A PNG image of four differently coloreddice with an 8-bit transparency channel, overlaid onto a checkered background, typically used ingraphics software to indicate transparency
PNG supports palette-based images (with palettes of 24-bitRGB or 32-bitRGBA colors),grayscale images (with or without analpha channel for transparency), and full-color non-palette-based RGB or RGBA images. The PNG working group designed the format for transferring images on theInternet, not for professional-quality print graphics; therefore, non-RGBcolor spaces such asCMYK are not supported. A PNG file contains a single image in an extensible structure ofchunks, encoding the basicpixels and other information such as textual comments andintegrity checks documented inRFC 2083.[7]
PNG files have the ".png"file extension and the "image/png"MIME media type.[8]PNG was published as aninformational RFC 2083 in March 1997 and as anISO/IEC 15948 standard in 2004.[1]
The motivation for creating the PNG format was the announcement on 28 December 1994 that implementations of theGraphics Interchange Format (GIF) format would have to pay royalties toUnisys due to theirpatent of theLempel–Ziv–Welch (LZW)data compression algorithm used in GIF.[9] This led to a flurry of criticism fromUsenet users. One of them was Thomas Boutell, who on 4 January 1995 posted a precursory discussion thread on theUsenet newsgroup "comp.graphics" in which he devised a plan for a free alternative to GIF. Other users in that thread put forth many propositions that would later be part of the final file format. Oliver Fromme, author of the popularJPEG viewerQPEG, proposed the PING name, eventually becoming PNG, arecursive acronym meaningPING is not GIF,[10] and also the .pngextension. Other suggestions later implemented included thedeflate compression algorithm and24-bit color support, the lack of the latter in GIF also motivating the team to create their file format. The group would become known as the PNG Development Group, and as the discussion rapidly expanded, it later used a mailing list associated with aCompuServe forum.[3][11]
The full specification of PNG was released under the approval ofWorld Wide Web Consortium (W3C) on 1 October 1996, and later asRFC 2083 on 15 January 1997. The specification was revised on 31 December 1998 as version 1.1, which addressed technical problems forgamma andcolor correction. Version 1.2, released on 11 August 1999, added the iTXt chunk as the specification's only change, and a reformatted version of 1.2 was released as a second edition of the W3C standard on 10 November 2003,[12] and as an International Standard (ISO/IEC 15948:2004) on 3 March 2004.[13][1]
Although GIF allows foranimation, it was initially decided that PNG should be a single-image format.[14] In 2001, the developers of PNG published theMultiple-image Network Graphics (MNG) format, with support for animation. MNG achieved moderate application support, but not enough among mainstream web browsers and no usage among web site designers or publishers. In 2008, certainMozilla developers published theAnimated Portable Network Graphics (APNG) format with similar goals. APNG is a format that is natively supported byGecko- andPresto-based web browsers and is also commonly used for thumbnails on Sony'sPlayStation Portable system (using the normal PNG file extension). In 2017, Chromium based browsers adopted APNG support. In January 2020,Microsoft Edge becameChromium based, thus inheriting support for APNG. With this all major browsers now support APNG.
The PNG Working Group has been chartered by the W3C, since September 14, 2021, to maintain and develop for the PNG specification. The third edition of PNG specification, which adds the proper support of APNG,high dynamic range (HDR) andExif data, was published as the first public working draft on October 25, 2022,[15] and ultimately as a W3C Recommendation on June 24, 2025.[16][17]
The original PNG specification was authored by an ad hoc group ofcomputer graphics experts and enthusiasts. Discussions and decisions about the format were conducted by email. The original authors listed on RFC 2083 are:[18]
Has the high bit set to detect transmission systems that do not support8-bit data and to reduce the chance that a text file is mistakenly interpreted as a PNG, or vice versa.
50 4E 47
InASCII, the lettersPNG, allowing a person to identify the format easily if it is viewed in a text editor.
0D 0A
ADOS-styleline ending (CRLF) to detect DOS-Unix line ending conversion of the data.
1A
A byte that stops display of the file under DOS when the commandtype has been used—theend-of-file character.
0A
A Unix-style line ending (LF) to detect Unix-DOS line ending conversion.
After the header, comes a series ofchunks,[20] each of which conveys certain information about the image. Chunks declare themselves ascritical orancillary, and a program encountering an ancillary chunk that it does not understand can safely ignore it. This chunk-based storage layer structure, similar in concept to acontainer format or toAmiga'sIFF, is designed to allow the PNG format to be extended while maintaining compatibility with older versions—it providesforward compatibility, and this same file structure (with different signature and chunks) is used in the associatedMNG,JNG, andAPNG formats.
A chunk consists of four parts: length (4 bytes,[21]big-endian), chunk type/name (4 bytes[22]), chunk data (length bytes) andCRC (cyclic redundancy code/checksum; 4 bytes[21]). The CRC is a network-byte-orderCRC-32 computed over the chunk type and chunk data, but not the length.
Length
Chunk type
Chunk data
CRC
4 bytes
4 bytes
Length bytes
4 bytes
Chunk types are given a four-lettercase sensitive ASCII type/name; compareFourCC. The case of the different letters in the name (bit 5 of the numeric value of the character) is abit field that provides thedecoder with some information on the nature of chunks it does not recognize.
The case of the first letter indicates whether the chunk is critical or not. If the first letter is uppercase, the chunk is critical; if not, the chunk is ancillary. Critical chunks contain information that is necessary to read the file. If a decoder encounters a critical chunk it does not recognize, it must abort reading the file or supply the user with an appropriate warning.
The case of the second letter indicates whether the chunk is "public" (either in the specification or the registry of special-purpose public chunks) or "private" (not standardized). Uppercase is public and lowercase is private. This ensures that public and private chunk names can never conflict with each other (although two private chunk names could conflict).
The third letter must be uppercase to conform to the PNG specification. It is reserved for future expansion. Decoders should treat a chunk with a lower case third letter the same as any other unrecognized chunk.
The case of the fourth letter indicates whether the chunk is safe to copy by editors that do not recognize it. If lowercase, the chunk may be safely copied regardless of the extent of modifications to the file. If uppercase, it may only be copied if the modifications have not touched any critical chunks.
IDAT contains the image, which may be split among multiple IDAT chunks. Such splitting slightly increases the file size, but makes it possible to generate a PNG in a streaming manner. The IDAT chunk contains the actual image data, which is the output stream of the compression algorithm.[23]
IEND marks the image end; the data field of the IEND chunk has 0 bytes/is empty.[24]
ThePLTE chunk is essential for color type 3 (indexed color). It is optional for color types two and six (truecolor and truecolor with alpha) and it must not appear for color types 0 and 4 (grayscale and grayscale with alpha).
Other image attributes that can be stored in PNG files includegamma values, background color, and textualmetadata information. PNG also supportscolor management through the inclusion ofICC color profiles.[25]
bKGD gives the default background color. It is intended for use when there is no better choice available, such as in standalone image viewers (but not web browsers; see below for more details).
cICP specifies the color space, transfer function and matrix coefficients as defined in ITU-TH.273.[26] It is intended for use withHDR imagery without requiring a color profile.[27]
gAMA specifiesgamma. The gAMA chunk contains only 4 bytes, and its value represents the gamma value multiplied by 100,000; for example, the gamma value 1/3.4 calculates to 29411.7647059 ((1/3.4)*(100,000)) and is converted to an integer (29412) for storage.[31]
hIST can store the histogram, or total amount of each color in the image.
iTXt contains a keyword andUTF-8 text, with encodings for possible compression and translations marked withlanguage tag. TheExtensible Metadata Platform (XMP) uses this chunk with a keyword 'XML:com.adobe.xmp'
pHYs holds the intended pixel size (or pixel aspect ratio); the pHYs contains "Pixels per unit, X axis" (4 bytes), "Pixels per unit, Y axis" (4 bytes), and "Unit specifier" (1 byte) for a total of 9 bytes.[32]
sBIT (significant bits) indicates the color-accuracy of the source data; this chunk contains a total of between 1 and 5 bytes, depending on the color type.[33][34][35]
sPLT suggests a palette to use if the full range of colors is unavailable.
sRGB indicates that the standardsRGB color space is used; the sRGB chunk contains only 1 byte, which is used for "rendering intent" (4 values—0, 1, 2, and 3—are defined for rendering intent).[36]
tEXt can store text that can be represented inISO/IEC 8859-1, with onekey-value pair for each chunk. The "key" must be between one and 79 characters long. Separator is a null character. The "value" can be any length, including zero up to the maximum permissible chunk size minus the length of the keyword and separator. Neither "key" nor "value" can contain null character. Leading or trailing spaces are also disallowed.
tIME stores the time that the image was last changed.
tRNS contains transparency information. For indexed images, it stores alpha channel values for one or more palette entries. For truecolor and grayscale images, it stores a single pixel value that is to be regarded as fully transparent.
zTXt contains compressed text (and a compression method marker) with the same limits astEXt.
Allowed combinations of color type and bit depth[12]
Color type
Channels
Bits per channel
1
2
4
8
16
Indexed
1
1
2
4
8
Grayscale
1
1
2
4
8
16
Grayscale and alpha
2
16
32
Truecolor
3
24
48
Truecolor and alpha
4
32
64
Pixels in PNG images are numbers that may be either indices of sample data in thepalette or the sample data itself. The palette is a separate table contained in the PLTE chunk. Sample data for a singlepixel consists of a tuple of between one and four numbers. Whether the pixel data represents palette indices or explicit sample values, the numbers are referred to aschannels and every number in the image is encoded with an identical format.
The permitted formats encode each number as an unsigned integer value using a fixed number of bits, referred to in the PNG specification as thebit depth. Notice that this is not the same ascolor depth, which is commonly used to refer to the total number of bits in each pixel, not each channel. The permitted bit depths are summarized in the table along with the total number of bits used for each pixel.
The number of channels depends on whether the image is grayscale or color and whether it has analpha channel. PNG allows the following combinations of channels, called thecolor type.
0 (0002)
grayscale
2 (0102)
red, green and blue: rgb/truecolor
3 (0112)
indexed: channel containing indices into a palette of colors
4 (1002)
grayscale and alpha: level ofopacity for each pixel
6 (1102)
red, green, blue and alpha
The color type is specified as an 8-bit value however only the low three bits are used and, even then, only the five combinations listed above are permitted. So long as the color type is valid it can be considered as a bit field as summarized in the adjacent table:
PNG color types
Color type
Name
Binary
Masks
A
C
P
0
Grayscale
0
0
0
0
2
Truecolor
0
0
1
0
color
3
Indexed
0
0
1
1
color, palette
4
Grayscale and alpha
0
1
0
0
alpha
6
Truecolor and alpha
0
1
1
0
alpha, color
bit value 1: the image data stores palette indices. This is only valid in combination with bit value 2;
bit value 2: the image samples contain three channels of data encodingtrichromaticcolors, otherwise the image samples contain one channel of data encodingrelative luminance,
bit value 4: the image samples also contain an alpha channel expressed as a linear measure of the opacity of the pixel. This is not valid in combination with bit value 1.
With indexed color images, the palette always stores trichromatic colors at a depth of 8 bits per channel (24 bits per palette entry). Additionally, an optional list of 8-bit alpha values for the palette entries may be included; if not included, or if shorter than the palette, the remaining palette entries are assumed to be opaque. The palette must not have more entries than the image bit depth allows for, but it may have fewer (for example, if an image with 8-bit pixels only uses 90 colors then it does not need palette entries for all 256 colors). The palette must contain entries for all the pixel values present in the image.
The standard allows indexed color PNGs to have 1, 2, 4 or 8 bits per pixel; grayscale images with no alpha channel may have 1, 2, 4, 8 or 16 bits per pixel. Everything else uses a bit depth per channel of either 8 or 16. The combinations this allows are given in the table above. The standard requires that decoders can read all supported color formats, but many image editors can only produce a small subset of them.
PNG offers a variety of transparency options. With true-color and grayscale images either a single pixel value can be declared as transparent or analpha channel can be added (enabling any percentage of partial transparency to be used). For paletted images, alpha values can be added to palette entries. The number of such values stored may be less than the total number of palette entries, in which case the remaining entries are considered fully opaque.
The scanning of pixel values for binary transparency is supposed to be performed before any color reduction to avoid pixels becoming unintentionally transparent. This is most likely to pose an issue for systems that can decode 16-bits-per-channel images (as is required for compliance with the specification) but only output at 8 bits per channel (the norm for all but the highest end systems).
Alphastorage can be "associated" ("premultiplied") or "unassociated", but PNG standardized[38] on "unassociated" ("non-premultiplied") alpha, which means that imagery is not alphaencoded; the emissions represented in RGB are not the emissions at the pixel level. This means that the over operation will multiply the RGB emissions by the alpha, and cannot represent emission and occlusion properly.
Compared to formats withlossy compression such as JPEG, choosing a compression setting higher than average delays processing, but often does not result in a significantly smaller file size.
PNG's filter method 0 can use the data in pixels A, B, and C to predict the value for X.A PNG with 256 colors, which is only 251 bytes large with pre-filter. The same image as a GIF would be more than thirteen times larger.
Before DEFLATE is applied, the data is transformed via a prediction method: a singlefilter method is used for the entire image, while for each image line, afilter type is chosen to transform the data to make it more efficiently compressible.[39] The filter type used for a scanline is prepended to the scanline to enable inline decompression.
There is only one filter method in the current PNG specification (denoted method 0), and thus in practice the only choice is which filter type to apply to each line. For this method, the filter predicts the value of each pixel based on the values of previous neighboring pixels, and subtracts the predicted color of the pixel from the actual value, as inDPCM. An image line filtered in this way is often more compressible than the raw image line would be, especially if it is similar to the line above, since the differences from prediction will generally be clustered around 0, rather than spread over all possible image values. This is particularly important in relating separate rows, since DEFLATE has no understanding that an image is a 2D entity, and instead just sees the image data as a stream of bytes.
There are five filter types for filter method 0; each type predicts the value of each byte (of the image data before filtering) based on the corresponding byte of the pixel to the left (A), the pixel above (B), and the pixel above and to the left (C) or some combination thereof, and encodes thedifference between the predicted value and the actual value. Filters are applied to byte values, not pixels; pixel values may be one or two bytes, or several values per byte, but never cross byte boundaries. The filter types are:[40]
Type byte
Filter name
Predicted value
0
None
Zero (so that the raw byte value passes through unaltered)
Compression is further improved by choosing filter types adaptively on a line-by-line basis. This improvement, and a heuristic method of implementing it commonly used by PNG-writing software, were created byLee Daniel Crocker, who tested the methods on many images during the creation of the format;[42] the choice of filter is a component of file size optimization, as discussed below.
If interlacing is used, each stage of the interlacing is filtered separately, meaning that the image can be progressively rendered as each stage is received; however, interlacing generally makes compression less effective.
An illustration of Adam7 interlacing over a 16×16 image
PNG offers an optional 2-dimensional, 7-passinterlacing scheme—theAdam7 algorithm. This is more sophisticated than GIF's 1-dimensional, 4-pass scheme, and allows a clearer low-resolution image to be visible earlier in the transfer, particularly if interpolation algorithms such asbicubic interpolation are used.[43]
However, the 7-pass scheme tends to reduce the data's compressibility more than simpler schemes.
An APNG (animated PNG) file (displays as static image insome web browsers)
The core PNG format does not support animation.MNG is an extension to PNG that does; it was designed by members of the PNG Group. MNG shares PNG's basic structure and chunks, but it is significantly more complex and has a different file signature, which automatically renders it incompatible with standard PNG decoders. This means that most web browsers and applications either never supported MNG or dropped support for it.
The complexity of MNG led to the proposal ofAPNG by developers at the Mozilla Foundation. It is based on PNG, supports animation and is simpler than MNG. APNG offers fallback to single-image display for PNG decoders that do not support APNG. Today, the APNG format is supported by all major web browsers.[44] APNG is supported inFirefox 3.0 and up,Pale Moon (all versions), andSafari 8.0 and up.[45] Chromium 59.0 added APNG support,[46][47] followed by Google Chrome.Opera supported APNG in versions 10–12.1, but support lapsed in version 15 when it switched to theBlink rendering engine; support was re-added in Opera 46 (inherited from Chromium 59).[48]Microsoft Edge has supported APNG since version 79.0, when it switched to a Chromium-based engine.
The PNG Group decided in April 2007 not to embrace APNG.[49] Several alternatives were under discussion, including ANG, aNIM/mPNG, "PNG in GIF" and its subset "RGBA in GIF".[50] However, currently only APNG has widespread support.
With the release of the third edition of the PNG specification in June 2025, now maintained by the PNG working group,[15] APNG is finally incorporated into the specification as an extension.[51]
DEFLATE compression method using a 256-byte window[52]
9
0xD7
ZLIB FCHECK value, no dictionary used, maximum compression algorithm[52]
10
0x63F8CFC00000
A compressed DEFLATE block using the static Huffman code that decodes to 0x00 0xFF 0x00 0x00[53]
16
0x03010100
The ZLIB check value: theAdler-32 checksum of the uncompressed data[52]
20
0x18DD8DB0
CRC of chunk's type and content (but not length)
Displayed in the fashion ofhex editors, with on the left side byte values shown inhex format, and on the right side their equivalent characters fromISO-8859-1 with unrecognized and control characters replaced with periods. Additionally the PNG signature and individual chunks are marked with colors. Note they are easy to identify because of their human readable type names (in this example PNG, IHDR, IDAT, and IEND).
Portability: Transmission is independent of the software and hardware platform.
Completeness: it's possible to represent true color, indexed-color, and grayscale images.
Coding and decoding in series: allows to generate and read data streams in series, that is, the format of the data stream is used for the generation and visualization of images at the moment through serial communication.
Progressive presentation: to be able to transmit data flows that are initially an approximation of the entire image and progressively they improve as the data flow is received.
Soundness to transmission errors: detects the transmission errors of the data stream correctly.
Losslessness: No loss: filtering and compression preserve all information.
Efficiency: any progressive image presentation, compression and filtering seeks efficient decoding and presentation.
Compression: images can be compressed efficiently and consistently.
Easiness: the implementation of the standard is easy.
Interchangeability: any PNG decoder that follows the standards can read all PNG data streams.
Flexibility: allows future extensions and private additions without affecting the previous point.
Freedom of legal restrictions: the algorithms used are free and accessible.
On small images,GIF can achieve greater compression than PNG (see thesection on filesize, below).
On most images, except for the above case, a GIF file has a larger size than an indexed PNG image.
PNG gives a much wider range of transparency options than GIF, includingalpha channel transparency.
Whereas GIF is limited to 8-bitindexed color, PNG gives a much wider range of color depths, including 24-bit (8 bits per channel) and 48-bit (16 bits per channel)truecolor, allowing for greater color precision, smoother fades, etc.[54] When an alpha channel is added, up to 64 bits per pixel (before compression) are possible.
When converting an image from the PNG format to GIF, the image quality may suffer due toposterization if the PNG image has more than 256 colors.
GIF intrinsically supports animated images. PNG supports animation only via extensions (see thesection on animation, above).
PNG images are less widely supported by older browsers. In particular, IE6 has limited support for PNG.[55]
Composite image comparing lossy compression in JPEG with lossless compression in PNG: the JPEG artifacts can be easily visible in the background of this kind of image data, where the PNG image has solid color.
TheJPEG (Joint Photographic Experts Group) format can produce a smaller file than PNG forphotographic (and photo-like) images, since JPEG uses alossy encoding method specifically designed for photographic image data, which is typically dominated by soft, low-contrast transitions, and an amount of noise or similar irregular structures. Using PNG instead of a high-quality JPEG for such images would result in a large increase in file size withnegligible gain in quality. In comparison, when storing images that contain text, line art, or graphics – images with sharp transitions and large areas of solid color – the PNG format can compress image data more than JPEG can. Additionally, PNG is lossless, while JPEG produces visual artifacts around high-contrast areas. (Such artifacts depend on the settings used in the JPG compression; they can be quite noticeable when a low-quality [high-compression] setting is used.) Where an image contains both sharp transitions and photographic parts, a choice must be made between the two effects. JPEG does not support transparency.
JPEG's lossy compression also suffers fromgeneration loss, where repeatedly decoding and re-encoding an image to save it again causes a loss of information each time, degrading the image. Because PNG is lossless, it is suitable for storing images to be edited. While PNG is reasonably efficient when compressing photographic images, there are lossless compression formats designed specifically for photographic images, losslessWebP andAdobe DNG (digital negative) for example. However these formats are either not widely supported, or are proprietary. An image can be stored losslessly and converted to JPEG format only for distribution, so that there is no generation loss.
While the PNG specification does not explicitly include a standard for embeddingExif image data from sources such as digital cameras, the preferred method for embedding EXIF data in a PNG is to use the non-critical ancillary chunk labeleXIf.[56]
Early web browsers did not support PNG images; JPEG and GIF were the main image formats. JPEG was commonly used when exporting images containing gradients for web pages, because of GIF's limited color depth. However, JPEG compression causes a gradient to blur slightly. A PNG format reproduces a gradient as accurately as possible for a given bit depth, while keeping the file size small. PNG became the optimal choice for small gradient images as web browser support for the format improved. No images at all are needed to display gradients in modern browsers, as gradients can be created usingCSS.
JPEG-LS is an image format by theJoint Photographic Experts Group, though far less widely known and supported than the other lossy JPEG format discussed above. It is directly comparable with PNG,[clarification needed] and has a standard set of test images.[57] On the Waterloo Repertoire ColorSet, a standard set of test images (unrelated to the JPEG-LS conformance test set), JPEG-LS generally performs better than PNG, by 10–15%, but on some images PNG performs substantially better, on the order of 50–75%.[58] Thus, if both of these formats are options and file size is an important criterion, they should both be considered, depending on the image.
JPEG XL is another, much improved, lossless or lossy format, that is unfortunately supported much less, developed to replace lossless formats like PNG.[59] JPEG XL is more than 50% smaller than JPEG, and that can happen while it's lossless, therefore making it even smaller than PNG.[60] It also supportshigh dynamic range, widecolour gamuts, and largecolour depths.[61] JPEG XL is also very efficient at decoding, and provides smooth transitions from the formats it intends to replace, losslessly able to convert from JPEG. It also excels at compressing without compromising on fidelity.[62]
Tag Image File Format (TIFF) is a format that incorporates an extremely wide range of options. While this makes TIFF useful as a generic format for interchange between professional image editing applications, it makes adding support for it to applications a much bigger task and so it has little support in applications not concerned with image manipulation (such as web browsers). The high level of extensibility also means that most applications provide only a subset of possible features, potentially creating user confusion and compatibility issues.
The most common general-purpose, lossless compression algorithm used with TIFF isLempel–Ziv–Welch (LZW). This compression technique, also used in GIF, was covered by patents until 2003. TIFF also supports the compression algorithm PNG uses (i.e.Compression Tag 000816 'Adobe-style') with medium usage and support by applications. TIFF also offers special-purpose lossless compression algorithms likeCCITT Group IV, which can compressbilevel images (e.g., faxes or black-and-white text) better than PNG's compression algorithm.
WebP is a format invented byGoogle that was intended to replace PNG, JPEG, and GIF.[63] WebP files allow for both lossy and lossless compression, while PNG only allows for lossless compression. WebP also supports animation, something that onlyGIF files could previously accomplish.[64]
The main improvements of WebP over PNG, however, are the large reduction in file size and therefore faster loading times when embedded into websites. Google claims that lossless WebP images are 26% smaller than PNG files.[65]
WebP has received criticism for being incompatible with various image editing programs and social media websites, unlike PNG.[66] WebP is also not supported across all web browsers, which may require web image hosters to create a fallback image to display to the user, negating the potential storage savings of WebP.[64]
AVIF is an image format developed by theAlliance for Open Media. AVIF was designed by the foundation to make up for the shortcomings of other image codecs, including PNG,GIF, andWebP.[67]
AVIF is generally smaller in size than both WebP and PNG.[68] AVIF supports animation while PNG previously did not.[69]
However, like WebP, AVIF is supported across fewer applications than PNG.[69]
Adobe Fireworks (formerly byMacromedia) uses PNG as its native file format, allowing other image editors and preview utilities to view the flattened image. However, Fireworks by default also stores metadata for layers, animation, vector data, text and effects. Such files should not be distributed directly. Fireworks can instead export the image as an optimized PNG without the extra metadata for use on web pages, etc.[citation needed]
Despite calls by theFree Software Foundation[72] and theWorld Wide Web Consortium (W3C),[73] tools such as gif2png,[74] and campaigns such as Burn All GIFs,[75] PNG adoption on websites was fairly slow due to late and buggy support in Internet Explorer, particularly regarding transparency.[76] PNG is the most used image file format on the web since 2018.[77]
Especially versions of Internet Explorer (Windows) below 9.0 (released 2011) had numerous problems which prevented it from correctly rendering PNG images.[78]
5.01 prints palette images with black (or dark gray) backgrounds under Windows 98, sometimes with radically altered colors.[82]
6.0 fails to display PNG images of 4097 or 4098 bytes in size.[83]
6.0 cannot open a PNG file that contains one or more zero-length IDAT chunks. This issue was first fixed in security update 947864 (MS08-024). For more information, see this article in the Microsoft Knowledge Base:947864 MS08-024: Cumulative Security Update for Internet Explorer.[84]
6.0 sometimes completely loses ability to display PNGs, but there are various fixes.[85]
6.0 and below have broken alpha-channel transparency support (will display the default background color instead).[86][87][88]
7.0 and below cannot combine 8-bit alpha transparency AND element opacity (CSS – filter: Alpha (opacity=xx)) without filling partially transparent sections with black.[89]
8.0 and below have inconsistent/broken gamma support.[78]
8.0 and below don't have color-correction support.[78]
PNG file size can vary significantly depending on how it is encoded and compressed; this is discussed and a number of tips are given inPNG: The Definitive Guide.[58]
Compared toGIF files, a PNG file with the same information (256 colors, no ancillary chunks/metadata), compressed by an effective compressor is normally smaller than a GIF image. Depending on the file and the compressor, PNG may range from somewhat smaller (10%) to significantly smaller (50%) to somewhat larger (5%), but is rarely significantly larger[58] for large images. This is attributed to the performance of PNG'sDEFLATE compared to GIF'sLZW, and because the added precompression layer of PNG's predictive filters take account of the 2-dimensional image structure to further compress files; as filtered data encodes differences between pixels, they will tend to cluster closer to 0, rather than being spread across all possible values, and thus be more easily compressed by DEFLATE. However, some versions ofAdobe Photoshop,CorelDRAW andMS Paint provide poor PNG compression, creating the impression that GIF is more efficient.[58]
PNG files vary in size due to a number of factors:
color depth
Color depth can range from 1 to 64 bits per pixel.
ancillary chunks
PNG supports metadata—this may be useful for editing, but unnecessary for viewing, as on websites.
interlacing
As each pass of the Adam7 algorithm is separately filtered, this can increase file size.[58]
filter
As a precompression stage, each line is filtered by a predictive filter, which can change from line to line. As the ultimate DEFLATE step operates on the whole image's filtered data, one cannot optimize this row-by-row; the choice of filter for each row is thus potentially very variable, though heuristics exist.[note 1]
compression
With additional computation, DEFLATE compressors can produce smaller files.
There is thus a filesize trade-off between high color depth, maximal metadata (including color space information, together with information that does not affect display), interlacing, and speed of compression, which all yield large files, with lower color depth, fewer or no ancillary chunks, no interlacing, and tuned but computationally intensive filtering and compression. For different purposes, different trade-offs are chosen: a maximal file may be best for archiving and editing, while a stripped down file may be best for use on a website, and similarly fast but poor compression is preferred when repeatedly editing and saving a file, while slow but high compression is preferred when a file is stable: when archiving or posting.Interlacing is a trade-off: it dramatically speeds up early rendering of large files (improves latency), but may increase file size (decrease throughput) for little gain, particularly for small files.[58]
Although PNG is a lossless format, PNG encoders can preprocess image data in a lossy fashion to improve PNG compression. For example, quantizing a truecolor PNG to 256 colors allows the indexed color type to be used for a likely reduction in file size.[92]
Some programs are more efficient than others when saving PNG files, this relates to implementation of the PNG compression used by the program.
Many graphics programs (such as Apple'sPreview software) save PNGs with large amounts ofmetadata and color-correction data that are generally unnecessary forWeb viewing. Unoptimized PNG files fromAdobe Fireworks are also notorious for this since they contain options to make the image editable in supported editors. Also CorelDRAW (at least version 11) sometimes produces PNGs which cannot be opened by Internet Explorer (versions 6–8).
Adobe Photoshop's performance on PNG files has improved in the CS Suite when using the Save For Web feature (which also allows explicit PNG/8 use).
Adobe's Fireworks saves larger PNG files than many programs by default. This stems from the mechanics of itsSave format: the images produced by Fireworks' save function include large, private chunks, containing complete layer and vector information. This allows further lossless editing. When saved with theExport option, Fireworks' PNGs are competitive with those produced by other image editors, but are no longer editable as anything but flattened bitmaps. Fireworks is unable to save size-optimized vector-editable PNGs.
Other notable examples of poor PNG compressors include:
Microsoft's Paint for Windows XP
Microsoft Picture It! Photo Premium 9
Poor compression increases the PNG file size but does not affect the image quality or compatibility of the file with other programs.
When the color depth of atruecolor image is reduced to an 8-bit palette (as in GIF), the resulting image data is typically much smaller. Thus a truecolor PNG is typically larger than a color-reduced GIF, although PNG could store the color-reduced version as a palettized file of comparable size. Conversely, some tools, when saving images as PNGs, automatically save them as truecolor, even if the original data use only 8-bit color, thus bloating the file unnecessarily.[58] Both factors can lead to the misconception that PNG files are larger than equivalent GIF files.
pngcrush is the oldest of the popular PNG optimizers. It allows for multiple trials on filter selection and compression arguments, and finally chooses the smallest one. This working model is used in almost every png optimizer.
advpng and the similar advdef utility in the AdvanceCOMP package recompress the PNG IDAT. Different DEFLATE implementations are applied depending on the selected compression level, trading between speed and file size: zlib at level 1, libdeflate at level 2,7-zip'sLZMA DEFLATE at level 3, andzopfli at level 4.
pngout was made with the author's own deflater (same to the author's zip utility, kzip), while keeping all facilities of color reduction / filtering. However, pngout doesn't allow for using several trials on filters in a single run. It's suggested to use its commercial GUI version, pngoutwin, or used with awrapper to automate the trials or to recompress using its own deflater while keep the filter line by line.[note 2]
zopflipng was also made with its own deflater, zopfli. It has all the optimizing features pngcrush has (including automating trials) while providing a very good, but slow deflater.
A simple comparison of their features is listed below.
0–4 or adaptive with 2 different algorithms, or with a brute way
Yes
Yes
zopfli
Beforezopflipng was available, a good way in practice to perform a png optimization is to use a combination of 2 tools in sequence for optimal compression: one which optimizes filters (and removes ancillary chunks), and one which optimizes DEFLATE. Although pngout offers both, only one type of filter can be specified in a single run, therefore it can be used with awrapper tool or in combination withpngcrush,[note 2] acting as a re-deflater, likeadvdef.
For removing ancillary chunks, most PNG optimization tools have the ability to remove all color correction data from PNG files (gamma, white balance, ICC color profile, standard RGB color profile). This often results in much smaller file sizes. For example, the following command line options achieve this withpngcrush:
pngcrush,pngout, andzopflipng all offer options applying one of the filter types 0–4 globally (using the same filter type for all lines) or with a "pseudo filter" (numbered 5), which for each line chooses one of the filter types 0–4 using an adaptive algorithm.zopflipng offers 3 different adaptive method, including a brute-force search that attempts to optimize the filtering.[note 7]
pngout andzopflipng provide an option to preserve/reuse[note 2][note 8] the line-by-line filter set present in the input image.
pngcrush andzopflipng provide options to try different filter strategies in a single run and choose the best. The freeware command line version ofpngout doesn't offer this, but the commercial version,pngoutwin, does.[note 9]
Zopfli and theLZMA SDK provideDEFLATE implementations that can produce highercompression ratios than thezlib reference implementation at the cost of performance. AdvanceCOMP'sadvpng andadvdef can use either of these libraries to re-compress PNG files. Additionally,PNGOUT contains its ownproprietary DEFLATE implementation.
advpng doesn't have an option to apply filters and always uses filter 0 globally (leaving the image data unfiltered); therefore it should not be used where the image benefits significantly from filtering. By contrast,advdef from the same package doesn't deal with PNG structure and acts only as a re-deflater, retaining any existing filter settings.
Sinceicons intended for Windows Vista and later versions may contain PNG subimages, the optimizations can be applied to them as well. At least oneicon editor, Pixelformer, is able to perform a special optimization pass while savingICO files, thereby reducing their sizes.
^The filtering is used to increase the similarity to the data, hence increasing the compression ratio. However, there is theoretically no formula for similarity, nor absolute relationship between the similarity and compressor, thus unless the compression is done, one can't tell one filter set is better than another.
^T. Boutell; et al. (March 1997).PNG (Portable Network Graphics) Specification Version 1.0. Network Working Group.doi:10.17487/RFC2083.RFC2083.Informational. sec. 8.4.PNG itself is strictly a single-image format. (...) In the future, a multiple-image format based on PNG may be defined. Such a format will be considered a separate file format
^abLaphroaig, Manul (31 October 2017).PoC or GTFO. No Starch Press.ISBN9781593278984.Each chunk consists of four parts: Length, a Chunk Type, the Chunk Data, and a 32-bit CRC. The Length is a 32-bit unsigned integer indicating the size of only the Chunk Data field
^Laphroaig, Manul (31 October 2017).PoC or GTFO. No Starch Press.ISBN9781593278984.Chunk Type is a 32-bit FourCC code such as IHDR, IDAT, or IEND.
^Roelofs 2003,Significant Bits (sBIT)"Grayscale images are the simplest; sBIT then contains a single byte indicating the number of significant bits in the source data"
