Subsurface scattering and ambient occlusion not playing well together

[R Reynolds]

Shown are three copies of the head of the Jungle Queen. Each head uses single attribute "skin" materials. The leftmost head material has subsurface scattering turned OFF while it's turned ON in the rightmost head. The centre head has subsurf. scat. turned OFF on it's right side while it's turned ON on it's left side. These three models are arranged in a chor that uses a single "sun" style light source and the global color (256, 256, 256) for ambient occlusion.

With subsurf. scat. turned off in the camera, all three heads are screen rendered as expected with just sun, just ambient or both illuminating it. When subsurf. scat. is enabled in the camera and the only illumination is the sun, the screen render is as expected. Turning on ambient occlusion in the chor and the screen render is not what I expected.

Results are identical in V19.0p and v19.5a.

subsurf_scatter_bug_embed.prj

[robcat2075]

I have a dim recollection that SSS only works in renders to file.

SSS+AO is also complicated. I think that is because SSS is very much about light direction while AO has no specific direction.

My inclination would be do the AO as a separate pass and composite it.

[R Reynolds]

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...SSS only works in renders to file

SSS is not calculated with a progressive screen render. But compare the attached, render to file image with the previous final screen render and they appear to have very similar results.

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...do the AO as a separate pass and composite it

You say that as though there is an SSS buffer that can be rendered separately. I started browsing the SSS sub-forum and found that this has been an issue since at least 2008 and in this https://forums.hash.com/topic/33040-rendering-sss-separately/ thread almost exactly 15 years ago you wrote;

Quote

I don't think A:M's OpenEXR render includes a separate AO buffer. You'd have to make one manually, which clever people do.

So I guess I'll have to be more clever 😄.

[robcat2075]

I should like to meet 2008 Robcat, he had so much excitement about this stuff.

 

The more clever tactic to get a separate AO channel, if it were not supported by the renderer, would be to set all to objects to white, turn off all other lights, and do an AO-only render.

However, it turns out EXR (most clever of all) can save an AO channel...

 

 

Here is the above PRJ, the EXR file and a render preset with the render settings I used.

AOTest04.zip

[robcat2075]

Here is a quick version of your PRJ using the composite method.

Changes:

The key light is set to 100 intensity%

The skin settings are:

 

The Composite "multiplies" the NoAO and OnlyAO together

[R Reynolds]

Thanks very much, Robert. This is all entirely new to me so it looks like it's time to climb that learning curve.

[robcat2075]

If you already have a dedicated compositing program like After Effects, or a paint program like Photoshop that includes compositing modes, that is the way to go with investigating this "post-processing" stuff, however... A:M does have some basic compositing powers and they work with OpenEXR!

There is a Tech Talk on A:M Composite projects https://forums.hash.com/topic/13249-tech-talk-series/

I don't recall what's in it. I'd have to ask 2008 Robcat about that.