Progressive Graphics File
PGF stands for Progressive Graphics File and is a graphics format for compressed raster graphics . It was designed to replace the aging JPEG compression. The data is saved either losslessly or lossy compressed.
properties
PGF, like JPEG and JPEG 2000, is designed for natural images. Artificially generated raster images such as screenshots or logos are better saved with PNG . JPEG 2000 and PGF are both based on the discrete wavelet transform (DWT). The compression rate of PGF does not quite come close to that of JPEG 2000 with the same image quality, but it is clearly better than that of JPEG. JPEG 2000 pays for this fact with a very high encoding and decoding time, whereas the encoding and decoding speed of PGF is comparable to the high speed of JPEG.
The advantage of PGF over many other image formats is the special arrangement of the data stream, which allows a progressive image structure. The image data are arranged in the data stream in such a way that the decoding of a first partial section is sufficient to be able to display a reduced version of the final image. The size of the picture can then be quadrupled with each further decoded section. PGF is therefore suitable for image transmission over a network, since the recipient receives a first impression of the transmitted image at an early stage.
With version 5, the optional extraction of image sections ( region of interest ) has also been introduced. This efficient type of section extraction enables a desired image section to be decoded without decoding the entire image. This enables access to small sections of the picture in a large picture in a short time. Such an image section extraction is only possible if the PGF file is saved in a slightly different structure.
The separately saved preview images in Windows Explorer or with digital cameras in Exif file format are also not necessary with PGF.
From version 5, PGF supports the following large number of different raster formats, in particular also transparency information analogous to PNG:
- Grayscale image with 1, 8, 16 or 31 bits
- color-indexed image with 256 colors
- RGB color image with 12, 16 (565), 24 or 48 bits per pixel
- ARGB -Farbbild with 32 bits per pixel
- L * a * b color image with 24 or 48 bits per pixel
- CMYK color image with 32 or 64 bits per pixel
Technical details
The PGF process chain comprises the following steps:
- Transformation of the color space (if color image)
- Discrete wavelet transform
- Quantization
- Progressive precoding
The whole procedure works exclusively with integer arithmetic and requires neither multiplications nor divisions. It is therefore suitable for direct hardware implementation.
Web links
- PGF website with lots of information and downloads
- xeraina Image Viewer - image viewer / converter for the most common image formats including PGF (freeware for non-commercial use)
swell
- ↑ PGF - A new progressive file format for lossy and lossless image compression ( Memento of the original from March 7, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 124 kB)
