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I have a complex scene that I have broken down to render in 3 layers, but in order to do a mist pass (needed to help control light), I am rendering the full scene on CPU in cycles with 1 sample. The pass renders, but some materials or objects are not being respected correctly by the mist pass.

Audience Mist pass

As you can see some elements in the scene are showing as white. I have looked at the materials and I can't see anything particularly special about them. They are hair meshes (not particle system) and they have fairly ordinary PBR textures applied. Ray visibility is on for each of the offending objects.

I'm stumped. What could this be caused by? This doesn't feel like a bug but something to do with controlling an object's effect on a mist pass.

Update, ok so it turns out that any material that has alpha information to control opacity of textures causes this. I guess this IS a bug....unless it doesn't matter? I think it will...depending on how its composited.

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    $\begingroup$ Probably if you can share your scene setup with just a single issued object ... so we can check, because it is not happening with my version, so it could be a bug. BTW I'mnot sure if I'm uptodate, but Mist pass is (or was) an issue to represent semitransparent pixel in render ... for that people use simple gradient shader under View Layer Properties > Override $\endgroup$
    – vklidu
    Commented Aug 7, 2023 at 7:32
  • $\begingroup$ Thanks for your comment. That's very very helpful. Could you give a little more detail? How do you use material override? If you set that does it simply ignore the materials for the mist pass? If so that would solve this issue. If it does, it really ought to be on by default don't you think? $\endgroup$ Commented Aug 8, 2023 at 7:45
  • $\begingroup$ To keep it organised here - share part of your scene (as suggested in my first comment) or delete your question and create a new one. We can't answer it (or close it as a bug) right now. Thank you ... Mat.Override has to be rendered as separate image for compositing (it overrides all materials for such render) so it is not the same like a mist pass, that can be composite in a single render run. $\endgroup$
    – vklidu
    Commented Aug 8, 2023 at 7:52
  • $\begingroup$ BTW since your hairs are a lot of overlapping meshes ... this is probably your issue blender.stackexchange.com/a/38113/2214 ... speaking to my semitransparent pixels see this answer blender.stackexchange.com/a/277370/2214 ... or just search on this site "mist" there is plenty of questions about this topic. $\endgroup$
    – vklidu
    Commented Aug 8, 2023 at 8:02

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