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| PGF | |
|---|---|
 | |
| Filename extension | .pgf |
| Internet media type | image/x-pgf |
| Magic number | 504746h (ASCII PGF) |
| Developed by | xeraina GmbH |
| Initial release | 2000; 26 years ago (2000) |
| Latest release | 7.21.7 2021; 5 years ago (2021) |
| Type of format | wavelet-basedbitmappedimage format |
| Extended from | JPEG,PNG |
| Open format? | LGPLv2[1] |
PGF (Progressive Graphics File) is awavelet-basedbitmappedimage format that employslossless andlossy data compression. PGF was created to improve upon and replace theJPEG format. It was developed at the same time asJPEG 2000 but with a focus on speed overcompression ratio.[citation needed]
PGF can operate at higher compression ratios without taking more encoding/decoding time and without generating the characteristic "blocky and blurry"artifacts of the originalDCT-based JPEG standard.[2] It also allows more sophisticatedprogressive downloads.[citation needed]
PGF supports a wide variety of color models:
PGF claims to achieve an improved compression quality over JPEG adding or improving features such as scalability. Its compression performance is similar to the original JPEG standard. Very low and very high compression rates (includinglossless compression) are also supported in PGF. The ability of the design to handle a very large range of effective bit rates is one of the strengths of PGF. For example, to reduce the number of bits for a picture below a certain amount, the advisable thing to do with the first JPEG standard is to reduce the resolution of the input image before encoding it — something that is ordinarily not necessary for that purpose when using PGF because of its wavelet scalability properties.
The PGF process chain contains the following four steps:
Initially, images have to be transformed from the RGBcolor space to another color space, leading to threecomponents that are handled separately. PGF uses a fully reversible modifiedYUV color transform. The transformation matrices are:
Thechrominance components can be, but do not necessarily have to be, down-scaled in resolution.
The color components are thenwavelet transformed to an arbitrary depth. In contrast to JPEG 1992 which uses an 8x8 block-sizediscrete cosine transform, PGF uses one reversible wavelet transform: a rounded version of the biorthogonalCDF 5/3wavelet transform. This wavelet filter bank is exactly the same as the reversible wavelet used in JPEG 2000. It uses only integer coefficients, so the output does not require rounding (quantization) and so it does not introduce any quantization noise.
After the wavelet transform, the coefficients are scalar-quantized to reduce the amount of bits to represent them, at the expense of a loss of quality. The output is a set of integer numbers which have to be encoded bit-by-bit. The parameter that can be changed to set the final quality is the quantization step: the greater the step, the greater is the compression and the loss of quality. With a quantization step that equals 1, no quantization is performed (it is used in lossless compression). In contrast to JPEG 2000, PGF uses only powers of two, therefore the parameter valuei represents a quantization step of 2i. Just using powers of two makes no need of integer multiplication and division operations.
The result of the previous process is a collection ofsub-bands which represent several approximation scales.A sub-band is a set ofcoefficients —integer numbers which represent aspects of the image associated with a certain frequency range as well as a spatial area of the image.
The quantized sub-bands are split further intoblocks, rectangular regions in the wavelet domain. They are typically selected in a way that the coefficients within them across the sub-bands form approximately spatial blocks in the (reconstructed) image domain and collected in a fixed sizemacroblock.
The encoder has to encode the bits of all quantized coefficients of a macroblock, starting with the most significant bits and progressing to less significant bits. In this encoding process, eachbit-plane of the macroblock gets encoded in two so-calledcoding passes, first encoding bits of significant coefficients, then refinement bits of significant coefficients. Clearly, in lossless mode all bit-planes have to be encoded, and no bit-planes can be dropped.
Only significant coefficients are compressed with an adaptiverun-length/Rice (RLR) coder, because they contain long runs of zeros. The RLR coder with parameterk (logarithmic length of a run of zeros) is also known as the elementaryGolomb code of order 2k.
There are several self-proclaimed advantages of PGF over the ordinary JPEG standard:[2]
The author publishedlibPGF via aSourceForge, under theGNU Lesser General Public License version 2.0.[1] Xeraina offers a freeWindows console encoder and decoder, and PGF viewers based onWIC for 32bit and 64bit Windows platforms. Other WIC applications including File Explorer are able to display PGF images after installing this viewer.[3]
Digikam is a popular open-source image editing and cataloging software that useslibPGF for its thumbnails. It makes use of the progressive decoding feature of PGF images to store a single version of each thumbnail, which can then be decoded to different resolutions without loss, thus allowing users to dynamically change the size of the thumbnails without having to recalculate them again.[citation needed]
File extension.pgf and theTLAPGF are also used for unrelated purposes:
.pgf withPortfolio Graphics.| Raster | |
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