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: 
I have several images like this: 
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!
