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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.

 

image.thumb.png.9eed89d66e93ee188fd9b7eb90c8a817.png

blend mode difference and alpha.afphoto

Mac mini M1 A2348

LG34WK950U-W, calibrated to DCI-P3 with LG Calibration Studio / Spider 5

iPad Air Gen 5 (2022) A2589

Special interest into procedural texture filter, edit alpha channel, RGB/16 and RGB/32 color formats, stacking, finding root causes for misbehaving files, finding creative solutions for unsolvable tasks, finding bugs in Apps.

 

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Mac mini M1 A2348

LG34WK950U-W, calibrated to DCI-P3 with LG Calibration Studio / Spider 5

iPad Air Gen 5 (2022) A2589

Special interest into procedural texture filter, edit alpha channel, RGB/16 and RGB/32 color formats, stacking, finding root causes for misbehaving files, finding creative solutions for unsolvable tasks, finding bugs in Apps.

 

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