GPU Frames Not Consistant

[christopher_rose]

I am trying to render an animation for a client (which is why I will be unable to upload a scene file) and I need to render with Vray GPU for speed reasons but I am facing an issue where looks as though a reflection is switching on and off on random frames throughout. This does not happen when comparing IPR renders and it does not happen when rendering with CPU, for these reasons I have ruled out any lighting, material or geometry issues.
The ‘flickering’ only seems to happen when the camera slowly moves in closer during the animation, when it’s stationary the frames are consistent.

I have tried rendering with GI set to ‘brute force’, changing the light cache from ‘still’ to ‘animation’, I have tried ‘bucket’ and ‘progressive’ samplers. Even though there are no hidden lights I have tried rendering with ‘full’, ‘adaptive’ and ‘light tree’. I have also saved the outputs as jpg, tiff and Vray RAW 32-bit exr. My Vray render log shows no errors so I am now stumped.

Here is an example of switching between two of the more obvious frames of the GPU render. It is the reflection pass that is causing this (the GI pass is affected but only faintly). All materials are Vray materials, lit with x4 standard Vray lights (no HDRi), and using a Vray camera.

I should also note that the areas being affected by the issue have ‘slice’ modifiers (I’m not sure if that is causing an issue).

Here is my render setup:

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Please help! I am at a loss and can’t afford to have to render the whole project using CPU.

[christopher_rose]

Okay, I think I have identified what could be a bug and resolved the issue and it was indeed to do with the slice modifier.

While this issue does not happen when rendering in CPU mode, when rendering in GPU mode I have to change the ‘Object Properties’ of the geometry that IS DOING the slicing. I have a simple animated plane that I am telling my slice modifier to use to make the cut. I had disabled all the ‘Render Controls’ apart from the ‘Renderable’ and ‘Inherit Visibility’ check boxes.

This works fine when CPU rendering:

However, when rendering in GPU mode I was getting strange flashing in the refection pass of the geometry that contained the slice modifier. I found that if I disabled all of the ‘Rendering Controls’ GPU mode then renders as intended with consistently lit frames:

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I hope this information will be of some use to someone.​

[Muhammed_Hamed]

Thank you for the report, and sharing the solution with us
We will take a look, I will see if I can replicate the issue on my side

Best,
Muhammed