this is my first post here!

I am working on a fish project - sand goby, that is semi transparent, see the extent of the fish transparency on different kind of background: additional images to show the transparency of the fish enter image description here enter image description here

I have several images like this: sand goby on white background

I would like to use the surrounding white background (to the left of the fish for example) to somehow proportionaly change the level of transparency of pixels within goby fish - where the fish is brighter there should be higher level of transparency, as a value of alpha channel for that pixel. So if I use the fish png as overlay on other color background the fish would get colored proportionaly in transparent areas

I was researching a bit, and found this How do I create a transparent image that is the diff of two images in gimp?

but that results in selecting the portions that are different from background, whereas I want the level of transparency -alpha channel in the whole region based on similarity with given background, so I could place the png on different background and get similar outcome as in above images of gobies having different coloration on different backgrounds.

Kind of like having a glass object, and trying to place it on different substrates...

Is this even possible in GIMP, what kind of tools would you suggest? I know some coding in openCV, PIL, but not to such extent.

Final goal would be to use the png as a texture for blender 3d model of sand goby.

Any help is highly welcome!

1 Answer 1


Possible technique:

  • Load the fish layer
  • Add a layer above it, fill it with the color of the background, either:

    • Sample the background with the color picker on a wide radius, and bucket fill your layer
    • Copy/paste a portion of the background and blur it heavily (you can use Filter>Blur>Pixelize and use the layer size to average it)
  • Set the top layer to Color erase mode
  • You can put the two layers in their own layer group
  • You can insert a background of your choice at the bottom

enter image description here

  • thank you very much! it works quite well!
    – Eager
    Apr 9, 2020 at 19:26
  • 1
    @Eager Please consider "accepting" the answer, then. This marks the question as having a satisfactory answer.
    – xenoid
    Apr 9, 2020 at 19:46
  • done! did not know how it works exactly
    – Eager
    Apr 20, 2020 at 13:29

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