PNG: The Definitive Guide

Chapter 9. Compression and Filtering

One of PNG's strengths, particularly in comparison to the GIF and TIFF image formats, is its compression. As I noted in Chapter 1, “An Introduction to PNG”, a primary motivation driving the design of the Portable Network Graphics format was to create a replacement for GIF that was not only free but also an improvement over it in essentially all respects. As a result, PNG compression is completely lossless--that is, the original image data can be reconstructed exactly, bit for bit--just as in GIF and most forms of TIFF.[67]

I wrote a longer, more technically detailed chapter on PNG compression for the

Lossless Compression Handbook, edited by Khalid Sayood and published in December 2002 by Academic Press (now Elsevier Science). It includes more rigorous test data, as well. A near-final draft is available in PDF format at the following link:

http://www.libpnq.orq/pub/pnq/book/LCH-pnq-chapter.pdf

I will update it to the final version and convert it to HTML format when time permits.

9.1. Filtering

We'll look at the compression engine itself shortly, but PNG's performance is not due solely to an improved compression algorithm. PNG also supports a precompression step called filtering. Filtering is a method of reversibly transforming the image data so that the main compression engine can operate more efficiently. As a simple example, consider a sequence of bytes increasing uniformly from 1 to 255. Since there is no repetition in the sequence, it compresses ...