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Editing GIFs
Editing GIF is possible, but... the GIF is an indexed-color format, pixel colors are coded on one byte (256 values) that are really indexes into a "color map". So you have a maximum of 256 colors across all frames(*). When you are in indexed-color format, there are several restrictions:
  • Many filters and tools won't work (and will be grayed out in the menus)
  • Layer opacity is binary (fully opaque, fully transparent, there is no partial opacity)
  • All the colors you add are mapped to the closest existing color in the color map, so your blue may well turn red if there is no blue in the initial GIF... and for the same reason, the colors of whatever you paste in are also mapped to colors in the color map.
So, in many cases, you will start by changing the image mode back to RGB (Image>Mode>RGB) which lifts the restriction above. But keep in mind that when you will export the GIF, these restrictions will apply again. In particular, if you added new colors, then some colors of the original image will be dropped from the color map and the corresponding pixels from the original image will be given another color. This will usually translate into a grainy look or jagged edges.

This isn't really Gimp's fault. GIF isn't an "edit" format, it is a "presentation" one. Editing a GIF, while possible, is a bit like editing a PDF instead of editing the original document (.DOC, .ODT...)

(*) a more recent GIF variant allows 256 colors per frame, but Gimp doesn't support it.

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