PNG-Tech

Technical Issues Regarding the PNG File Format

This site contains a collection of comments, impressions, comparisons, experiments and ideas regarding the PNG (Portable Network Graphics) file format, and related technologies: ZLIB (Data Compression Format and Library), and MNG (Multiple-Image Network Graphics). It is focused on the compression algorithms used in PNG.

New! OptiPNG-0.5.5 is now available at SourceForge.

PNG uses ZLIB streams [RFC1950] as compressed datastreams. The compression algorithm, called Deflate [RFC1951], was designed by Phil Katz and used initially in PKZIP.
Jean-loup Gailly and Mark Adler rewrote the code, avoiding any patented algorithms. This code, freely available in source, can be found here. Additional technical details are provided here.
A good explanation of the Deflate algorithm by Antaeus Feldspar is available here.
We use the term ZLIB (written with capital letters) when we refer to the standard or to standard-compliant streams, and the term zlib (written with small letters) when we refer to the reference implementation.

Any comments, suggestions and contributed articles are welcome.

• ZLIB-related articles
  • Proposal for correcting the ZLIB specification
    In the current ZLIB specification, described in RFC1950, some boundary conditions regarding the size of the sliding window are incomplete. The proposal addresses this issue.
  • Bug fix for 256-byte sliding windows in zlib-1.1.3
    In the zlib implementations up to version 1.1.3, there is a problem with 256-byte sliding windows. This window size is disabled in zlib-1.1.4. It is unknown when a complete fix will be available.
    Upgrading to a newer zlib is strongly recommended, due to a security problem that exists in zlib prior to 1.1.4.
  • Proposal for a new zlib strategy: Z_RLE
    Z_RLE is a fast strategy that has a high speed and a low memory overhead, like Z_HUFFMAN_ONLY, but achieves a better compression ratio.
    New! The Z_RLE strategy is implemented in the zlib-1.2.x series.
  • TOO_FAR in zlib is not too far...
    A parameter used in the zlib source code should be set to a higher value, in order to achieve a slightly better compression.
• PNG-related articles
  • A guide to PNG optimization
    PNG compression does not depend only on the image statistics, but it may vary within large limits, depending on several compression parameters.
  • Brute-force vs. heuristic filtering
    The filtering heuristics proposed in the PNG specification yield good results in most of the cases. There are a few cases, however, when these heuristics are less than optimal.
  • LZW-GIF can be sometimes better than Deflate
    The Deflate compression used by PNG is superior to the LZW compression used by GIF. There are very few isolated cases when LZW is marginally better, and this article presents such examples.
  • 8 bits per pixel in paletted images
    This article shows an example in which the encoding of an image using as few bits per pixel as possible, is suboptimal.
• MNG-related articles
  • Here is the current draft of the PNG, JNG and MNG chunk ordering specification. This specification is written as an EBNF grammar.
• Links
  • Books
    Although many books (mostly about computer imaging) mention PNG and MNG, only a few explain how these formats actually work. If you are looking for such details, you may find this list useful.
  • Related articles
    Some others sites also contain material directly related to the purpose of PNG-Tech.
  • Corpora
    Uncompressed or PNG-compressed images that can be freely distributed, and can be used in compression tests.

Copyright © 2000-2007

Appeared: 16 Jan 2000. Last updated: 28 Jan 2007.