Web Design in a Nutshell, 3rd Edition

Filters

Before PNG compresses an image, it first runs the image data, row by row, through one of five filters (Sub, Up, Average, Paeth, or Adaptive). The filters use different methods for finding patterns in the image information that can then be condensed more efficiently. The process is similar to how LZW compression takes advantage of horizontal repetition in GIFs, but PNG can look for vertical repetition as well.

In most applications, the filters are applied internally and are hidden from the end user (as they should be). If your tool provides filter options, there are only two you need to remember:

• Use None for all indexed color images (or grayscale images with fewer than 16 shades).

• Use Adaptive for all other image types.