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Motion blur and Velocity

Unread postPosted: Wed Jun 15, 2022 3:30 pm
by blazelet
Hello!

If a cache is set to "read velocity" but there is no velocity attribute on the geo to read, does it fall back to calculate or does it render without blur?

I am getting mixed results on some tests and am just curious what I should be seeing. Thanks!

Re: Motion blur and Velocity

Unread postPosted: Wed Jun 15, 2022 5:52 pm
by anemoff
Hi,

Yes, that's correct. If the asset doesn't have any velocity information baked in it, "Read Velocities" will fall back to "Compute Velocities".
What file format are you using?

Feel free to share assets and/or screenshots.

Re: Motion blur and Velocity

Unread postPosted: Wed Jun 15, 2022 6:02 pm
by blazelet
Thank you for the confirmation!

In this case I have 2 different things that I'm using, alembic points with scattered geo and an alembic geo sequence, both without velocity attributes. I was getting the same result whether I used calculate or read velocity, and this explains why.

A follow up then, if my geo is set to "read velocities" and there is no baked in velocity attribute, it'll default to "calculate velocity". Under what circumstances will that not give me a blur result? I assume changing point count would be one circumstance, are there others?

Unfortunately I can't share the scene or screenshots. Thank you!

Re: Motion blur and Velocity

Unread postPosted: Wed Jun 15, 2022 9:13 pm
by mdkai
Hi
in order to get a proper result you should have a presistent ID for the points or they will fail to provide a proper calculation as they need this for the pre and post frame calculations (or what shutter mode you are using)

In addition some older versions of Clarisse had issues with motionblur and you may need to check what version you are using.
The best way is to have animation cached out with a substancial subframe count (like 6-8) and then use that mode for the calculation.. since motion blur, specifically radial ones are highly depending on a proper curvature only possible by subframe calcuation.

Subframes are much heavier on the memory of course but will give the most accurate results.

You can also check then by disabling the frame snapping and have Clarisse show the time in substeps so you can verify arcs much better.
Cheers

Re: Motion blur and Velocity

Unread postPosted: Wed Jun 15, 2022 9:37 pm
by sam
Normally you won't get motion blur if the geometry is exported as changing topology and no motion vectors are available. I don't see any other cases where that could happen.