How to render multimatte fast with VRay GPU

[D3Pixel]

Hi

I am struggling to render out an image sequence of a single multimatte element from an 18 mil poly scene over 300 frames. The scene also uses motion blur.
Is there a workflow I can use to get this out quickly? I do not need any other element, just the multimatte that is on one object but do not need the RGB, just the matte.
Please note I can not use the RenderMask as that is broken in VRay GPU and I do not want to switch to CPU in case the motion blur changes thus making the matte not line up.

I managed to get it down to 30 seconds per frame but then all the files in the folder were the first frame of the animation x 300 so I did something wrong or this is also broken too!

Can anybody suggest a fast render GPU workflow to get this done?

Thanks for any advice.

[D3Pixel]

Never mind, got it working hopefully, using very low preview settings, it saves the RGB but I can just discard it. 2 hours to complete the job.

[hermit.crab]

Since you only needed the MultiMatte pass, you could have disabled all lighting and GI so that the RGB pass is rendered black, which would in terms decrease the render time.

[D3Pixel]

Hi Aleksander

Thanks for the reply. The motion blur on the original footage did not play well with the multimatte so I tried to use a render mask instead but that crashed when trying to save the scene as a .vrscene (vray script errors). Then max crashed (repeatable) when trying to search for an object from an exclude list. I was at this point rather frustrated so exported the scene as FBX and rendered the whole thing again in Blender Cycles.

Edit: I said Cryptomatte and meant Render Mask. I'm tired.

[hermit.crab]

Regarding the 'include/exclude' list - this issue is a known 3ds max (2019.2) bug, which has been fixed in the latest 3ds max update (2019.3).

[D3Pixel]

Regarding the 'include/exclude' list - this issue is a known 3ds max (2019.2) bug, which has been fixed in the latest 3ds max update (2019.3).

Thanks for the tip Aleksandar. Will get that updated.