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Operating system: Windows 10 x64

Graphics card: NVidia Quadro RTX 3000 and NVidia RTX 3080

Blender Version: 3.4.1

Issue: it looks like current caustics only supports one object, which makes it impossible to properly render any scene with liquids/bottles of liquid etc.

Exact steps for others to reproduce the error:

  1. Create a refractive object like glass.
  2. Use Glass BSDF or Principled BSDF on it with transmission/roughness maxed out.
  3. Put another object (liquid) inside of it, use similar Glass/Principled BSDF with transmission on it.
  4. Put ground plane with default Principled BSDF
  5. Enable "cast caustics" for both objects.
  6. Enable "receive caustics" on plane.
  7. Add light source and enable "shadow caustics" for it.
  8. Enable "reflective and refractive caustics" in render settings.
  9. Separate "liquid" and "glass" objects to test them separately from each other.
  10. Check results - Caustic shadow shows as a dark opaque shadow for liquid inside glass. Caustic shadow renders properly for separated objects on their own.

enter image description here

Added another image where I removed glossy filtering and clamping, set light bounces to 64, and increased samples to 1000000. Same result.

enter image description here

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  • $\begingroup$ You probably need to increase light bounces and unclamp some values that were implemented to control noise caused by glass etc. blender.stackexchange.com/a/219957/110840 $\endgroup$
    – Allen Simpson
    Commented Feb 10, 2023 at 18:26
  • $\begingroup$ I tried removing clamping and glossy filtering, as well as increasing light bounces to 64 and samples to 1000000 (which is a bit insane). It does add extra caustics to single objects same way it's done in your link. But the liquid inside the glass still produces dark shadow with no caustics. So no, it does not work and it looks like Cycles issue. $\endgroup$
    – MMM
    Commented Feb 10, 2023 at 18:42

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Alright, I received an official response on that issue from Blender Projects mod.

The rendering technique used to speed up the rendering of caustics is based on MNEE. There are a number of limitations with this technique. See https://docs.blender.org/manual/en/latest/render/cycles/object_settings/object_data.html#caustics

I believe you are seeing this limitation.

So unfortunately this is a limitation of the technique and it is not something that can simply be fixed :\

So look like Cycles is unable to properly render caustic liquid/refractive object nested in another refractive object. I suggest using a different render engine like LuxCore to achieve realistic results.

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  • $\begingroup$ Well, caustics have always been a weakness of Cycles and when I saw the first videos on what's new in Blender after this new method came up, the general conclusion I heard on this topic was, still far away from perfect but better than before... so I didn't expect too great results anyway. $\endgroup$
    – Gordon Brinkmann
    Commented Feb 10, 2023 at 21:08

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