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Really struggling with figuring this one out! I have some transparent/translucent materials I'd like to render on different passes. I am using some mask holdouts to get around misbehaving booleans for a faux "fluid sim" that involves filling a complex shape (deserves another post..) From the display inside Blender, they appear fine. However, when saving as a PNG, EXR, or multilayer EXR, the alpha channel is speckled.

Funnily enough, this is the same alpha I get when checking "convert premultiplied" in the compositor when I try using the alpha over node. Same speckled appearance when I have the alpha convert set to premultiplied. Note that I dont want to use the blender compositor for my final file export.

Blender Viewer

PNG viewer on file system or in After effects - tried all the alpha modes Exported PNG - notice the material on the right

Same result when converting to premultiplied... interesting... enter image description here

Result is OK with nothing in the node tree or with the alpha over node: enter image description here enter image description here

Blend - https://we.tl/t-ijQ126oytW It just seems that blender's viewer is handling the alpha channel differently then when you try to save a file out of it. Thanks!

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  • $\begingroup$ Noise is harder to see on the dark background - that's all. $\endgroup$ Apr 12, 2023 at 17:41

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Alpha channel is not denoised so it's noisy. You can use Denoise node to denoise only alpha and output it with File Output node. That's in Add -> Filter -> Denoise in the Compositor node editor.

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