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I have a following problem: when I render my final image with mist pass, the mist pass itself becomes noisy, see in these images:

  1. Mist pass combined in image:

  1. Mist Pass itself:

Could the problem be that I am using complicated objects in my scene (grass particles, volumetric fog, etc.)? How one would proprely render mist pass?

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    $\begingroup$ Please use the built in tools to add images to your posts. Use any image editor or online optimizer if you must resize an image. Information should be displayed in this page to help those who would answer your question. $\endgroup$
    – susu
    Commented Jan 9, 2021 at 21:26
  • $\begingroup$ Since there is a part of grass without noise, it seems to be something with material. Try to disable some elements in the scene to see if issue persists. $\endgroup$
    – vklidu
    Commented Jan 9, 2021 at 22:57
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    $\begingroup$ T66149 T65887 possibly related to these two bugs. $\endgroup$
    – HISEROD
    Commented Jan 10, 2021 at 1:39
  • $\begingroup$ @HISEROD Yes, seems like that. I was thinking about fogy clouds in the scene so I placed a plane with transparency into scene and it results like his screen. imgur.com/FwxzlHk $\endgroup$
    – vklidu
    Commented Jan 10, 2021 at 9:48
  • $\begingroup$ Seems that removing these particle systems and fog helps. So it is probably a bug then. $\endgroup$ Commented Jan 11, 2021 at 11:33

2 Answers 2

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I had a similar problem once with a grass particle system. The issue arises when there are insufficient light paths for rendering. Try increasing the light path counts. Depending on your materials, it varies which one you have to tweak.

enter image description here

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  • $\begingroup$ If you don't want to increase the overall rendertime you can add a Denoise Node right after the Mist path, before you mix Mist and RGB together. (I wouldn't recommend this) $\endgroup$ Commented Nov 21, 2022 at 12:42
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My solution was to add a brightness and contrast to the alpha of my leaves (in the shader node, bump the contrast up to a high value) and the noise disappears completely. The only problem is that the alpha becomes alpha test with hard edges.

There might be a way to reblur it in post, but I am not a shader expert. My problem was that the leaves had white edges around them in the mist pass. So it seems any transparency will turn out noisy in the mist pass.Try to use transmission instead where possible, hence my use of the brightness and contrast trick for leaves with alpha

enter image description here

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