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Peter Werner

Overflow artifacts in Apply Image

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Trying out the awesome new Equations feature in "Apply Image", I found that expressions that lead to out-of-bounds values are not handled gracefully (8-bit mode).

 

My test was a simple color difference greenscreen keyer:

DA = SG - max(SR, SB)

The result has artifacts where this expression yields negative results. 

 

Doing the clamping manually works fine.

DA = max(0, SG-max(SR, SB))

It would make more sense to always have this clamping performed automatically and implicitly for non-float documents. This would also ensure that more expressions and macros created for unbounded floating point images will work with 8-bit and 16-bit documents without adjustments.

 

I have also not been able to find out how to access the current pixel's coordinates inside Apply Image (seems to work for "Distort > Equations" expressions) and what the syntax for modulo operators is, but that's likely either due to my stupidity or it not being implemented yet and not a bug. Also, I noticed a strange 1-pixel diagonal line pattern when choosing a source layer and then navigating to an adjustment layer in the Layers panel.

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