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I'm trying to remake some "2001 : A Space Odyssey" shot and I already have this issue with other project.

You can see on the screenshot, my non-compressed files saved isn't as good as the render displayed. I think it's just something that I don't know.

enter image description here

Duarte Farrajota Ramos
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IamBoubou
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    Sorry, but without any settings or more information we can just guess around. So providing your blend file or giving us all necessary information would help. – Chris Oct 04 '21 at 05:37

1 Answers1

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This is a limitation of PNG format itself, which uses unassociated (straight) Alpha format, which handles badly pixels that are both emissive and transparent: as the halo gets near to full transparency (on the outside of the star), its alpha values drop toward zero; in unassociated alpha format they are then multiplied with the RGB values, so the result gets black or nearly black as soon as the alpha value gets 0 or nearly 0.

To solve use associated (premultiplied) alpha formats, like EXR does.

See this answer (which is not Cycles related but general) for further infos: Why should I never ever use "Convert Premul" on Cycles renders?

Try to investigate more deeply on the associated/unassociated alpha problem, which is not trivial because both formats have their pro and cons.

josh sanfelici
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