How do I get rid of these small black dots in an underwater scene (environment volume)?

[nwoolridge]

I've created a test scene for my students to show one potential solution for creating the effect of an underwater scene. It includes an environmental volume, and a light casting fake caustics via a gobo light filter (the gobo is based on a 2d C4D noise). The resulting renders exhibit single pixel black dots across the image. Changing various sampling values (light sampling, Camera AA, Volume, etc.) does not seem to eliminate them, but removing the gobo from the light does get rid of them (and, of course, eliminates the caustic rays and pattern on the seafloor).

Does anyone know how this might be fixed?

Underwater Test Scene.c4d.zip

[lee_griggs]

Replacing the C4D noise with a cell_noise fixed the issue. Example scene file here.

[nwoolridge]

Unfortunately this destroys the caustics rays cast in the atmosphere volume. Is there a way to force the cell_noise node to be 2d, or in UV space rather than UVW?

I think this should work with the C4D noises (I don't have the same issue with C4D's standard engines or Redshift), so I think this should be classed as a bug (probably in Arnold's implementation of the C4D noises?).

[nwoolridge]

Ah, I just saw that I can set the space to "UV" in the coordinates tab of the cell_noise. That solves that problem...

It would be great to solve the issue/bug in using C4D's noises...

[peter.horvath6V6K3 · Accepted solution]

The problem is not the C4D Noise shader actually, but the color_correct. In those black pixels the noise shader outputs a low color (e.g. 0.00009). The color correct applies a 0.1 gamma on this and it basically explodes.

Clipping the noise output should solve the problem. For instance add a range shader between the noise and the color correct and set the Output min to something like 0.01.