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.