Progressive Graphics File

PGF stands for Progressive Graphics File and is a graphics format for compressed raster graphics. It was designed as a replacement for the somewhat dated JPEG compression. The data is stored compressed either lossless or lossy.

Properties

PGF is like JPEG and JPEG 2000 designed for natural images. Artificially generated raster images such as screenshots or logos are better stored with PNG. JPEG 2000 and PGF are both based on the discrete wavelet transform (DWT). The compression rate of PGF is the same image quality does not quite match that of JPEG 2000 approach but is clearly better than JPEG. JPEG 2000 paid this circumstance with a high degree of encoding and decoding time, whereas the encoding and decoding speed of PBA to the high speed of JPEG is comparable.

The advantage of PGF compared to many other image formats is the special arrangement of the data stream, which allows a progressive image construction. The image data are arranged in the data stream so that the decoding of a first section sufficient to represent a scaled down version of the final image can. With each decoded section then the image can be quadrupled in size. Thus, PGF is suitable for image transmission over a network, as the receiver has timely a first impression of the transmitted image.

With version 5, the optional extraction of the Image ( Region of Interest) has been introduced. This efficient way of cutting extraction enables decoding of a desired image part without decoding the whole picture. The result is access to small image cutouts in a large image possible in a short time. However, such an image section extraction is only possible if the PGF file is stored in a slightly different structure.

The separately stored thumbnails in Windows Explorer or in Digital cameras Exif file format are not required for PGF.

PGF supported from version 5, the following large number of different raster formats, especially transparency information analogous to PNG:

  • Grayscale image with 1, 8, 16, or 31 bits
  • Color- indexed image with 256 colors
  • RGB Farbild 12, 16 ( 565 ), 24 or 48 bits per pixel,
  • ARGB color image with 32 bits per pixel
  • L * a * b * color image having 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 color space (if color image)
  • Discrete Wavelet Transform
  • Quantization
  • Progressive Präkodierung

The whole process only works with integer arithmetic and requires no multiplications or divisions. It is therefore suitable for direct hardware implementation.

Image compression Transparency (graphic)
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