Web Site Optimization: How To Speed Up Your Web Site By Minimizing Your GIF and PNG Image "Bit-Depth"

Are bloated images slowing down your website and causing you lost business? Images comprise over 50% of the average web page so putting them on a diet is essential to improving web performance. One of the best ways to optimize GIFs and PNGs is to minimize the bit-depth or the number of colors within your images.

For palette-based formats like GIF and PNG, file size is directly related to the size of the color palette, or the number of colors in the image. As the number of colors in an image crosses a power of two, the file size jumps. A 33-color image must make use of a six-bit palette, while a 32-color image can make use of a 5-bit palette. Smaller palettes mean smaller codes (representing pixel patterns), which makes for smaller files. So minimizing the number of colors in index-color images like GIFs and PNGs will minimize file size.

The compression schemes used in GIFs (LZW) and PNGs (Deflate) are dictionary-based encoding algorithms. Dictionary based compression algorithms substitute shorter codes for longer patterns of strings within the data stream. Pixel patterns (substrings) in the data stream found in the dictionary are replaced with a single code. If a substring is not found in the dictionary, a new code is created and added to the dictionary. Compression is achieved when smaller codes are substituted for longer patterns of data.

The code size is based on the number of bits per pixel, and depends on the maximum length code within your dictionary. Thus the smaller your bit-depth, the smaller the codes representing your pixel pattern strings, and the smaller the file.

Choosing the right format for your images is the first step in optimizing web graphics. At lower bit-depths, banding can appear in smooth-toned images. Try quantizing your image to different palettes to minimize banding.

PNG Compression

PNG uses the deflate compression algorithm which is a newer, more efficient cousin of LZW. Deflate is designed to be costless of patent problems. Deflate is a combination of the LZ77 algorithm found in zip programs like WinZIP and gzip, and Huffman coding. Deflate typically compresses files 20 to 30 percent smaller than LZW, which is the difference youll find between gzip and compress. PNGs utilize a similar substitution technique, but are commonly 5 to 25 percent smaller than GIFs, although there are exceptions. In addition to horizontal "scan-line" pattern substitution PNGs sometimes substitute for vertical patterns. PNG might also truncate color palettes. In general, the smaller the color palette, the smaller the overal file size.

Conclusion

To minimize the file size of palette-based image formats like GIF and PNG, minimize their bit-depth, or number of colors. Each measure you pass a power of two, document size jumps higher. By minimizing the number of colors, and maximizing the flat-color areas you will create smaller, more workable images destined for the Web.

Further Reading:

LZW Data Compression
http://www.dogma.net/markn/articles/lzw/lzw.htm
Mark Nelson breaks down the LZW compression algorithm for Dr. Dobbs Journal of October 1989.

Multimedia: Use Lossy Compression for Smaller GIFs and PNGs http://www.websiteoptimization.com/speed weak/lossy/
Lossy compression lets you squeeze more bytes out of your GIFs and PNGs. Lossy compression enhances identical pixel patterns to improve compression in indexed-color images.

Multimedia: Use Weighted Optimization
http://www.websiteoptimization.com/speed weak/weighted/
Weighted optimization or regional compression applies different degrees of compression to different areas of your image. By varying the quality within images you may improve document size.

Optimizing Web Graphics
http://www.websiteoptimization.com/speed/12/
Chapter summary from Speed Up Your Site, shows how to correct, optimize, and compress images for minimum document size and maximum quality.

PNG Home Site
http://www.libpng.org/pub/png/
Greg Roelofs site devoted to all things PNG.