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]

[67] And as a corollary, PNG file sizes are usually considerably larger than ordinary JPEG, since the latter uses lossy compression--that is, it throws away some information. TIFF also supports JPEG compression as one of its many options, but the more common methods are lossless and based on either run-length encoding (RLE) or the same LZW algorithm used in GIF.