In GIMP, when I use "Color to Alpha" to remove the dull blue background of this image: dull blue

It removes some blue from the entire image, so most of the image looks slightly transparent:

dull blue How can I stop this so that only "the dull blue" is removed?


Use a selection so that color-to-alpha is only applied where needed:

  • using the fuzzy selection, select the background. Shift-click any closed areas (inside of 'O','P','D,'A', etc....) to add it
  • Select>Grow by 2 pixels to include all the edge pixels in the selection
  • Color>Color to alpha.

PS; You have strange artifacts around the big squares (the background isn't as uniform as it looks) so using C2A alone will leave some opacity in the background. A more complete approach can be:

  • Make fuzzy selection as above
  • Save selection to channel
  • Select>Shrink by 2 pixels and [delete]. This insure that the background is completely clean
  • Restore original selection from channel
  • Select>Grow by 2px and C2A.

This more complete method is partially automated in the ofn-erase-background script.

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