Uknown shading artefacts from Mantra rendering

[ProceduralFrankie]

Hello everyone,

i am having some serious rendering problems with a farily simple scene and materials, this black squares appear randomly and change each frame.
It would sound like a bug becasue sometimes the same frame renders cleanly from an other machine or after reeboot, I checked everything i could think about but no improvement. The only thing that seemed to be working, but only in Mplay renders,wes the rising of Raytracing bias, Z importance and Shading Qiality Multiplier. It was initially happening only in one file, but now is on two different scenes which share most of th esame assets and i don't know where to go. I would share a scenefile but there is a lot of stuff attached to it.
Any help or suggestion will be very much appreciated since is the last week of our master project! 

Francesco

[toadstorm]

have you isolated which AOVs/image planes the noise is showing up on?

have you tried rendering with simpler materials (fully diffuse or simple non-glossy reflective)? with a simpler light setup?

if it's happening everywhere regardless of lights/materials, do you maybe have coplanar surfaces in there? that would explain the ray bias parameter changing the results slightly.

[ProceduralFrankie]

I opened the .exr file in Nuke, but the black artefacts are in all the passes, in some stronger than in others. We  also tried to run the render after putting on the geometry a Polydoctor node and a fuse node with really low fuse number to see if there was any coplanar or overlapping surfaces. I also tried  with a basic principal shader, but nothing changed. Anything we try seems to give us different outcomes, also is really difficult to test because the result is different between the render view, Mplay and the actual render. Sometimes It fixes the problem and is not showing up anymore in mplay or in the render view, but only in the output files. 

[mawi]

Big/small scenescale? Houdini version? Scaled camera/lights? Engine(pbr, raytrace)?

Is there any part of the scene we can look at?

[Atom]

It may be duplicate geometry in the same place. z-fighting. Or try adding a normal node at the end of the SOP network.

Edited August 14, 2018 by Atom

[davpe]

i've seen this a few times already. not sure what it is or if it's the same thing but for me it helped to disable adaptive sampling.