: "Replace" image (gradient) in image with alpha Consider the following graphic: Note that the first image's background is transparent, and that the blue background in the second and third images

@XinRu324

If you have imagemagick installed (if you don't and are on a mac i'd recommend using homebrew) there is a super-easy way that will get you close...

see this article here : www.imagemagick.org/Usage/compose/#changemask
Basically from your two images you can call this command...

convert original.png background.png -compose ChangeMask -composite result.png


with the image with everything being original.png, the blue gradient background being background.png and the output (transparent) being result.png (or whatever you want it to be called).



You can see the result above... it successfully strips the background away, but the anti-aliasing of the items is still somewhat blue.

Maybe this is enough for you... if not things become much more difficult quickly.

The problem is that it isn't obvious with those edge pixels what colour/transparency combination they are.

There are techniques to help with this which you can read about in more details here:

www.imagemagick.org/Usage/photos/#chroma_key http://www.imagemagick.org/Usage/photos/#green_screen


But it is non-trivial to achieve as far as I know.

To get a really good result ideally you have two versions of the image sat on different backgrounds (white & black is classic) and then this technique works:
www.imagemagick.org/Usage/masking/#two_background
Anyway, fingers crossed the first option might be enough for your needs.

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