Colors that should not be dithered during RGB->Indexed conversion
Submitted by Raphaël Quinet
It would be nice to have a way to select one or several colors that should not be dithered during the RGB->Indexed conversion.
Currently, the user can only choose between no dithering at all and various dithering methods (including "reduced color bleeding" but this is not perfect). But there is no way to tell the Gimp to dither the image except for the areas that have a specific color.
The dithering algorithm (Floyd-Steinberg or positioned dithering) could be modified to be able to skip over some areas if they match a color taken from some palette. The error vector used when dithering would then be propagated to the next line without any modification or would be reset to 0.
In the Indexed Color Conversion window, there is already an option "Enable Dithering of Transparency". There could be an additional option "Disable Dithering of Colors from Custom Palette: [palette menu]".
Combined with the enhancement proposed in bug #66258, this would make it possible to create some indexed images containing flat areas of web-safe colors (or other "important" colors) together with other areas in which dithering is allowed.
Current workaround for this problem: convert the same RGB image to indexed mode twice: once with dithering and once without dithering (using the same settings for the palette in both cases). Then copy some parts of the non-dithered image into the dithered version, or vice-versa.
Version: git master