I have tried many ways using a layer mask (such as this one) to accomplish this but could not get the result I expect. I have this image:

enter image description here

I would like to make the square outline semi-transparent say alpha=0.5 but leave everything else unchanged. The steps I took were:

  1. open image in GIMP
  2. duplicate layer
  3. add layer mask initialized as black
  4. select upper layer & use magic wand to select the gray square outline
  5. select layer mask & use bucket fill tool to make selection gray (#7f7f7f)

But the outline remains solid gray instead of 50% opaque gray. Why is that & how can I get the result I want? What am I missing? Thanks

  • 2
    From your comments, it sounds like the base layer is still opaque. Doesn't matter what you do with transparency on upper layers, if the bottom isn't transparent, the whole final image won't be transparent.
    – TunaMaxx
    Commented Dec 15, 2016 at 18:13
  • 1
    like tunamaxx said, your base layer is still opaque. to fix this, keep the selection that you used and delete it from the base layer. As an FYI for workflow, you could alternatively fill the selection with white, and adjust the top layers opacity rather than the opacity on the mask. Its a more easily adjustable method later on. Commented Dec 15, 2016 at 21:25

1 Answer 1


Thanks to TunaMaxx & Tyler for pointing the way, what I missed was precisely the base layer remained solid. Deleting the square outline in the base layer then applying the layer mask gave me the result I was expecting.

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