On 08/25/2013 07:36 AM, Steve Kinney wrote:
Hot tip: The menu at Image > Mode will allow you to convert from Indexed (GIF native format) to RGB (a much more flexible format). When you have to edit the content of GIF frames, make sure the image is set to RGB format; many filters and tools either don't work, or work very badly, in Indexed mode. The GIMP will convert an RGB image to Indexed format when you export it as a GIF file. Have fun! :o) Steve

IMHO the real hot tip is to not edit GIFs...

Converting the image to RGB loses the colormap, and, if you introduce new colors with your processing, and export to GIF with the defaults, Gimp will create a new color map, and some of the old colors not in the new color map will be approximated with some other color, which can severely degrade the image quality (this often leads to jaggy lines).

If you don' t want introduce new colors, first export the colormap to a palette(*), then convert to RGB, and, before exporting, explicitly convert to indexed using the saved palette/colormap.

When you mix two images, all bets are off if they combined colormaps total more than 256 colors.

(*) In the Palettes list, right click, "Import palette", and select "Image" source.

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