Indexed gradients are now handled by the Gradient class and used by the PNGIO plugin to generate indexed PNGs (some consts in the code can toggle it back to non-indexed images). This leads to a significant speedup and large file size reduction. On a DEM test case, while reading a 2D ARC grid and writing it back, the following have been measured:

After commit 758, support for indexed PNGs. This leads to major speed and size improvements:

version                  duration                   file size
full color                 0.530                       617k    
reduced to 30        0.460   13%             156k   75%
indexed, 30           0.352   34%               90k   85%
indexed, 30           0.320   40%             123k   80%
(optimized for speed)