I managed to reproduce it today on the Mushroom scene. It looks like some form of correlation but right now that is just a theory. Do you have any other scenes that it happens on that you could send? They would be useful in pinning it down.
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Great that you can reproduce the problems.
That mushroom scene is not by me, but I double-checked the scene / geometry etc. and it’s all perfectly clean. No inverted or weird normals, no extreme scaling, no strange attributes or settings that might cause it.
So far it’s clearly connected to the sampling pattern (Sobol is totally fine, PMJ is broken). I hope the culprit can be found soon.
EDIT: Thanks for confirming and submitting this 
… and for fixing it!
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No problem it still needs review but it’s getting there
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Now with the new features of former Cycles-X and new default settings (adaptive sampling, 4096 samples, threshold 0.01) and the threshold becoming the main quality adjustment instead of sample count as before, I guess it might be cool to offer an override for the threshold on a per-view-layer basis.
Currently this is what it looks like:
You can override the materials of a whole view layer with a global one and you can also override the max samples. But wouldn’t it be more logical to (also) offer to override the threshold?
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hello @leesonw, i’m talking about what is described this way in the issue linked by @Alaska
T91540 Cycles: improve subsurface scattering for some meshes shapes
“* Self-intersection meshes give dark borders. Counting back/front facing intersections could help determine inside/outside better to avoid such artifacts.”
So not related to you fix on instancing
More way to procedurally override material data would be most welcome.
We just did a show where we had to render volumetric lighting in a seperate pass/scene with a dumb holdout shader to get proper speedup rendering wise. Would have been great to override on a viewlayer basis or even collection basis too.
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A bit more advanced possibilities to override materials etc. on a view layer or even collection basis would indeed be very welcome.
I suggested a much simpler thing though, just adding the threshold to the overridable settings. This should hopefully be rather simple to implement.
With this closed and categorized as a known limitation, I would like to know if there’s an approved way to calculate distance to the nearest surface in volumes. I’d been doing a good many aura effects using the AO node in volumes as a rudimentary distance-to-surface metric, and since it looks like that’s no longer going to be possible, I’d like to know if there’s anything that can replace it and salvage my projects (ideally something better I should have been doing from the beginning!)
Edit: alternatively, spline curvilinear coordinates would be even better for some particular situations - is that possible?
Are the current Cycles-X builds (like cycles-x.342cdb03ee06) different from the 3.0 nightlies given it was merged into 3.0 recently? I am just wondering if we should keep testing the Cycles-X builds or just test the regular 3.0 builds.
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If you want the latest ‘Cycles X’ build you should download Blender 3.0 Alpha daily build.
The question remains: why do cycles-x branch builds still exist??
Most of the (even very) old branches still exist, that’s business as usual.
But so far noone told the Buildbot to stop building the Cycles-X branch on a daily basis although the last commit there was from September 20th.
( History · rB ).
This might confuse people… 
P.S.: Just for fun I scrolled through the list of existing branches and the oldest one received its last update in January… of 2006! 
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Branches are usually not deleted, as they contain the history of all the single changes.
Cycles-X was merged to master, so the branch is not relevant for users anymore. I will check to update the Buidlbot.
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Hi Thomas, nice to see you back on Cycles business. I lost track of you last few years (Am I wrong?)
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I’m working on this scene (still lots of things to do), which consists on Cycles X heavy volumetrics and new OIDN denoising, everything using the Compositor (which is faster on 3.0) and Geometry Nodes as well. I plan to work on some comparassions in a few days.
Pre 3.0, my computer couldn’t even render the scene* at this resolution (2560 x 1080 - two RTX 2070 Super and 32GB DDR4). Thanks for the improvement.
*No render layers and compositing tricks, everything as “raw” as possible.
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Hey is there a way to make the “Tile Highlighting” more visible? because at the moment in some scenes you can’t even see.:
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My version 3.0 crashes when I have assigned a second material to an object and switch to render (cycles) view in the viewport. Am I doing something wrong or is this a bug?
(Sorry I am a blender newcomer). Beside of that, great work. Cycles-X runs smooth on my GPU! 
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Crashes are always something you should report to the bug tracker, as long as you can share with developers a .blend file where the problem can be reproduced. You make sure that you are using the latest available buildbot 3.0 Alpha:
To report you must be registered on the Bug Tracker site. Then you open Blender, and from the “Help” menu you select Report a Bug.
@Florian-10 , Compared to what? In 2.93 does it look different? Have you tried different “look” Color Management settings from the Render tab? Even with “Standard” in View Transform.
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I tested todays 3.0a build including the recently merged Cycles X update with a scene that was created in 2.93 (can’t share it due to some of the models). It has a lot of foliage scattered around and some volumes.
Compared to 2.93, I experienced a significantly higher consumption of vram without speed ups in spite of using the same settings (resolution, samples, noise threshold, denoising).
2.93: 5589MB
3.0a: 10743MB (tiled rendering)
3.0a: 18293MB (progressive refine)
Are these differences normal as it’s still in alpha stage or am I missing something?
Also, viewport navigation seems to be way slower although all scatter systems are turned off (not related to Cycles).
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Hi.
Since Cycles X is now in master, maybe that part of the text needs to be edited now?
So people report problems according to bug tracker requirements, sharing the corresponding .blend file and hardware description.
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