Technical Discussion
The aim of PGF is not only improved compression quality over JPEG but also adding (or improving) features such as scalability. In fact, PGF's improvement in compression performance relative to the original JPEG standard is actually rather modest and should not ordinarily be the primary consideration for evaluating the design. Moreover, very low and very high compression rates (including lossless compression) are also supported in PGF. In fact, 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:
- Color space transform (in case of color images)
- Discrete Wavelet Transform
- Quantization (in case of lossy data compression)
- Hierarchical bit-plane run-length encoding
Read more about this topic: Progressive Graphics File
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