When I remove the DraftTransformation optimizer, I get the same issue in preview as well. I removed Transformation optimizer, and the issue was still there. So the optimizer isn’t the problem. The thing is that DraftTransformation sets the interpolation to nearest, which by default is cubic.
I think the issue is that cubic resample is broken(I am not sure since that is not part of the bisected commit). If I force it to be nearest resample(on this line) then also it works.
Edit: Changing rectangle layers blend method to alpha works. Not really sure why it breaks with Alpha Brighten. The only difference between final and preview renderer is the use of DraftTransformation optimizer, which sets the interpolation mode to nearest instead of cubic.
Edit 2: I think I found the issue, it might be a floating point precision issue with Alpha Brighten and Darken.
Similar thing with backdrop.sif example. Look at the mountains, in the final render they are offset and double. Another weird thing is that the second frame changes color and alpha when changing zoom in viewport.
But I do think your discovery seems very convincing and I might have looked at the wrong place. I tried doing an extensive search and found something, yet now I’m not sure if I was even close to get it right… (yeah I got lost while searching the source of my issue, not gonna lie )
But for now let’s just sort this one out first and see if it affects the other one as well~
Edit: Should I write an addendum on my issue thread?..