Motion Vector Render Element - How to control sampling?

[jeffkrigstein]

Hi all,

I am trying to do a render of just an animation's motion vector pass. I have done the standard render-speed-up tactics like applying a very basic material, disabling GI, disabling lights etc. The result ends up being that the motion vector render element looks incredibly noisy.

What is the easiest way to correct this and force V-Ray's progressive or adaptive sampling modes to consider the Motion Vector render element when comparing the noise thresholds?

I know I can render out by increasing the Min Subdivs on Adaptive sampling or just rendering with Fixed sampling, but this makes the whole frame take a lot longer and isn't particularly efficient when the object isn't large in the frame.

Thanks for any assistance with this.

-Jeff

[boyan]

Hey, Jeff,

Thank you for the post. V-Ray is easy to use --- my advice is to forget all of the old school stuff by tweaking options and etc. All you need is "Adaptive mode " (for Bucket rendering CPU/GPU) and Progressive mode. So -> V-Ray Main tab -> Image Sampler - Sampler type -> use Adaptive mode.Set Max. Subdivs to 50 and Threshold to 0.008 - 0.01 and compare the result with the previous one. If you want even faster render for Motion Blur render element only you can set Min.shading rate to 1. NEXT --> GI settings --> BF/LC which is by default is fast with V-Ray Next and you can only tweak the Light Cache settings for an animation. Please check the Retrace threshold option and set it to 8 in order to avoid flickering.
If you want to change the global motion blur settings when you're using the "Replicator" item in your scene there is an option in "Motion blur samples" tab -> "Override motion blur samples".

I hope this info is helping you.

Cheers,

[vladimir.nedev]

The result ends up being that the motion vector render element looks incredibly noisy.

Where is the noise ? Is it the edges of the moving objects ?

What is the easiest way to correct this and force V-Ray's progressive or adaptive sampling modes to consider the Motion Vector render element when comparing the noise thresholds?

It's not possible at the moment.
The V-Ray core has an option "Consider for AA" which does roughly what you want, but in Modo it's exposed only for some of the render elements, and the motion vector is not one of them.
It seems we don't have it exposed in V-Ray for Maya either, so I am not sure it will help.

Greetings,
Vladimir Nedev

[jeffkrigstein]

Boyan and Vlad, thank you very much for your replies!

The noise is all over the image. See image below...

Adaptive sampler
Min shading rate - 6
Min subdivs - 1
Max subdivs - 24
Threshold - 0.01
Render time - 2.7s

With what you've said above, does that mean currently the only option is to increase the minimum Subdivs and increase render time across the whole image?

Thanks for your advice.

-Jeff

[boyan]

Hey, Jeff,

Currently, yes it is. I think your Final render will be noisy, too not only the motion vector RE. So double the max Subdivs and decrease the Threshold. I think this might fix the noise.

Cheers,

[vladimir.nedev]

The noise is all over the image. See image below...

It looks like noise from the motion blur on the moving object.

You don't need motion blur enabled when you want to output a motion vector pass.
When you have a motion vector render element, V-Ray for Modo will export several transformation/geometry samples,
just as if motion blur was enabled, and with the same duration.

I don't have experience with generating motion blur from a motion vector pass, but I think you shouldn't have motion blur in there anyway.

So, just disable the motion blur, and the noise should be significantly less and easier to clear.

Greetings,
Vladimir Nedev

[jeffkrigstein]

So, just disable the motion blur, and the noise should be significantly less and easier to clear.

I don't have any camera motion blur enabled on the image at render time. I am only applying it to the other passes in post using this motion vector pass output. That's why I'm trying to work out how to render just the motion vector pass for the whole animation at the most optimal speed. When I do a test render of the beauty pass, it has no camera motion blur applied.

Is there anything else I can try?

Thanks again for your time.

-Jeff

[vladimir.nedev]

Hm, that's strange. In this case, we will need the scene to check what that noise is.

Greetings,
Vladimir Nedev