Reflection artifact problem

[Evox_Productions]

We are having a problem where the reflections of our animations render out blotchy on random frames (See attached sequential frames). The team originally thought it was a GI problem, but it is definitely only the reflection passes. It also shows up in the beauty pass. We are using the carpaint shader and thought it was an issue with the flakes, however the issue still occurs with flakes turned off. This only seems to happen when rendering on our farm. A re-render of the frame solves the problem most of the time, even if rendered on the same rendernode that produced the offending frame. We are running a stable build from dec 12. We did not have this issue prior to installing this stable build. Any info on this is much appreciated.

[pechart]

First thing that comes to my mind is, that one of your machines is using a different build, maybe. Can you isolate the problem to specific machines?

[paulDryzen]

We had the same issue on some of our renderings.

The dmc sampler has some problems with HDRI image with high contrast areas. For example you have a white and a black pixel without any gradient in between them, if the dmc sampler is hitting the black pixel no more rays are fired in that direction, but if it hits the white pixel it sends more rays to that area.
Our sollution was a really small blur on the hdri or sometimes a glow effect on the bright ares to avoid having pure black areas in the HDRI itself.

This forced the dmc sampler to always sample the hdri a bit more and the rendering didn´t had these kind of problem anymore.

Hope this helps.

Paul

[Evox_Productions]

Thanks for the info. It is strange because I have been using this scene and hdri since august and have not had any problems until a week ago. I am pretty sure the farm all has the same build but i will check it out.

Would switching from adaptive DMC to Adaptive subdivision make any difference? Would server lag have any effect?

[vlado]

Server lag should have nothing to do with it; the sampler will of course change things (for better or worse, I don't know ) But it's hard to say what's going on without the scene to look at the settings.

Best regards,
Vlado