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I've run into this problem on multiple different projects, and can't figure out what's causing it. Images below.

It seems to be the result of rendering a scene with semi-transparent (opacity at 0.5) objects with a transparent background, and is only apparent after saving the image as a PNG (EXR is fine). The results in the transparent objects with transparency behind them having visually different brightness/visibility levels than objects with another object behind them. The issue is especially visible when a semi-transparent object is in front of a black object.

Does anybody know of a solution for this? Thanks!

This is an EXR composited over black with Alpha Over, working as expected: enter image description here

This is the PNG, showing clear errors/artifacts with the alpha channel: enter image description here

Taliesin River
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    This may be another variation on this common problem with use of alpha in PNGs? : https://blender.stackexchange.com/a/46197/73773 – NeverConvex Jan 07 '22 at 03:08
  • Ah thanks - If you don't mind clarifying, do I understand correctly that this is basically just an inherent limitation of PNGs and the only way to avoid it is to use other formats? – Taliesin River Jan 07 '22 at 18:09
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    I'm not an expert on the PNG standard, but that is my understanding from the BSE post I linked and a few other resources, yes. There's a bunch of jargon thrown around, but if I've understood correctly, PNG throws away some alpha information, and does this in a way that makes non-zero color behave strangely in low-alpha (especially in the exact alpha=0 case) pixels, and as a result some other image format needs to be used when this is important – NeverConvex Jan 07 '22 at 19:56

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