The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.


Palette-based images, also known as colormapped or index-color images, use the PLTE chunk and are supported in four pixel depths: 1, 2, 4, and 8 bits, corresponding to a maximum of 2, 4, 16, or 256 palette entries. Unlike GIF images, however, fewer than the maximum number of entries may be present. On the other hand, GIF does support pixel depths of 3, 5, 6, and 7 bits; 6-bit (64-color) images, in particular, are common on the World Wide Web.

TIFF also supports palette images, but baseline TIFF allows only 4- and 8-bit pixel depths. Perhaps a more useful comparison is with the superset of baseline TIFF that is supported by Sam Leffler's free libtiff, which has become the software industry's unofficial standard for TIFF decoding. libtiff supports palette bit depths of 1, 2, 4, 8, and 16 bits. Unlike PNG and GIF, however, the TIFF palette always uses 16-bit integers for each red, green, and blue value, and as with GIF, all 2bit depth entries must be present in the file. Nor is there any provision for compression of the palette data--so a 16-bit TIFF palette would require 384 KB all by itself.

Last Update: 2010-Nov-26