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I get strange artefacts from the smoke domain where density should be zero but the domain box is over other collision objects in cycles (Blender 2.92 and 2.93 beta). Res seems high enough. For some reason, it seems to be caused by reflected rays of textured surfaces that are inside the smoke domain box.

I'm not sure what causes this to happen but I am trying to find a way around this. If I disable the material input node, it goes away. If I render with no smoke the objects are ok. So the domain seems to affect the non-smoke area. Any ideas?

enter image description here

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  • $\begingroup$ Please Edit your question to include additional details and screenshots. As it stands I don't think there is enough information to be able to provide a meaningful answer. $\endgroup$ May 9, 2021 at 21:25
  • $\begingroup$ I think I might have to render the image twice, once with and once without then combine in a graphics editor. $\endgroup$
    – James
    May 9, 2021 at 22:50
  • $\begingroup$ If you do need to render twice and combine the images you can use different Render Layers within Blender and combine in the Compositor within Blender. This is needed for certain situations and results. However, in your case, check that your Domain geometry doesn't exactly coincide with another mesh (whether another domain or volume or transparent surface). If it does exactly coinside, move it very slightly so that it does not. This might help since if surfaces exactly coincide in space it can cause problems with rays as they pass through that surface as Blender can't always tell when it does $\endgroup$ May 10, 2021 at 6:07
  • $\begingroup$ I thought maybe the auto smoothing with weighted normals might be causing an issue but it did not. I thought that trying a much higher res on the domain might help but it did not. The rendering, I have set two views so it renders one then renders the other. I can then combine after. Because I need the smoke I might have to blend both layers in a graphics program. It's just easier to get the areas I want. It is a shame that the domain is causing the distortion as it would be nice to only have to render one view layer. $\endgroup$
    – James
    May 10, 2021 at 18:55
  • $\begingroup$ Note, that the artefacts seem to be linked to reflected textures through the smoke domain. I don't know the specific relevance in the code, and I doubt I would easily tracking it myself. I've not looked at the source code but it must have lots of crazy physics stuff. $\endgroup$
    – James
    May 10, 2021 at 18:58

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Without a screenshot it's hard to tell, but I often see artifacts in smoke simulations even in tutorials because of too low transparency settings. Try to increase the Transparency value under Render Settings > Light Paths > Max Bounces. Maybe 32 or 64 or even 128. Important: Transparency, not Transmission (image is from a different question, the red framed part is important).

bounce settings

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  • $\begingroup$ I have 'full global illumination set'. I'm thinking it might be a bug with the domain bounding box. But I'm not sure. $\endgroup$
    – James
    May 9, 2021 at 22:12
  • $\begingroup$ How much bounces do you have for Transparency? I don't know the default values for full global illumination. I think my Blender has a default of 12, which is much too few usually. If setting it higher doesn't help, I can't tell what the real problem is without a screenshot of how it actually looks or a blend file to explore the settings. $\endgroup$ May 9, 2021 at 22:23
  • $\begingroup$ Full global is 128 for each. The smoke part is fine. It comes out well it's just really weird artefacts on the other objects that should not have any smoke. It looks almost like inverted normals on some of the verts but the normals are fine. $\endgroup$
    – James
    May 9, 2021 at 22:25
  • $\begingroup$ I'm sorry, I can hardly recognize anything in those screenshots... I don't see any objects, just an off-white blur which admittedly is not the same left and right. But it seems I don't know what's going on. $\endgroup$ May 10, 2021 at 6:48

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