I have two images that are very similar, let's say image A is the base image and image B is the image that slightly differs. I need to extract all the parts of image B that differs from the base image A and turn them into a third image that has transparency in all the areas that were the same in both images. The differing pixels will come from image B. Basically, just cutting out image B to create an overlay that will later fit on top of image A. I am using GIMP.
- Load both images as layers of the same image
- Put the top layer in Difference mode: everything common will be black
- Make that view a new layer: Layer>New from visible
- Use the Threshold tool with a threshold of 1, you should get white everywhere there was a difference between the two layers
- Open the Channels dialogs, right click on any of the R, G or B channels and Channel to selection
- Hide/remove the "difference" layer
- Activate the layer with Image B and use the selection to Edit>Copy, then use Layer>To new layer