Hi,
the "difference" blend mode is great to spot any differneces between 2 layers. Identical pixel blend to pure black, any difference are rendered to non-black, which can be amplified by a levels adjustment.
Unfortunately, this will only work when alpha is 1 (pure white). Otherwise, he blend formula will lead to non-zero results for color and below-one results for alpha, making this useless for spoting differences.
My question: Is this by design?
To be useful for spotting differneces, i would expect it to deliver RGB=0 and alpha=1 if both layers have equal RGB and alpha values, for those cases where alpha <> 1, too.
In the picture, you can find a gradient in the bottom from white to black, 100% opacity
In the top, a copy of the gradient with 50% opacity
On the left, the original gradients. On the right, i added a copy with blend mode "difference".
As you can see, only the right bottom quarter shows pure black as expected. The right upper quarter shows non-black, even if the layers are 100% identical both in RGB and alpha.
blend mode difference and alpha.afphoto