNG encoder receives sample values that have already been quantized into integer values, there is no point in doinggamma encoding on them; that would just result in further loss of information. The encoder should just write the sample values to the PNG datastream. This does not imply that thegAMA chunk should contain agamma value of 1.0 because the desired end-to-endtransfer function from scene intensity to display output intensity is not necessarily linear. However, the desiredgamma value is probably not far from 1.0. It may depend on whether the scene being rendered is a daylight scene or an indoor scene, etc.

When the sample values come directly from a piece of hardware, the correctgAMA value can, in principle, be inferred from thetransfer function of the hardware and lighting conditions of the scene. In the case of video digitizers ("frame grabbers"), the samples are probably in the sRGB color space, because the sRGB specification was designed to be compatible with modern video standards. Image scanners are less predictable. Their output samples may be proportional to the input light intensity since CCD sensors themselves are linear, or the scanner hardware may have already applied a power function designed to compensate for dot gain in subsequent printing (an exponent of about 0.57), or the scanner may have corrected the samples for display on a monitor. It may be necessary to refer to the scanner's manual or to scan a calibrated target in order to determine the characteristics of a particular scanner. It should be remembered thatgamma relates samples to desired display output, not to scanner input.

Datastream format converters generally should not attempt to convert supplied images to a differentgamma. The data should be stored in the PNG datastream without conversion, and thegamma value should be deduced from information in the source datastream if possible.Gamma alteration at datastream conversion time causes re-quantization of the set of intensity levels that are represented, introducing further roundoff error with little benefit. It is almost always better to just copy the sample values intact from the input to the output file.

If the source datastream describes thegamma characteristics of the image, a datastream converter is strongly encouraged to write agAMA chunk. Some datastream formats specify the display exponent (the exponent of the function which maps image samples to display output rather than the other direction). If the source file'sgamma value is greater than 1.0, it is probably a display exponent, and the reciprocal of this value should be used for the PNGgamma value. If the source file format records the relationship between image samples and a quantity other than display output, it will be more complex than this to deduce the PNGgamma value.

If a PNG encoder or datastream converter knows that the image has been displayed satisfactorily using a display system whosetransfer function can be approximated by a power function with exponentdisplay_exponent, the image can be marked as having thegamma value:

gamma=1/display_exponent

It is better to write agAMA chunk with a value that is approximately correct than to omit the chunk and force PNG decoders to guess an approximategamma value. If a PNG encoder is unable to infer thegamma value, it is preferable to omit thegAMA chunk. If a guess has to be made this should be left to the PNG decoder.

gamma does not apply to alpha samples; alpha is always represented linearly.

See also13.13Decoder gamma handling.

SeeC.Gamma and chromaticity for references to color issues.

PNG encoders capable of full color management will perform more sophisticated calculations than those described here and may choose to use theiCCP chunk. If it is known that the image samples conform to the sRGB specification [SRGB], PNG encoders are strongly encouraged to use thesRGB chunk.

If it is possible for the encoder to determine the chromaticities of the source display primaries, or to make a strong guess based on the origin of the image, or the hardware running it, the encoder is strongly encouraged to output thecHRM chunk. If this is done, thegAMA chunk should also be written; decoders can do little with acHRM chunk if thegAMA chunk is missing.

There are a number of recommendations and standards for primaries andwhite points, some of which are linked to particular technologies, for example the CCIR 709 standard [ITU-R-BT.709] and the SMPTE-C standard [SMPTE-170M].

There are three cases that need to be considered:

1. the encoder is part of the generation system;
2. the source image is captured by a camera or scanner;
3. the PNG datastream was generated by translation from some other format.

In the case of hand-drawn or digitally edited images, it is necessary to determine what monitor they were viewed on when being produced. Many image editing programs allow the type of monitor being used to be specified. This is often because they are working in some device-independent space internally. Such programs have enough information to write validcHRM andgAMA chunks, and are strongly encouraged to do so automatically.

If the encoder is compiled as a portion of a computer image renderer that performs full-spectral rendering, the monitor values that were used to convert from the internal device-independent color space to RGB should be written into thecHRM chunk. Any colors that are outside the gamut of the chosen RGB device should be mapped to be within the gamut; PNG does not store out-of-gamut colors.

If the computer image renderer performs calculations directly in device-dependent RGB space, acHRM chunk should not be written unless the scene description and rendering parameters have been adjusted for a particular monitor. In that case, the data for that monitor should be used to construct acHRM chunk.

A few image formats store calibration information, which can be used to fill in thecHRM chunk. For example, TIFF 6.0 files [TIFF-6.0] can optionally store calibration information, which if present should be used to construct thecHRM chunk.

Video created with recent video equipment probably uses the CCIR 709 primaries and D65white point [ITU-R-BT.709], which are given inTable29.

An older but still very popular video standard is SMPTE-C [SMPTE-170M] given inTable30.

It isnot recommended that datastream format converters attempt to convert supplied images to a different RGB color space. The data should be stored in the PNG datastream without conversion, and the source primary chromaticities should be recorded if they are known. Color space transformation at datastream conversion time is a bad idea because of gamut mismatches and rounding errors. As withgamma conversions, it is better to store the data losslessly and incur at most one conversion when the image is finally displayed.

See13.14Decoder color handling.

The alpha channel can be regarded either as a mask that temporarily hides transparent parts of the image, or as a means for constructing a non-rectangular image. In the first case, the color values of fully transparent pixels should be preserved for future use. In the second case, the transparent pixels carry no useful data and are simply there to fill out the rectangular image area required by PNG. In this case, fully transparent pixels should all be assigned the same color value for best compression.

Image authors should keep in mind the possibility that a decoder will not support transparency control in full (see13.16Alpha channel processing). Hence, the colors assigned to transparent pixels should be reasonable background colors whenever feasible.

For applications that do not require a full alpha channel, or cannot afford the price in compression efficiency, thetRNS transparency chunk is also available.

If the image has a known background color, this color should be written in thebKGD chunk. Even decoders that ignore transparency may use thebKGD color to fill unused screen area.

If the original image has premultiplied (also called "associated") alpha data, it can be converted to PNG's non-premultiplied format by dividing each sample value by the corresponding alpha value, then multiplying by the maximum value for the image bit depth, and rounding to the nearest integer. In valid premultiplied data, the sample values never exceed their corresponding alpha values, so the result of the division should always be in the range 0 to 1. If the alpha value is zero, output black (zeroes).

When encoding input samples that have a sample depth that cannot be directly represented in PNG, the encoder shall scale the samples up to a sample depth that is allowed by PNG. The most accurate scaling method is the linear equation:

output = floor((input * MAXOUTSAMPLE / MAXINSAMPLE) + 0.5)

where the input samples range from 0 toMAXINSAMPLE and the outputs range from 0 toMAXOUTSAMPLE (which is 2^sampledepth-1).

A close approximation to the linear scaling method is achieved by "left bit replication", which is shifting the valid bits to begin in the most significant bit and repeating the most significant bits into the open bits. This method is often faster to compute than linear scaling.

Assume that 5-bit samples are being scaled up to 8 bits. If the source sample value is 27 (in the range from 0-31), then the original bits are:

4 3 2 1 0---------1 1 0 1 1

Left bit replication gives a value of 222:

7 6 5 4 3  2 1 0----------------1 1 0 1 1  1 1 0|=======|  |===|    |      Leftmost Bits Repeated to Fill Open Bits    |Original Bits

which matches the value computed by the linear equation. Left bit replication usually gives the same value as linear scaling, and is never off by more than one.

A distinctly less accurate approximation is obtained by simply left-shifting the input value and filling the low order bits with zeroes. This scheme cannot reproduce white exactly, since it does not generate an all-ones maximum value; the net effect is to darken the image slightly. This method is not recommended in general, but it does have the effect of improving compression, particularly when dealing with greater-than-8-bit sample depths. Since the relative error introduced by zero-fill scaling is small at high sample depths, some encoders may choose to use it. Zero-fill shallnot be used for alpha channel data, however, since many decoders will treat alpha values of all zeroes and all ones as special cases. It is important to represent both those values exactly in the scaled data.

When the encoder writes ansBIT chunk, it is required to do the scaling in such a way that the high-order bits of the stored samples match the original data. That is, if thesBIT chunk specifies a sample depth of S, the high-order S bits of the stored data shall agree with the original S-bit data values. This allows decoders to recover the original data by shifting right. The added low-order bits are not constrained. All the above scaling methods meet this restriction.

When scaling up sourceimage data, it is recommended that the low-order bits be filled consistently for all samples; that is, the same source value should generate the same sample value at any pixel position. This improves compression by reducing the number of distinct sample values. This is not a mandatory requirement, and some encoders may choose not to follow it. For example, an encoder might instead dither the low-order bits, improving displayed image quality at the price of increasing file size.

In some applications the original source data may have a range that is not a power of 2. The linear scaling equation still works for this case, although the shifting methods do not. It is recommended that ansBIT chunk not be written for such images, sincesBIT suggests that the original data range was exactly 0..2^S-1.

Suggested palettes may appear assPLT chunks in any PNG datastream, or as aPLTE chunk intruecolor PNG datastreams. In either case, the suggested palette is not an essential part of theimage data, but it may be used to present the image on indexed-color display hardware. Suggested palettes are of no interest to viewers running ontruecolor hardware.

When ansPLT chunk is used to provide a suggested palette, it is recommended that the encoder use the frequency fields to indicate the relative importance of the palette entries, rather than leave them all zero (meaning undefined). The frequency values are most easily computed as "nearest neighbor" counts, that is, the approximate usage of each RGBA palette entry if no dithering is applied. (These counts will often be available "for free" as a consequence of developing the suggested palette.) Because the suggested palette includes transparency information, it should be computed for the un-composited image.

Even for indexed-color images,sPLT can be used to define alternative reduced palettes for viewers that are unable to display all the colors present in thePLTE chunk. If thePLTE chunk appears without thebKGD chunk in an image ofcolor type 6, the circumstances under which the palette was computed are unspecified.

An older method for including a suggested palette in atruecolor PNG datastream uses thePLTE chunk. If this method is used, the histogram (frequencies) should appear in a separatehIST chunk. ThePLTE chunk does not include transparency information. Hence for images ofcolor type 6 (truecolor with alpha), it is recommended that abKGD chunk appear and that the palette and histogram be computed with reference to the image as it would appear after compositing against the specified background color. This definition is necessary to ensure that useful palette entries are generated for pixels having fractional alpha values. The resulting palette will probably be useful only to viewers that present the image against the same background color. It is recommended thatPNG editors delete or recompute the palette if they alter or remove thebKGD chunk in an image ofcolor type 6.

For images ofcolor type 2 (truecolor), it is recommended that thePLTE andhIST chunks be computed with reference to the RGB data only, ignoring any transparent-color specification. If the datastream uses transparency (has atRNS chunk), viewers can easily adapt the resulting palette for use with their intended background color (see13.17Histogram and suggested palette usage).

For providing suggested palettes, thesPLT chunk is more flexible than thePLTE chunk in the following ways:

1. WithsPLT multiple suggested palettes may be provided. A PNG decoder may choose an appropriate palette based on name or number of entries.
2. In a PNG datastream ofcolor type 6 (truecolor with alpha channel), thePLTE chunk represents a palette alreadycomposited against thebKGD color, so it is useful only for display against that background color. ThesPLT chunk provides an un-composited palette, which is useful for display against backgrounds chosen by the PNG decoder.
3. Since thesPLT chunk is an ancillary chunk, aPNG editor may add or modify suggested palettes without being forced to discard unknown unsafe-to-copy chunks.
4. Whereas thesPLT chunk is allowed in PNG datastreams forcolor types 0, 3, and 4 (greyscale andindexed-color), thePLTE chunk cannot be used to provide reduced palettes in these cases.
5. More than 256 entries may appear in thesPLT chunk.

A PNG encoder that uses thesPLT chunk may choose to write a suggested palette represented byPLTE andhIST chunks as well, for compatibility with decoders that do not recognize thesPLT chunk.

This specification defines two interlace methods, one of which is no interlacing. Interlacing provides a convenient basis from which decoders can progressively display an image, as described in13.10Interlacing and progressive display.

For images ofcolor type 3 (indexed-color), filter type 0 (None) is usually the most effective. Color images with 256 or fewer colors should almost always be stored inindexed-color format;truecolor format is likely to be much larger.

Filter type 0 is also recommended for images of bit depths less than 8. For low-bit-depth greyscale images, in rare cases, better compression may be obtained by first expanding the image to 8-bit representation and then applying filtering.

Fortruecolor andgreyscale images, any of the five filters may prove the most effective. If an encoder uses a fixed filter, the Paeth filter type is most likely to be the best.

For best compression oftruecolor andgreyscale images, and if compression efficiency is valued over speed of compression, the recommended approach is adaptive filtering in which a filter type is chosen for each scanline. Each unique image will have a different set of filters which perform best for it. An encoder could try every combination of filters to find what compresses best for a given image. However, when an exhaustive search is unacceptable, here are some general heuristics which may perform well enough: compute the output scanline using all five filters, and select the filter that gives the smallest sum of absolute values of outputs. (Consider the output bytes as signed differences for this test.) This method usually outperforms any single fixed filter type choice.

Filtering according to these recommendations is effective in conjunction with either of the two interlace methods defined in this specification.

The encoder may divide the compressed datastream intoIDAT chunks however it wishes. (MultipleIDAT chunks are allowed so that encoders may work in a fixed amount of memory; typically the chunk size will correspond to the encoder's buffer size.) A PNG datastream in which eachIDAT chunk contains only one data byte is valid, though remarkably wasteful of space. (Zero-lengthIDAT chunks are also valid, though even more wasteful.)

A nonempty keyword shall be provided for each text chunk. The generic keyword "Comment" can be used if no better description of the text is available. If a user-supplied keyword is used, encoders should check that it meets the restrictions on keywords.

TheiTXt chunk uses the UTF-8 encoding of Unicode and thus can store text in any language. ThetEXt andzTXt chunks use the Latin-1 (ISO 8859-1) character encoding, which limits the range of characters that can be used in these chunks. Encoders should preferiTXt totEXt andzTXt chunks, in order to allow a wide range of characters without data loss. Encoders must convert characters that use locallegacy character encodings to the appropriate encoding when storing text.

When creatingiTXt chunks, encoders should followUTF-8 encode inEncoding Standard.

Encoders should discourage the creation of single lines of text longer than 79 Unicodecode points, in order to facilitate easy reading. It is recommended that text items less than 1024 bytes in size should be output using uncompressed text chunks. It is recommended that the basic title and author keywords be output using uncompressed text chunks. Placing large text chunks after theimage data (after theIDAT chunks) can speed up image display in some situations, as the decoder will decode theimage data first. It is recommended that small text chunks, such as the image title, appear before theIDAT chunks.

This clause gives some requirements and recommendations for PNG decoder behavior and viewer behavior. A viewer presents the decoded PNG image to the user. Since viewer and decoder behavior are closely connected, decoders and viewers are treated together here. The only absolute requirement on a PNG decoder is that it successfully reads any datastream conforming to the format specified in the preceding chapters. However, best results will usually be achieved by following these additional recommendations.

PNG decoders shall support all valid combinations of bit depth,color type, compression method,filter method, and interlace method that are explicitly defined in this International Standard.

Errors in a PNG datastream will fall into two general classes, transmission errors and syntax errors (see4.10Error handling).

Examples of transmission errors are transmission in "text" or "ascii" mode, in which byte codes 13 and/or 10 may be added, removed, or converted throughout the datastream; unexpected termination, in which the datastream is truncated; or a physical error on a storage device, in which one or more blocks (typically 512 bytes each) will have garbled or random values. Some examples of syntax errors are an invalid value for a row filter, an invalid compression method, an invalid chunk length, the absence of aPLTE chunk before the firstIDAT chunk in an indexed image, or the presence of multiplegAMA chunks. A PNG decoder should handle errors as follows:

1. Detect errors as early as possible using the PNG signature bytes and CRCs on each chunk. Decoders should verify that all eight bytes of the PNG signature are correct. A decoder can have additional confidence in the datastream's integrity if the next eight bytes begin anIHDR chunk with the correct chunk length. ACRC should be checked before processing the chunk data. Sometimes this is impractical, for example when a streaming PNG decoder is processing a largeIDAT chunk. In this case theCRC should be checked when the end of the chunk is reached.
2. Recover from an error, if possible; otherwise fail gracefully. Errors that have little or no effect on the processing of the image may be ignored, while those that affect critical data shall be dealt with in a manner appropriate to the application.
3. Provide helpful messages describing errors, including recoverable errors.

Three classes of PNG chunks are relevant to this philosophy. For the purposes of this classification, an "unknown chunk" is either one whose type was genuinely unknown to the decoder's author, or one that the author chose to treat as unknown, because default handling of that chunk type would be sufficient for the program's purposes. Other chunks are called "known chunks". Given this definition, the three classes are as follows:

1. known chunks, which necessarily includes all of the critical chunks defined in this specification (IHDR,PLTE,IDAT,IEND)
2. unknown critical chunks (bit 5 of the first byte of the chunk type is 0)
3. unknown ancillary chunks (bit 5 of the first byte of the chunk type is 1)

See5.4Chunk naming conventions for a description of chunk naming conventions.

PNG chunk types are marked "critical" or "ancillary" according to whether the chunks are critical for the purpose of extracting a viewable image (as withIHDR,PLTE, andIDAT) or critical to understanding the datastream structure (as withIEND). This is a specific kind of criticality and one that is not necessarily relevant to every conceivable decoder. For example, a program whose sole purpose is to extract text annotations (for example, copyright information) does not require a viewable image but shoulddecode UTF-8 correctly. Another decoder might consider thetRNS andgAMA chunks essential to its proper execution.

Syntax errors always involve known chunks because syntax errors in unknown chunks cannot be detected. The PNG decoder has to determine whether a syntax error is fatal (unrecoverable) or not, depending on its requirements and the situation. For example, most decoders can ignore an invalidIEND chunk; a text-extraction program can ignore the absence ofIDAT; an image viewer cannot recover from an emptyPLTE chunk in an indexed image but it can ignore an invalidPLTE chunk in atruecolor image; and a program that extracts the alpha channel can ignore an invalidgAMA chunk, but may consider the presence of twotRNS chunks to be a fatal error. Anomalous situations other than syntax errors shall be treated as follows:

1. Encountering an unknown ancillary chunk is never an error. The chunk can simply be ignored.
2. Encountering an unknown critical chunk is a fatal condition for any decoder trying to extract the image from the datastream. A decoder that ignored a critical chunk could not know whether the image it extracted was the one intended by the encoder.
3. A PNG signature mismatch, aCRC mismatch, or an unexpected end-of-stream indicates a corrupted datastream, and may be regarded as a fatal error. A decoder could try to salvage something from the datastream, but the extent of the damage will not be known.

When a fatal condition occurs, the decoder should fail immediately, signal an error to the user if appropriate, and optionally continue displaying anyimage data already visible to the user (i.e. "fail gracefully"). The application as a whole need not terminate.

When a non-fatal error occurs, the decoder should signal a warning to the user if appropriate, recover from the error, and continue processing normally.

When decoding an indexed-color PNG, if out-of-range indexes are encountered, decoders have historically varied in their handling of this error.Displaying the pixel as opaque black is one common error recovery tactic, and is now required by this specification. Older implementations will vary, and so the behavior must not be relied on by encoders.

Decoders that do not compute CRCs should interpret apparent syntax errors as indications of corruption (see also13.2Error checking).

Errors in compressed chunks (IDAT,zTXt,iTXt,iCCP) could lead to buffer overruns. Implementors ofdeflate decompressors should guard against this possibility.

APNG is designed to allow incremental display of frames before the entiredatastream has been read. This implies that some errors may not be detected until partway through the animation. It is strongly recommended that when any error is encountered decoders should discard all subsequent frames, stop the animation, and revert to displaying the static image. A decoder which detects an error before the animation has started should display the static image. An error message may be displayed to the user if appropriate.

Decoders shall treat out-of-orderAPNG chunks as an error.APNG-awarePNG editors should restore them to correct order, using the sequence numbers.

The PNG error handling philosophy is described in13.1Error handling.

An unknown chunk type isnot to be treated as an error unless it is a critical chunk.

The chunk type can be checked for plausibility by seeing whether all four bytes are in the range codes 41-5A and 61-7A (hexadecimal); note that this need be done only for unrecognized chunk types. If the total datastream size is known (from file system information, HTTP protocol, etc), the chunk length can be checked for plausibility as well. If CRCs are not checked, dropped/added data bytes or an erroneous chunk length can cause the decoder to get out of step and misinterpret subsequent data as a chunk header.

For known-length chunks, such asIHDR, decoders should treat an unexpected chunk length as an error. Future extensions to this specification will not add new fields to existing chunks; instead, new chunk types will be added to carry new information.

Unexpected values in fields of known chunks (for example, an unexpected compression method in theIHDR chunk) shall be checked for and treated as errors. However, it is recommended that unexpected field values be treated as fatal errors only incritical chunks. An unexpected value in an ancillary chunk can be handled by ignoring the whole chunk as though it were an unknown chunk type. (This recommendation assumes that the chunk'sCRC has been verified. In decoders that do not check CRCs, it is safer to treat any unexpected value as indicating a corrupted datastream.)

Standard PNG images shall be compressed with compression method 0. The compression method field of theIHDR chunk is provided for possible future standardization or proprietary variants. Decoders shall check this byte and report an error if it holds an unrecognized code. See10.Compression for details.

A PNG datastream is composed of a collection of explicitly typed chunks. Chunks whose contents are defined by the specification could actually contain anything, including malicious code. Similarly there could be data after theIEND chunk which could contain anything, including malicious code. There is no known risk that such malicious code could be executed on the recipient's computeras a result of decoding the PNG image. However, a malicious application might hide such code inside an innocent-looking image file and then execute it.

The possible security risks associated with future chunk types cannot be specified at this time. Security issues will be considered when defining future public chunks. There is no additional security risk associated with unknown or unimplemented chunk types, because such chunks will be ignored, or at most be copied into another PNG datastream.

TheiTXt,tEXt, andzTXt chunks contain keywords and data that are meant to be displayed as plain text. TheiCCP andsPLT chunks contain keywords that are meant to be displayed as plain text. It is possible that if the decoder displays such text without filtering out control characters, especially the ESC (escape) character, certain systems or terminals could behave in undesirable and insecure ways. It is recommended that decoders filter out control characters to avoid this risk; see13.7Text chunk processing.

For theeXIf chunk, the Exif Specification [CIPA-DC-008] does not contain an express requirement that tag "value offset" pointers must actually point to a valid address within the file. This requirement is merely implied. (See Paragraph 4.6.2, which describes the Exif IFD structure.) Regardless, decoders should be prepared to encounter invalid pointers and to handle them appropriately.

Every chunk begins with a length field, which makes it easier to write decoders that are invulnerable to fraudulent chunks that attempt to overflow buffers. TheCRC at the end of every chunk provides a robust defence against accidentally corrupted data. The PNG signature bytes provide early detection of common file transmission errors.

A decoder that fails to check CRCs could be subject to data corruption. The only likely consequence of such corruption is incorrectly displayed pixels within the image. Worse things might happen if theCRC of theIHDR chunk is not checked and the width or height fields are corrupted. See13.2Error checking.

A poorly written decoder might be subject to buffer overflow, because chunks can be extremely large, up to 2³¹-1 bytes long. But properly written decoders will handle large chunks without difficulty.

Some image editing tools have historically performed redaction by merely setting the alpha channel of the redacted area to zero, without also removing the actual image data. Users who rely solely on the visual appearance of such images run a privacy risk because the actual image data can be easily recovered.

Similarly, some image editing tools have historically performed clipping by rewriting the width and height inIHDR without re-encoding the image data, which thus extends beyond the new width and height and may be recovered.

Images witheXIf chunks may contain automatically-included data, such as photographic GPS coordinates, which could be a privacy risk if the user is unaware that the PNG image contains this data. (Other image formats that contain EXIF, such as JPEG/JFIF, have the same privacy risk).

Decoders shall recognize chunk types by a simple four-byte literal comparison; it is incorrect to perform case conversion on chunk types. A decoder encountering an unknown chunk in which the ancillary bit is 1 may safely ignore the chunk and proceed to display the image. A decoder trying to extract the image, upon encountering an unknown chunk in which the ancillary bit is 0, indicating a critical chunk, shall indicate to the user that the image contains information it cannot safely interpret.

Decoders should test the properties of an unknown chunk type by numerically testing the specified bits. Testing whether a character is uppercase or lowercase is inefficient, and even incorrect if a locale-specific case definition is used.

Decoders should not flag an error if the reserved bit is set to 1, however, as some future version of the PNG specification could define a meaning for this bit. It is sufficient to treat a chunk with this bit set in the same way as any other unknown chunk type.

Decoders do not need to test the chunk type private bit, since it has no functional significance and is used to avoid conflicts between chunks defined byW3C and those defined privately.

All ancillary chunks are optional; decoders may ignore them. However, decoders are encouraged to interpret these chunks when appropriate and feasible.

Non-square pixels can be represented (see11.3.4.3pHYs Physical pixel dimensions), but viewers are not required to account for them; a viewer can present any PNG datastream as though its pixels are square.

Where the pixel aspect ratio of the display differs from the aspect ratio of the physical pixel dimensions defined in the PNG datastream, viewers are strongly encouraged to rescale images for proper display.

When thepHYs chunk has a unit specifier of 0 (unit is unknown), the behavior of a decoder may depend on the ratio of the two pixels-per-unit values, but should not depend on their magnitudes. For example, apHYs chunk containing(ppuX, ppuY, unit) = (2, 1, 0) is equivalent to one containing(1000, 500, 0); both are equally valid indications that the image pixels are twice as tall as they are wide.

One reasonable way for viewers to handle a difference between the pixel aspect ratios of the image and the display is to expand the image either horizontally or vertically, but not both. The scale factors could be obtained using the following floating-point calculations:

image_ratio = pHYs_ppuY / pHYs_ppuXdisplay_ratio = display_ppuY / display_ppuXscale_factor_X = max(1.0, image_ratio/display_ratio)scale_factor_Y = max(1.0, display_ratio/image_ratio)

Because other methods such as maintaining the image area are also reasonable, and because ignoring thepHYs chunk is permissible, authors should not assume that all viewing applications will use this scaling method.

As well as making corrections for pixel aspect ratio, a viewer may have reasons to perform additional scaling both horizontally and vertically. For example, a viewer might want to shrink an image that is too large to fit on the display, or to expand images sent to a high-resolution printer so that they appear the same size as they did on the display.

If practical, PNG decoders should have a way to display to the user all theiTXt,tEXt, andzTXt chunks found in the datastream. Even if the decoder does not recognize a particular text keyword, the user might be able to understand it.

When processingtEXt andzTXt chunks, decoders could encounter characters other than those permitted. Some can be safely displayed (e.g., TAB, FF, and CR, hexadecimal 09, 0C, and 0D, respectively), but others, especially the ESC character (hexadecimal 1B), could pose a security hazard (because unexpected actions may be taken by display hardware or software). Decoders should not attempt to directly display any non-Latin-1 characters (except for newline and perhaps TAB, FF, CR) encountered in atEXt orzTXt chunk. Instead, they should be ignored or displayed in a visible notation such as "\nnn". See13.3Security considerations.

When processingiTXt chunks, decoders should followUTF-8 decode inEncoding Standard.

Even though encoders are recommended to represent newlines as linefeed (hexadecimal 0A), it is recommended that decoders not rely on this; it is best to recognize all the common newline combinations (CR, LF, and CR-LF) and display each as a single newline. TAB can be expanded to the proper number of spaces needed to arrive at a column multiple of 8.

Decoders running on systems with a non-Latin-1legacy character encoding should remap character codes so that Latin-1 characters are displayed correctly. Unsupported characters should be replaced with a system-appropriate replacement character (such as U+FFFD REPLACEMENT CHARACTER, U+003F QUESTION MARK, or U+001A SUB) or mapped to a visible notation such as "\nnn". Characters should be only displayed if they are printable characters on the decoding system. Some byte values may be interpreted by the decoding system as control characters; for security, decoders running on such systems should not display these control characters.

Decoders should be prepared to display text chunks that contain any number of printing characters between newline characters, even though it is recommended that encoders avoid creating lines in excess of 79 characters.

The compression technique used in this specification does not require the entire compressed datastream to be available before decompression can start. Display can therefore commence before the entire decompressed datastream is available. It is extremely unlikely that any general purpose compression methods in future versions of this specification will not have this property.

It is important to emphasize thatIDAT chunk boundaries have no semantic significance and can occur at any point in the compressed datastream. There is no required correlation between the structure of theimage data (for example, scanline boundaries) anddeflate block boundaries orIDAT chunk boundaries. The completeimage data is represented by a singlezlib datastream that is stored in some number ofIDAT chunks; a decoder that assumes any more than this is incorrect. Some encoder implementations may emit datastreams in which some of these structures are indeed related, but decoders cannot rely on this.

To reverse the effect of a filter, the decoder may need to use the decoded values of the prior pixel on the same line, the pixel immediately above the current pixel on the prior line, and the pixel just to the left of the pixel above. This implies that at least one scanline's worth ofimage data needs to be stored by the decoder at all times. Even though some filter types do not refer to the prior scanline, the decoder will always need to store each scanline as it is decoded, since the next scanline might use a filter type that refers to it. See7.3Filtering.

Decoders are required to be able to read interlaced images. If the reference image contains fewer than five columns or fewer than five rows, some passes will be empty. Encoders and decoders shall handle this case correctly. In particular, filter type bytes are associated only with nonempty scanlines; no filter type bytes are present in an empty reduced image.

When receiving images over slow transmission links, viewers can improve perceived performance by displaying interlaced images progressively. This means that as each reduced image is received, an approximation to the complete image is displayed based on the data received so far. One simple yet pleasing effect can be obtained by expanding each received pixel to fill a rectangle covering the yet-to-be-transmitted pixel positions below and to the right of the received pixel. This process can be described by the following ISO C code [ISO_9899]:

/*    variables declared and initialized elsewhere in the code:        height, width    functions or macros defined elsewhere in the code:        visit(), min() */int starting_row[7]  = {0,0,4,0,2,0,1 };int starting_col[7]  = {0,4,0,2,0,1,0 };int row_increment[7] = {8,8,8,4,4,2,2 };int col_increment[7] = {8,8,4,4,2,2,1 };int block_height[7]  = {8,8,4,4,2,2,1 };int block_width[7]   = {8,4,4,2,2,1,1 };int pass;long row, col;pass =0;while (pass <7){    row = starting_row[pass];while (row < height)    {        col = starting_col[pass];while (col < width)        {visit(row, col,min(block_height[pass], height - row),min(block_width[pass], width - col));            col = col + col_increment[pass];        }        row = row + row_increment[pass];    }    pass = pass +1;}

The functionvisit(row,column,height,width) obtains the next transmitted pixel and paints a rectangle of the specified height and width, whose upper-left corner is at the specified row and column, using the color indicated by the pixel. Note that row and column are measured from 0,0 at the upper left corner.

If the viewer is merging the received image with a background image, it may be more convenient just to paint the received pixel positions (thevisit() function sets only the pixel at the specified row and column, not the whole rectangle). This produces a "fade-in" effect as the new image gradually replaces the old. An advantage of this approach is that proper alpha or transparency processing can be done as each pixel is replaced. Painting a rectangle as described above will overwrite background-image pixels that may be needed later, if the pixels eventually received for those positions turn out to be wholly or partially transparent. This is a problem only if the background image is not stored anywhere offscreen.

To achieve PNG's goal of universal interchangeability, decoders shall accept all types of PNG image:indexed-color,truecolor, andgreyscale. Viewers running on indexed-color display hardware need to be able to reducetruecolor images to indexed-color for viewing. This process is called "color quantization".

A simple, fast method for color quantization is to reduce the image to a fixed palette. Palettes with uniform color spacing ("color cubes") are usually used to minimize the per-pixel computation. For photograph-like images, dithering is recommended to avoid ugly contours in what should be smooth gradients; however, dithering introduces graininess that can be objectionable.

The quality of rendering can be improved substantially by using a palette chosen specifically for the image, since a color cube usually has numerous entries that are unused in any particular image. This approach requires more work, first in choosing the palette, and second in mapping individual pixels to the closest available color. PNG allows the encoder to supply suggested palettes, but not all encoders will do so, and the suggested palettes may be unsuitable in any case (they may have too many or too few colors). Therefore, high-quality viewers will need to have a palette selection routine at hand. A large lookup table is usually the most feasible way of mapping individual pixels to palette entries with adequate speed.

Numerous implementations of color quantization are available. The PNG sample implementation, libpng (http://www.libpng.org/pub/png/libpng.html), includes code for the purpose.

Decoders may wish to scale PNG data to a lesser sample depth (data precision) for display. For example, 16-bit data will need to be reduced to 8-bit depth for use on most present-day display hardware. Reduction of 8-bit data to 5-bit depth is also common.

The most accurate scaling is achieved by the linear equation

output = floor((input * MAXOUTSAMPLE / MAXINSAMPLE) + 0.5)

where

MAXINSAMPLE = (2sampledepth)-1
MAXOUTSAMPLE = (2desired_sampledepth)-1

A slightly less accurate conversion is achieved by simply shifting right by(sampledepth - desired_sampledepth) places. For example, to reduce 16-bit samples to 8-bit, the low-order byte can be discarded. In many situations the shift method is sufficiently accurate for display purposes, and it is certainly much faster. (But ifgamma correction is being done, sample rescaling can be merged into thegamma correction lookup table, as is illustrated in13.13Decoder gamma handling.)

If the decoder needs to scale samples up (for example, if theframe buffer has a greater sample depth than the PNG image), it should use linear scaling or left-bit-replication as described in12.4Sample depth scaling.

When ansBIT chunk is present, the referenceimage data can be recovered by shifting right to the sample depth specified bysBIT. Note that linear scaling will not necessarily reproduce the original data, because the encoder is not required to have used linear scaling to scale the data up. However, the encoder is required to have used a method that preserves the high-order bits, so shifting always works. This is the only case in which shifting might be said to be more accurate than linear scaling. A decoder need not pay attention to thesBIT chunk; the stored image is a valid PNG datastream of the sample depth indicated by theIHDR chunk; however, usingsBIT to recover the original samples before scaling them to suit the display often yields a more accurate display than ignoringsBIT.

When comparing pixel values totRNS chunk values to detect transparent pixels, the comparison shall be done exactly. Therefore, transparent pixel detection shall be done before reducing sample precision.

SeeC.Gamma and chromaticity for a brief introduction togamma issues.

Viewers capable of full color management will perform more sophisticated calculations than those described here.

For an image display program to produce correct tone reproduction, it is necessary to take into account the relationship between samples and display output, and thetransfer function of the display system. This can be done by calculating:

sample = integer_sample / (2sampledepth - 1.0)
display_output = sample1.0/gamma
display_input = inverse_display_transfer(display_output)
framebuf_sample = floor((display_input * MAX_FRAMEBUF_SAMPLE)+0.5)

whereinteger_sample is the sample value from the datastream,framebuf_sample is the value to write into theframe buffer, andMAX_FRAMEBUF_SAMPLE is the maximum value of aframe buffer sample (255 for 8-bit, 31 for 5-bit, etc). The first line converts an integer sample into a normalized floating point value (in the range 0.0 to 1.0), the second converts to a value proportional to the desired display output intensity, the third accounts for the display system'stransfer function, and the fourth converts to an integerframe buffer sample. Zero raised to any positive power is zero.

A step could be inserted between the second and third to adjustdisplay_output to account for the difference between the actual viewing conditions and the reference viewing conditions. However, this adjustment requires accounting for veiling glare, black mapping, and color appearance models, none of which can be well approximated by power functions. Such calculations are not described here. If viewing conditions are ignored, the error will usually be small.

The displaytransfer function can typically be approximated by a power function with exponentdisplay_exponent, in which case the second and third lines can be merged into:

display_input = sample1.0/(gamma * display_exponent) = sampledecoding_exponent

so as to perform only one power calculation. For color images, the entire calculation is performed separately for R, G, and B values.

Thegamma value can be taken directly from thegAMA chunk. Alternatively, an application may wish to allow the user to adjust the appearance of the displayed image by influencing thegamma value. For example, the user could manually set a parameteruser_exponent which defaults to 1.0, and the application could set:

gamma= gamma_from_file / user_exponentdecoding_exponent=1.0 / (gamma * display_exponent)= user_exponent / (gamma_from_file * display_exponent)

The user would setuser_exponent greater than 1 to darken the mid-level tones, or less than 1 to lighten them.

AgAMA chunk containing zero is meaningless but could appear by mistake. Decoders should ignore it, and editors may discard it and issue a warning to the user.

It isnot necessary to perform a transcendental mathematical computation for every pixel. Instead, a lookup table can be computed that gives the correct output value for every possible sample value. This requires only 256 calculations per image (for 8-bit accuracy), not one or three calculations per pixel. For an indexed-color image, a one-time correction of the palette is sufficient, unless the image uses transparency and is being displayed against a nonuniform background.

If floating-point calculations are not possible,gamma correction tables can be computed using integer arithmetic and a precomputed table of logarithms. Example code appears in [PNG-EXTENSIONS].

When the incoming image has unknowngamma value (gAMA,sRGB, andiCCP all absent), standalone image viewers should choose a likely defaultgamma value, but allow the user to select a new one if the result proves too dark or too light. The defaultgamma value may depend on other knowledge about the image, for example whether it came from the Internet or from the local system. For consistency, viewers for document formats such as HTML, or vector graphics such as SVG, should treat embedded or linked PNG images with unknowngamma value in the same way that they treat other untagged images.

In practice, it is often difficult to determine what value of display exponent should be used. In systems with no built-ingamma correction, the display exponent is determined entirely by theCRT. A display exponent of 2.2 should be used unless detailed calibration measurements are available for the particularCRT used.

Many modernframe buffers have lookup tables that are used to performgamma correction, and on these systems the display exponent value should be the exponent of the lookup table andCRT combined. It may not be possible to find out what the lookup table contains from within the viewer application, in which case it may be necessary to ask the user to supply the display system's exponent value. Unfortunately, different manufacturers use different ways of specifying what should go into the lookup table, so interpretation of the systemgamma value is system-dependent.

The response of real displays is actually more complex than can be described by a single number (the display exponent). If actual measurements of the monitor's light output as a function of voltage input are available, the third and fourth lines of the computation above can be replaced by a lookup in these measurements, to find the actualframe buffer value that most nearly gives the desired brightness.

SeeC.Gamma and chromaticity for references to color issues.

In many cases, theimage data in PNG datastreams will be treated as device-dependent RGB values and displayed without modification (except for appropriategamma correction). This provides the fastest display of PNG images. But unless the viewer uses exactly the same display hardware as that used by the author of the original image, the colors will not be exactly the same as those seen by the original author, particularly for darker or near-neutral colors. ThecHRM chunk provides information that allows closer color matching than that provided bygamma correction alone.

ThecHRM data can be used to transform theimage data from RGB to XYZ and thence into a perceptually linear color space such as CIE LAB. The colors can be partitioned to generate an optimal palette, because the geometric distance between two colors in CIE LAB is strongly related to how different those colors appear (unlike, for example, RGB or XYZ spaces). The resulting palette of colors, once transformed back into RGB color space, could be used for display or written into aPLTE chunk.

Decoders that are part of image processing applications might also transformimage data into CIE LAB space for analysis.

In applications where color fidelity is critical, such as product design, scientific visualization, medicine, architecture, or advertising, PNG decoders can transform theimage data from source RGB to the display RGB space of the monitor used to view the image. This involves calculating the matrix to go from source RGB to XYZ and the matrix to go from XYZ to display RGB, then combining them to produce the overall transformation. The PNG decoder is responsible for implementing gamut mapping.

Decoders running on platforms that have a Color Management System (CMS) can pass theimage data,gAMA, andcHRM values to the CMS for display or further processing.

PNG decoders that provide color printing facilities can use the facilities in Level 2 PostScript to specifyimage data in calibrated RGB space or in a device-independent color space such as XYZ. This will provide better color fidelity than a simple RGB to CMYK conversion. The PostScript Language Reference manual [PostScript] gives examples. Such decoders are responsible for implementing gamut mapping between source RGB (specified in thecHRM chunk) and the target printer. The PostScript interpreter is then responsible for producing the required colors.

PNG decoders can use thecHRM data to calculate an accurate greyscale representation of a color image. Conversion from RGB to grey is simply a case of calculating the Y (luminance) component of XYZ, which is a weighted sum of R, G, and B values. The weights depend upon the monitor type, i.e. the values in thecHRM chunk. PNG decoders may wish to do this for PNG datastreams with nocHRM chunk. In this case, a reasonable default would be the CCIR 709 primaries [ITU-R-BT.709]. The original NTSC primaries shouldnot be used unless the PNG image really was color-balanced for such a monitor.

The background color given by thebKGD chunk will typically be used to fill unused screen space around the image, as well as any transparent pixels within the image. (Thus,bKGD is valid and useful even when the image does not use transparency.) If nobKGD chunk is present, the viewer will need to decide upon a suitable background color. When no other information is available, a medium grey such as 153 in the 8-bit sRGB color space would be a reasonable choice. Transparent black or white text and dark drop shadows, which are common, would all be legible against this background.

Viewers that have a specific background against which to present the image (such as web browsers) should ignore thebKGD chunk, in effect overridingbKGD with their preferred background color or background image.

The background color given by thebKGD chunk is not to be considered transparent, even if it happens to match the color given by thetRNS chunk (or, in the case of anindexed-color image, refers to a palette index that is marked as transparent by thetRNS chunk). Otherwise one would have to imagine something "behind the background" tocomposite against. The background color is either used as background or ignored; it is not an intermediate layer between the PNG image and some other background.

Indeed, it will be common that thebKGD andtRNS chunks specify the same color, since then a decoder that does not implement transparency processing will give the intended display, at least when no partially-transparent pixels are present.

The alpha channel can be used tocomposite a foreground image against a background image. The PNG datastream defines the foreground image and the transparency mask, but not the background image. PNG decoders arenot required to support this most general case. It is expected that most will be able to support compositing against a single background color.

The equation for computing acomposited sample value is:

output = alpha * foreground + (1-alpha) * background

where alpha and the input and output sample values are expressed as fractions in the range 0 to 1. This computation should be performed with intensity samples (notgamma-encoded samples). For color images, the computation is done separately for R, G, and B samples.

The following code illustrates the general case of compositing a foreground image against a background image. It assumes that the original pixel data are available for the background image, and that output is to aframe buffer for display. Other variants are possible; see the comments below the code. The code allows the sample depths andgamma values of foreground image and background image all to be different and not necessarily suited to the display system. In practice no assumptions about equality should be made without first checking.

This code is ISO C [ISO_9899], with line numbers added for reference in the comments below.

01  int foreground[4];/* image pixel: R, G, B, A */02  int background[3];/* background pixel: R, G, B */03  int fbpix[3];/* frame buffer pixel */04  int fg_maxsample;/* foreground max sample */05  int bg_maxsample;/* background max sample */06  int fb_maxsample;/* frame buffer max sample */07  int ialpha;08  float alpha, compalpha;09  float gamfg, linfg, gambg, linbg, comppix, gcvideo;/* Get max sample values in data and frame buffer */10  fg_maxsample = (1 << fg_sample_depth) -1;11  bg_maxsample = (1 << bg_sample_depth) -1;12  fb_maxsample = (1 << frame_buffer_sample_depth) -1;/*     * Get integer version of alpha.     * Check for opaque and transparent special cases;     * no compositing needed if so.     *     * We show the whole gamma decode/correct process in     * floating point, but it would more likely be done     * with lookup tables.     */13  ialpha = foreground[3];14if (ialpha ==0) {/*         * Foreground image is transparent here.         * If the background image is already in the frame         * buffer, there is nothing to do.         */15      ;16  }elseif (ialpha == fg_maxsample) {/*         * Copy foreground pixel to frame buffer.         */17for (i =0; i <3; i++) {18          gamfg = (float) foreground[i] / fg_maxsample;19          linfg =pow(gamfg,1.0 / fg_gamma);20          comppix = linfg;21          gcvideo =pow(comppix,1.0 / display_exponent);22          fbpix[i] = (int) (gcvideo * fb_maxsample +0.5);23      }24  }else {/*         * Compositing is necessary.         * Get floating-point alpha and its complement.         * Note: alpha is always linear; gamma does not         * affect it.         */25      alpha = (float) ialpha / fg_maxsample;26      compalpha =1.0 - alpha;27for (i =0; i <3; i++) {/*             * Convert foreground and background to floating             * point, then undo gamma encoding.             */28          gamfg = (float) foreground[i] / fg_maxsample;29          linfg =pow(gamfg,1.0 / fg_gamma);30          gambg = (float) background[i] / bg_maxsample;

31          linbg =pow(gambg,1.0 / bg_gamma);/*             * Composite.             */32          comppix = linfg * alpha + linbg * compalpha;/*             * Gamma correct for display.             * Convert to integer frame buffer pixel.             */33          gcvideo =pow(comppix,1.0 / display_exponent);34          fbpix[i] = (int) (gcvideo * fb_maxsample +0.5);35      }36  }

Variations:

1. If output is to another PNG datastream instead of aframe buffer, lines 21, 22, 33, and 34 should be changed along the following lines

   /* * Gamma encode for storage in output datastream. * Convert to integer sample value. */gamout =pow(comppix, outfile_gamma);outpix[i] = (int) (gamout * out_maxsample +0.5);

   Also, it becomes necessary to process background pixels when alpha is zero, rather than just skipping pixels. Thus, line 15will need to be replaced by copies of lines 17-23, but processing background instead of foreground pixel values.
2. If the sample depths of the output file, foreground file, and background file are all the same, and the threegamma values also match, then the no-compositing code in lines 14-23 reduces to copying pixel values from the input file to the output file if alpha is one, or copying pixel values from background to output file if alpha is zero. Since alpha is typically either zero or one for the vast majority of pixels in an image, this is a significant saving. Nogamma computations are needed for most pixels.
3. When the sample depths andgamma values all match, it may appear attractive to skip thegamma decoding and encoding (lines 28-31, 33-34) and just perform line 32 usinggamma-encoded sample values. Although this does not have too bad an effect on image quality, the time savings are small if alpha values of zero and one are treated as special cases as recommended here.
4. If the original pixel values of the background image are no longer available, only processedframe buffer pixels left by display of the background image, then lines 30 and 31 need to extract intensity from theframe buffer pixel values using code such as

   /* * Convert frame buffer value into intensity sample. */gcvideo = (float) fbpix[i] / fb_maxsample;linbg =pow(gcvideo, display_exponent);

   However, some roundoff error can result, so it is better to have the original background pixels available if at all possible.
5. Note that lines 18-22 are performing exactly the samegamma computation that is done when no alpha channel is present. If the no-alpha case is handled with a lookup table, the same lookup table can be used here. Lines 28-31 and 33-34 can also be done with (different) lookup tables.
6. Integer arithmetic can be used instead of floating point, providing care is taken to maintain sufficient precision throughout.

When displaying a PNG image with full alpha channel, it is important to be able tocomposite the image against some background, even if it is only black. Ignoring the alpha channel will cause PNG images that have been converted from an associated-alpha representation to look wrong. (Of course, if the alpha channel is a separate transparency mask, then ignoring alpha is a useful option: it allows the hidden parts of the image to be recovered.)

Even if the decoder does not implement true compositing logic, it is simple to deal with images that contain only zero and one alpha values. (This is implicitly true forgreyscale andtruecolor PNG datastreams that use atRNS chunk; forindexed-color PNG datastreams it is easy to check whether thetRNS chunk contains any values other than 0 and 255.) In this simple case, transparent pixels are replaced by the background color, while others are unchanged.

If a decoder contains only this much transparency capability, it should deal with a full alpha channel by treating all nonzero alpha values as fully opaque or by dithering. Neither approach will yield very good results for images converted from associated-alpha formats, but this is preferable to doing nothing. Dithering full alpha to binary alpha is very much like dithering greyscale to black-and-white, except that all fully transparent and fully opaque pixels should be left unchanged by the dither.

For viewers running on indexed-color hardware attempting to display atruecolor image, or an indexed-color image whose palette is too large for theframe buffer, the encoder may have provided one or more suggested palettes insPLT chunks. If one of these is found to be suitable, based on size and perhaps name, the PNG decoder can use that palette. Suggested palettes with a sample depth different from what the decoder needs can be converted using sample depth rescaling (see13.12Sample depth rescaling).

When the background is a solid color, the viewer shouldcomposite the image and the suggested palette against that color, then quantize the resulting image to the resulting RGB palette. When the image uses transparency and the background is not a solid color, no suggested palette is likely to be useful.

Fortruecolor images, a suggested palette might also be provided in aPLTE chunk. If the image has atRNS chunk and the background is a solid color, the viewer will need to adapt the suggested palette for use with its desired background color. To do this, the palette entry closest to thetRNS color should be replaced with the desired background color; or alternatively a palette entry for the background color can be added, if the viewer can handle more colors than there arePLTE entries.

For images ofcolor type 6 (truecolor with alpha), anyPLTE chunk should have been designed for display of the image against a uniform background of the color specified by thebKGD chunk. Viewers should probably ignore the palette if they intend to use a different background, or if thebKGD chunk is missing. Viewers can use a suggested palette for display against a different background than it was intended for, but the results may not be very good.

If the viewer presents a transparenttruecolor image against a background that is more complex than a uniform color, it is unlikely that the suggested palette will be optimal for thecomposite image. In this case it is best to perform atruecolor compositing step on thetruecolor PNG image and background image, then color-quantize the resulting image.

Intruecolor PNG datastreams, if bothPLTE andsPLT chunks appear, the PNG decoder may choose from among the palettes suggested by both, bearing in mind the different transparency semantics described above.

The frequencies in thesPLT andhIST chunks are useful when the viewer cannot provide as many colors as are used in the palette in the PNG datastream. If the viewer has a shortfall of only a few colors, it is usually adequate to drop the least-used colors from the palette. To reduce the number of colors substantially, it is best to choose entirely new representative colors, rather than trying to use a subset of the existing palette. This amounts to performing a new color quantization step; however, the existing palette and histogram can be used as the input data, thus avoiding a scan of theimage data in theIDAT chunks.

If no suggested palette is provided, a decoder can develop its own, at the cost of an extra pass over theimage data in theIDAT chunks. Alternatively, a default palette (probably a color cube) can be used.

See also12.5Suggested palettes.

Authors are encouraged to look existing chunk types in both this specification and [PNG-EXTENSIONS] before considering introducing a new chunk types. The chunk types at [PNG-EXTENSIONS] are expected to be less widely supported than those defined in this specification.

Two examples ofPNG editors are a program that adds or modifies text chunks, and a program that adds a suggested palette to atruecolor PNG datastream. Ordinary image editors are notPNG editors because they usually discard all unrecognized information while reading in an image.

To allow new chunk types to be added to PNG, it is necessary to establish rules about the ordering requirements for all chunk types. Otherwise aPNG editor does not know what to do when it encounters an unknown chunk.

EXAMPLE Consider a hypothetical new ancillary chunk type that is safe-to-copy and is required to appear afterPLTE ifPLTE is present. If a program attempts to add aPLTE chunk and does not recognize the new chunk, it may insert thePLTE chunk in the wrong place, namely after the new chunk. Such problems could be prevented by requiringPNG editors to discard all unknown chunks, but that is a very unattractive solution. Instead, PNG requires ancillary chunks not to have ordering restrictions like this.

To prevent this type of problem while allowing for future extension, constraints are placed on both the behavior ofPNG editors and the allowed ordering requirements for chunks. The safe-to-copy bit defines the proper handling of unrecognized chunks in a datastream that is being modified.

1. If a chunk's safe-to-copy bit is 1, the chunk may be copied to a modified PNG datastream whether or not thePNG editor recognizes the chunk type, and regardless of the extent of the datastream modifications.
2. If a chunk's safe-to-copy bit is 0, it indicates that the chunk depends on theimage data. If the program has madeany changes tocritical chunks, including addition, modification, deletion, or reordering of critical chunks, then unrecognized unsafe chunks shallnot be copied to the output PNG datastream. (Of course, if the programdoes recognize the chunk, it can choose to output an appropriately modified version.)
3. APNG editor is always allowed to copy all unrecognized ancillary chunks if it has only added, deleted, modified, or reorderedancillary chunks. This implies that it is not permissible for ancillary chunks to depend on other ancillary chunks.
4. PNG editors shall terminate on encountering an unrecognized critical chunk type, because there is no way to be certain that a valid datastream will result from modifying a datastream containing such a chunk. (Simply discarding the chunk is not good enough, because it might have unknown implications for the interpretation of other chunks.) The safe/unsafe mechanism is intended for use with ancillary chunks. The safe-to-copy bit will always be 0 for critical chunks.

The rules governing ordering of chunks are as follows.

1. When copying an unknownunsafe-to-copy ancillary chunk, aPNG editor shall not move the chunk relative to any critical chunk. It may relocate the chunk freely relative to other ancillary chunks that occur between the same pair of critical chunks. (This is well defined since the editor shall not add, delete, modify, or reorder critical chunks if it is preserving unknown unsafe-to-copy chunks.)
2. When copying an unknownsafe-to-copy ancillary chunk, aPNG editor shall not move the chunk from beforeIDAT to afterIDAT or vice versa. (This is well defined becauseIDAT is always present.) Any other reordering is permitted.
3. When copying aknown ancillary chunk type, an editor need only honour the specific chunk ordering rules that exist for that chunk type. However, it may always choose to apply the above general rules instead.

These rules are expressed in terms of copying chunks from an input datastream to an output datastream, but they apply in the obvious way if a PNG datastream is modified in place.

See also5.4Chunk naming conventions.

PNG editors that do not change theimage data should not change thetIME chunk. The Creation Time keyword in thetEXt,zTXt, andiTXt chunks may be used for a user-supplied time.

This updates the existingimage/png Internet Media type, under theimage top level type. This appendix is in conformance withBCP 13 andW3CRegMedia.

Media type name:

image

Media subtype name:

png

Required parameters:

None

Optional parameters:

None

Encoding considerations:

binary

Security considerations:

A PNG document is composed of a collection of explicitly typed "chunks". For each of the chunk types defined in the PNG specification (except forgIFx), the only effect associated with those chunks is to cause an image to be rendered on the recipient's display or printer.

ThegIFx chunk type is used to encapsulate Application Extension data, and some use of that data might present security risks, though no risks are known. Likewise, the security risks associated with future chunk types cannot be evaluated, particularly unregistered chunks. However, it is the intention of the PNG Working Group to disallow chunks containing "executable" data to become registered chunks.

The text chunks,tEXt,iTXT andzTXt, contain data that can be displayed in the form of comments, etc. Some operating systems or terminals might allow the display of textual data with embedded control characters to perform operations such as re-mapping of keys, creation of files, etc. For this reason, the specification recommends that the text chunks be filtered for control characters before direct display.

The PNG format is specifically designed to facilitate early detection of file transmission errors, and makes use of cyclical redundancy checks to ensure the integrity of the data contained in its chunks.

Interoperability considerations:

Network byte order used throughout.

Published specification:

Portable Network Graphics (PNG) Specification,https://www.w3.org/TR/PNG/

Applications which use this media:

PNG is widely implemented in all Web browsers, image viewers, and image creation tools

Fragment identifier considerations:

N/A

Restrictions on usage:

N/A

Provisional registration? (standards tree only):

No

Additional information:

Deprecated alias names for this type:

N/A

Magic number(s):

89 50 4E 47 0D 0A 1A 0A

File extension(s):

.png

Macintosh file type code:

PNGf

Object Identifiers:

N/A

General Comments:

This registration updates the earlier one:

1. The old one points to an expired Internet Draft. This updated registration points to aW3C Recommendation.
2. The old contact person is sadly deceased. The new contact email is a publicly archivedW3C mailing list for the PNG Working Group.
3. Change controller isW3C

Person to contact for further information:

Name:

PNG Working Group

Email:

public-png@w3.org

Intended usage:

Common

Author/Change controller:

W3C

This appendix is in conformance withBCP 13 andW3CRegMedia.

Media type name:

image

Media subtype name:

apng

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

binary

Security considerations:

AnAPNG document is composed of a collection of explicitly typed "chunks". For each of the chunk types defined in the PNG specification (except forgIFx), the only effect associated with those chunks is to cause an animated image to be rendered on the recipient's display.

ThegIFx chunk type is used to encapsulate Application Extension data, and some use of that data might present security risks, though no risks are known. Likewise, the security risks associated with future chunk types cannot be evaluated, particularly unregistered chunks. However, it is the intention of the PNG Working Group to disallow chunks containing "executable" data to become registered chunks.

The text chunks,tEXt,iTXt andzTXt, contain data that can be displayed in the form of comments, etc. Some operating systems or terminals might allow the display of textual data with embedded control characters to perform operations such as re-mapping of keys, creation of files, etc. For this reason, the specification recommends that the text chunks be filtered for control characters before direct display.

The PNG format is specifically designed to facilitate early detection of file transmission errors, and makes use of cyclical redundancy checks to ensure the integrity of the data contained in its chunks.

If one creates anAPNG file with unrelated static image and animated image chunks, somebody using a tool not supporting theAPNG format would only see the static image and be unaware of the additional content. This could be used e.g. to bypass moderation.

Interoperability considerations:

None

Published specification:

Portable Network Graphics (PNG) Specification,https://www.w3.org/TR/png/

Applications which use this media:

Animated PNG (APNG) is widely implemented in all Web browsers, and is increasingly available in image viewers, and animation and image creation tools

Fragment identifier considerations:

N/A

Restrictions on usage:

N/A

Provisional registration? (standards tree only):

No

Additional information:

Deprecated alias names for this type:

image/vnd.mozilla.apng

Magic number(s):

89 50 4E 47 0D 0A 1A 0A

File extension(s):

.apng

Object Identifiers:

N/A

General Comments:

image/apng has been in widespread, unregistered use since 2015. Animated PNG was not part of the official PNG specification until 2022. This registration, plus the PNG specification (3rd Edition) brings official documentation into alignment with already widely-deployed reality.

Person to contact for further information:

Name:

PNG Working Group

Email:

public-png@w3.org

Intended usage:

Common

Author/Change controller:

W3C

This annex gives the locations of some Internet resources for PNG software developers. By the nature of the Internet, the list is incomplete and subject to change.

ICC profile specifications are available at:https://www.color.org/

There is a World Wide Web site for PNG athttp://www.libpng.org/pub/png/. This page is a central location for current information about PNG and PNG-related tools.

Additional documentation and portable C code fordeflate, and an optimized implementation of theCRC algorithm are available from the zlib web site,https://www.zlib.net/.

A sample implementation in portable C,libpng, is available athttp://www.libpng.org/pub/png/libpng.html. Sample viewer and encoder applications of libpng are available athttp://www.libpng.org/pub/png/book/sources.html and are described in detail inPNG: The Definitive Guide [ROELOFS]. Test images can also be accessed from the PNG web site.

• Credited original authors of theeXIf chunk
• Removed pointless constraint that unsigned integers cannot be negative (!)
• Moved "Collection" keyword from extensions document to main specification
• Copied original security note regarding EXIF, from Extensions document
• Updated reference to latest version of ITU H.273

• RenamedmDCv tomDCV
• RenamedcLLi tocLLI
• Consistently require thatacTL only comes beforeIDAT, notPLTE

• Clarified that bit depth and color type fields can take private values
• Listed both decimal and hexadecimal values in MaxCLL and MaxFALL examples
• Corrected terminology in mDCV section
• Corrected "PNG image" in cHRM section
• Consolidated color chunk precedence information
• Added order of precedence for color space chunks
• Clarified color chunk priorities description
• Mark SMPTE RP 2077 as informative, since it is not freely available
• Added color chunk priority table
• Clarified alpha channels are always full-range
• AddedEOTF andOETF definitions
• Added recommendation for handling negative values resulting from narrow-range images using an extended range transfer function
• Clarified "video" from "video full range flag) is used to match H.273 wording but still applies to still images
• Added link to IANA subtag registry
• Fixed links to alhpa compaction and alpha separation sections
• Fixed broken tRNS link
• Added Display P3 cICP example
• Changed "perceived" wording to "measured", as luminance and chromaticity don't define a perceived color
• Clarified which metadata forms part of HDR10
• Updated link to latest version of ITU-R BT.2390
• Noted that currently there is no published standard to adapt an SDR image from default viewing conditions (display luminance and ambient illumination) to those given inmDCV
• Noted that the adaptation of BT.2100 HLG images to differing viewing conditions, given in BT.2390, may also be used with SDR images.
• Noted that tone mapping, to adjust the luminance levels given incLLI to those of a target display to prevent clipping, is particularly important for formats such as BT.2100 PQ which use absolute luminance.
• Noted that use of full-range BT.709 values, while common, is not part of the BT.709 standard
• Added mention of protected code values for Serial Digital Interface (baseband video) in description of narrow-range and full-range video
• Added reference to ITU-R-BT.2390
• Changed description ofmDCV to focus on BT.2100 PQ, as use with other formats is currently rare
• Clarified matching requirements for 16-bittRNS matching
• Used more precise "primary chromaticities" rather than just "primaries"
• Added requirement thatmDCV requires an accompanyingcICP to fit with expectations of existing HDR10 workflows
• Better explanation ofVideo Full Range Flag
• Clarified ordering and encoding of sequence numbers, clarifiedfdAT concatenation is in order of sequence number
• Added missing mentions ofcICP in descriptions of related chunks
• Fully specified the ordering and byte-order of fields inmDCV
• Added missing definition of PNG two-byte unsigned integer
• Used Display P3, rather than sRGB, in SDRmDCV examples
• Added Chris Needham as co-author
• Updated examples formDCV, added reference to SMPTE-ST-2086.
• Added reference to Display P3
• Explicitly mentioned that the concatenated data fromIDAT andfdAT may include chunks with zero-length data
• Assorted non-substantive improvements to markup, styling, internal cross-linking, correction of typos, punctuation, grammar, consistent use of US English, etc

• Clarified descriptions ofmDCV andcLLi
• Added note to Security Considerations about potentially malicious data afterIEND.
• Clarified thatcLLI is for HDR content
• Added an informative reference to Smith & Zink "On the Calculation and Usage of HDR Static Content Metadata"
• Added an informative reference to CTA-861.3-A for static HDR metadata
• Fixed broken reference to ITU-R BT.2100
• Updated reference to SMPTE-ST-2067-21
• Added guidance on calculating MaxCLL and MaxFALL values
• Added example (live streaming) wherecLLI could not be pre-calculated
• Added definitions for stop, SDR, HDR, HLG and PQ
• Clarified definition of narrow-range
• Updated ITU-T H Suppl. 19 reference to latest version
• Added Simon Thompson as an author
• Mandated current browser handling of out-of-range palette indices
• Updated "Additional Information" table to addmDCV andcLLI.

• Explained preferable handling of trailing bytes in the finalIDAT chunk for encoders and decoders.
• Linked to open issue on tone-mappingHDR [ITU-R-BT.2100] images in the presence ofmDCV.
• Follow the Encoding Standard on UTF-8 encode and decode.
• Added definition of a frame.
• Required the Matrix Coefficients incICP to be zero (RGB data).
• Added known Privacy issue with recoverable data that only appears to have been redacted.
• Improved advice on choosing filters.
• Added links to color image type definitions.
• Clarified that MaxFALL uses the values of the frame with highest mean luminance.
• Clarified luminance units.
• Prefer RFC 3339 format for Creation Time.
• Improved the definition ofmDCV, with better descriptions, default values, and reference to SMPTE standards.
• Refactored the terms and definitions, for clarity.
• Improved definitions of source, reference, and PNG images.
• Moved concepts from the terms and definitions section to the main prose.
• Corrected error ineXIf chunk, which conflicted with thechunk ordering section.
• SimplifiedScope section to remove redundant detail described elsewhere.
• Redrew chunk-ordering lattice diagrams to be clearer and more consistent.
• Added a new chunk,cLLI, to describe the Maximum Single-Pixel and Frame-Average Luminance Levels for both static and animatedHDR [ITU-R-BT.2100] PNG images.
• Updated external links to latest versions, preferring https over http.
• Specified interoperable handling of extra sample bits, beyond the specified bit depth, intRNS andbKGD chunks.
• Added a new chunk,mDCV to describe the color volume of the mastering display used to gradeHDR [ITU-R-BT.2100] content.
• Used correct Unicode character names.
• Changed chunk type codes to use hexadecimal, rather than decimal.
• Described textual chunk processing more clearly.
• RecommendediTXt for new content.
• Clarifications on the language tag field of theiTXt chunk, corrected examples to conform to BCP47.
• Updated image/apng registration appendix.APNG MIME type registered with IANA.
• Converted ACII-art figures to more accessible diagrams.

• The three previously defined, but unofficial, chunks for Animated PNG (APNG) have been added:

  • acTL Animation Control Chunk
  • fcTL Frame Control Chunk
  • fdAT Frame Data Chunk

  This brings the PNG specification into alignment with widely deployed industry practice.

• Added thecICP chunk, Coding-independent code points for video signal type identification, to contain image format metadata defined in [ITU-T-H.273] which enables PNG to contain [ITU-R-BT.2100] High Dynamic Range (HDR) and Wide Color Gamut (WCG) images.

• For chunks which define the image color space, the order of precedence is clearly defined, if more than one is present.

• The previously definedeXIf chunk has been moved from the PNG-Extensions document [PNG-EXTENSIONS] into the main body of this specification, to reflect its increasing use.

• To help with tonemapping HDR content, added themDCV chunk, which contains metadata about the display used in mastering, andcLLI, which contains metadata about peak and average light levels. This enabled more accurate color matching on heterogeneous platforms

• Clarified that theiCCP chunk, which contains an ICC profile, can contain profiles conforming to any version of the ICC.1 specification. PNG Second Edition only referenced the then-current v2 of ICC.1, although it has since become industry practice to also used higher versions.

• Clarified handling of out-of-range indexes, for indexed-color PNG

• Clarified error recovery for unknown and invalid ancillary chunks

• Incorporation of allPNG Second Edition Errata. Notably, clarified that PNG images with unknown gamma value, when embedded in formats such as HTML or SVG, must be treated asuntagged images

• Various editorial clarifications in response to community feedback

• References updated to latest versions

• Markup corrections and link fixes

• Document source reformatted to use ReSpec

For the list of changes betweenW3C RecommendationPNG Specification Version 1.0 andPNG Second Edition, seePNG Second Edition changelist

[BCP47]

Tags for Identifying Languages. A. Phillips, Ed.; M. Davis, Ed. IETF. September 2009. Best Current Practice. URL:https://www.rfc-editor.org/rfc/rfc5646

[CIPA-DC-008]

Exchangeable image file format for digital still cameras: Exif Version 2.32. Camera & Imaging Products Association. 2019-05-17. URL:https://www.cipa.jp/std/documents/download_e.html?DC-008-Translation-2019-E

[COLORIMETRY]

Colorimetry, Fourth Edition. CIE 015:2018. CIE. 2018. URL:http://www.cie.co.at/publications/colorimetry-4th-edition

[Display-P3]

Display P3. Apple, Inc. ICC. 2022-02. URL:https://www.color.org/chardata/rgb/DisplayP3.xalter

[ENCODING]

Encoding Standard. Anne van Kesteren. WHATWG. Living Standard. URL:https://encoding.spec.whatwg.org/

[ICC]

ICC.1:2022 (Profile version 4.4.0.0). International Color Consortium. May 2022. URL:http://www.color.org/specification/ICC.1-2022-05.pdf

[ICC-2]

Specification ICC.2:2019 (Profile version 5.0.0 - iccMAX). International Color Consortium. 2019. URL:https://www.color.org/specification/ICC.2-2019.pdf

[ISO_15076-1]

ISO 15076-1:2010 Image technology colour management — Architecture, profile format and data structure — Part 1: Based on ICC.1:2010. ISO. 2010-12. URL:https://www.iso.org/standard/54754.html

[ISO_8859-1]

ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1.. ISO. 1998.

[ISO_9899]

ISO/IEC 9899:2018 Information technology — Programming languages — C. ISO. 2018-6. URL:https://www.iso.org/standard/74528.html

[ISO-3309]

ISO/IEC 3309:1993, Information Technology — Telecommunications and information exchange between systems — High-level data link control (HDLC) procedures — Frame structure.. ISO. 1993.

[ISO646]

Information technology — ISO 7-bit coded character set for information interchange. International Organization for Standardization (ISO). December 1991. Published. URL:https://www.iso.org/standard/4777.html

[ITU-R-BT.2100]

ITU-R BT.2100, SERIES BT: BROADCASTING SERVICE (TELEVISION). Image parameter values for high dynamic range television for use in production and international programme exchange. ITU. 2018-07. URL:https://www.itu.int/rec/R-REC-BT.2100

[ITU-R-BT.709]

ITU-R BT.709, SERIES BT: BROADCASTING SERVICE (TELEVISION). Parameter values for the HDTV standards for production and international programme exchange. ITU. 2015-06. URL:https://www.itu.int/rec/R-REC-BT.709

[ITU-T-H.273]

ITU-T H.273, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services – Coding of moving video. Coding-independent code points for video signal type identification. ITU. 2024-07-14. URL:https://www.itu.int/rec/T-REC-H.273

[ITU-T-Series-H-Supplement-19]

Series H: Audio Visual and Multimedia Systems - Usage of video signal type code points. ITU. 2021-04. URL:https://www.itu.int/ITU-T/recommendations/rec.aspx?rec=14652

[ITU-T-V.42]

ITU-T V.42, SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK. Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion. ITU. 2002-03-29. URL:https://www.itu.int/rec/T-REC-V.42-200203-I/en

[JPEG]

JPEG File Interchange Format. Eric Hamilton. C-Cube Microsystems. Milpitas, CA, USA. September 1992. URL:https://www.w3.org/Graphics/JPEG/jfif3.pdf

[Paeth]

Image File Compression Made Easy, in Graphics Gems II, pp. 93-100. Paeth, A.W. Academic Press. 1991. URL:https://www.sciencedirect.com/science/article/pii/B9780080507545500293

[PNG-EXTENSIONS]

Extensions to the PNG Third Edition Specification, Version 1.6.0. W3C. 2021. URL:https://w3c.github.io/png/extensions/Overview.html

[rfc1123]

Requirements for Internet Hosts - Application and Support. R. Braden, Ed. IETF. October 1989. Internet Standard. URL:https://www.rfc-editor.org/rfc/rfc1123

[rfc1950]

ZLIB Compressed Data Format Specification version 3.3. P. Deutsch; J-L. Gailly. IETF. May 1996. Informational. URL:https://www.rfc-editor.org/rfc/rfc1950

[RFC1951]

DEFLATE Compressed Data Format Specification version 1.3. P. Deutsch. IETF. May 1996. Informational. URL:https://www.rfc-editor.org/rfc/rfc1951

[RFC1952]

GZIP file format specification version 4.3. P. Deutsch. IETF. May 1996. Informational. URL:https://www.rfc-editor.org/rfc/rfc1952

[RFC2119]

Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL:https://www.rfc-editor.org/rfc/rfc2119

[rfc3339]

Date and Time on the Internet: Timestamps. G. Klyne; C. Newman. IETF. July 2002. Proposed Standard. URL:https://www.rfc-editor.org/rfc/rfc3339

[rfc3629]

UTF-8, a transformation format of ISO 10646. F. Yergeau. IETF. November 2003. Internet Standard. URL:https://www.rfc-editor.org/rfc/rfc3629

[RFC8174]

Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL:https://www.rfc-editor.org/rfc/rfc8174

[SMPTE-170M]

Television — Composite Analog Video Signal — NTSC for Studio Applications. Society of Motion Picture and Television Engineers. 2004-11-30. URL:https://standards.globalspec.com/std/892300/SMPTE%20ST%20170M

[SMPTE-ST-2086]

Mastering Display Color Volume Metadata Supporting High Luminance and Wide Color Gamut Images. Society of Motion Picture and Television Engineers. 27 April 2018. URL:https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8353899

[SRGB]

Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB. IEC. URL:https://webstore.iec.ch/publication/6169

[XMP]

Extensible metadata platform (XMP) specification -- Part 1. Adobe Systems Incorporated. ISO/IEC. April 2019. URL:https://www.iso.org/standard/75163.html

[Ziv-Lempel]

A Universal Algorithm for Sequential Data Compression, IEEE Transactions on Information Theory, vol. IT-23, no. 3, pp. 337 - 343. J. Ziv; A. Lempel. IEEE. 1977-05. URL:https://ieeexplore.ieee.org/document/1055714

[COLOR-FAQ]

Color FAQ. Poynton, C.2009-10-19. URL:https://poynton.ca/ColorFAQ.html

[CTA-861.3-A]

HDR Static Metadata Extensions (CTA-861.3-A). Consumer Technology Association. 2015-01. URL:https://shop.cta.tech/products/hdr-static-metadata-extensions

[EBU-R-103]

Video Signal Tolerance in Digital Television Systems. EBU. 2020-05. URL:https://tech.ebu.ch/docs/r/r103.pdf

[GAMMA-FAQ]

Gamma FAQ. Poynton, C.1998-08-04. URL:https://poynton.ca/GammaFAQ.html

[GIF]

Graphics Interchange Format. CompuServe Incorporated. 31 July 1990. URL:https://www.w3.org/Graphics/GIF/spec-gif89a.txt

[HDR-Static-Meta]

On the Calculation and Usage of HDR Static Content Metadata. Smith, Michael D.; Zink, Michael. Society of Motion Picture and Television Engineers. 2021-08-05. URL:https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9508136

[HDR10]

HDR10 Media Profile. URL:https://en.wikipedia.org/wiki/HDR10

[Hill]

Comparative analysis of the quantization of color spaces on the basis of the CIELAB color-difference formula. Hill, B.; Roger, Th.; Vorhagen, F.W.1997-04. URL:https://dl.acm.org/doi/10.1145/248210.248212

[ITU-R-BT.2020]

Parameter values for ultra-high definition television systems for production and international programme exchange. ITU. URL:https://www.itu.int/rec/R-REC-BT.2020

[ITU-R-BT.2390]

High dynamic range television for production and international programme exchange. ITU. URL:https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BT.2390-11-2023-PDF-E.pdf

[Kasson]

An Analysis of Selected Computer Interchange Color Spaces. Kasson, J.; W. Plouffe. 1992. URL:https://dl.acm.org/doi/abs/10.1145/146443.146479

[Luminance-Chromaticity]

Luminance and Chromaticity. Color Usage Research Lab, NASA Ames Research Center. URL:https://colorusage.arc.nasa.gov/lum_and_chrom.php

[PostScript]

PostScript Language Reference Manual. Adobe Systems Incorporated. Addison-Wesley. 1990.

[ROELOFS]

PNG: The Definitive Guide. Roelofs, G. O'Reilly & Associates Inc. 1999-06-11. URL:http://www.libpng.org/pub/png/pngbook.html

[SMPTE-RP-177]

Derivation of Basic Television Color Equations. Society of Motion Picture and Television Engineers. 1 November 1993. URL:https://standards.globalspec.com/std/1284890/smpte-rp-177

[SMPTE-RP-2077]

Full-Range Image Mapping. Society of Motion Picture and Television Engineers. 2013-01-01. URL:https://doi.org/10.5594/SMPTE.RP2077.2013

[SMPTE-ST-2067-21]

Interoperable Master Format — Application #2E. Society of Motion Picture and Television Engineers. 2023-02-20. URL:https://doi.org/10.5594/SMPTE.ST2067-21.2023

[TIFF-6.0]

TIFF Revision 6.0. 3 June 1992. URL:https://www.loc.gov/preservation/digital/formats/fdd/fdd000022.shtml