SSS differences Houdini21 and Houdini22

Forums Solaris and Karma SSS differences Houdini21 and Houdini22

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houdiniUsers: Member; 12 posts; Joined: May 2020; Offline

Oct. 18, 2026 5:03 a.m.

Hi,
Is someone aware of changes on sss rendering in Karma between Houdini21 and Houdini22? My test is a simple tube with a png file and color space set to srgb_texture. Texture is connected to the sss color and it looks much darker in Houdini22. I uploaded the file in case anyone wants to check. This happens if I use thin walled in my openPbr shader.

Edited by houdiniUsers - Oct. 18, 2026 05:04:56

Attachments:
sss.hiplc (589.0 KB)
h22.jpg (168.2 KB)
h21.jpg (177.9 KB)


freshbaked: Member; 162 posts; Joined: June 2020; Offline

Oct. 18, 2026 1:40 p.m.

If you're absolutely sure it's not a colorspace issue, it could be related to this: https://github.com/academysoftwarefoundation/materialx/issues/3027 [github.com]

In which case you could run your texture through MtlxSqrt before plugging into SSS color. But I dunno, even just using an SSS color of 0.5 produces different results with thin-walled SSS in CPU vs XPU so troubleshooting this stuff is always a nightmare for me.

Edited by freshbaked - Oct. 19, 2026 11:44:28


houdiniUsers: Member; 12 posts; Joined: May 2020; Offline

Oct. 19, 2026 4:40 a.m.

Ah, thanks a lot, this sounds exactly like my problem. I'll have a look how we use the fix.


freshbaked: Member; 162 posts; Joined: June 2020; Offline

Oct. 19, 2026 1:11 p.m.

For what it's worth, during my limited testing this darkening issue only seemed to be present in Karma CPU and square rooting the SSS color/texture map did seem to fix it for me. XPU seems to not have this issue though if you're able to use it instead.


houdiniUsers: Member; 12 posts; Joined: May 2020; Offline

Oct. 20, 2026 3:58 a.m.

Good to know, but since we only use CPU we'll use the mtlxSqrt node to fix the problem.


houdiniUsers: Member; 12 posts; Joined: May 2020; Offline

Oct. 20, 2026 5:05 a.m.

It turns out that the problem appears not only on thinwalled geometry but as well on thin translucent nomal geometry. I could see it clearly on the ears of our characters, illuminated from behind. As soon as the color is connected to the sss color, the sss behaves differently and is much darker in H22. And here the mtlxSqrt cannot be applied since it would affect the whole sss color.


freshbaked: Member; 162 posts; Joined: June 2020; Offline

Oct. 20, 2026 10:56 a.m.

Hm do you have an example? I can't reproduce this on typical thick geo with thin-walled off. But you could paint a mask and mix between your regular texture and the squarerooted texture, I don't see why that wouldn't work.


dlee: Staff; 490 posts; Joined: Sept. 2016; Offline

Oct. 20, 2026 11:14 a.m.

Hi. Can you file a bug please? (for the double-multiply of thin-walled SSS colour)
This is actually a long standing bug in the reference implementation of OpenPBR shadergraph that ships with MaterialX. We've been manually patching it for 21.0 and older, but it slipped us by when we upgraded MaterialX library to the newer version.


houdiniUsers: Member; 12 posts; Joined: May 2020; Offline

Oct. 20, 2026 2:46 p.m.

Just reported it.

I attached the file with different versions. All look darker in H22.

Edited by houdiniUsers - Oct. 20, 2026 14:47:45

Attachments:
sss.zip (2.4 MB)

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