Progressive Graphics File
 |
|
| Filename extension | .pgf |
|---|---|
| Magic number |
504746h (ASCII PGF) |
| Developed by | xeraina GmbH |
| Initial release | 2000 |
| Latest release |
7.15.25
(2015) |
| Type of format | wavelet-based bitmapped image format |
| Extended from | JPEG, PNG |
| Open format? | LGPLv2 |
PGF (Progressive Graphics File) is a wavelet-based bitmapped image format that employs lossless and lossy data compression. PGF was created to improve upon and replace the JPEG format. It was developed at the same time as JPEG 2000 but with a focus on speed over compression ratio.
PGF can operate at higher compression ratios without taking more encoding/decoding time and without generating the characteristic "blocky and blurry" artifacts of the original DCT-based JPEG standard. It also allows more sophisticated progressive downloads.
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 (including lossless 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 RGB color space to another color space, leading to three components that are handled separately. PGF uses a fully reversible modified YUV color transform. The transformation matrices are:
The chrominance components can be, but do not necessarily have to be, down-scaled in resolution.
The color components are then wavelet transformed to an arbitrary depth, in contrast to JPEG 1992 which uses an 8x8 block-size discrete cosine transform. PGF uses one reversible wavelet transform: a rounded version of the biorthogonal CDF 5/3 wavelet 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.
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