Karma shadow artifacts when rendering with motion blur

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Hi;

I have some trees that I'm generating via the Labs Trees tools; the leaves are copied as packed prims onto the branches and are then animated.

I'm exporting a single static model of my trees as a series of USD files (1 file per tree variant), then exporting a second USD file per tree that contains only the animation data for the leaves.

In Solaris, I bring in the static trees, then sublayer on the animation. The leaves are represented by a point instancer. I then use a second point instancer to copy various trees around in an area.

This all seems to work great, and when I render a sequence of frames, I see this:



However, when I enable Motion Blur on my Karma Render Settings LOP, and render again, I see these weird shadow-like artifacts:



In trying to pare back my scene to try to understand where the breakage is occurring, it seems to be with the use of motion blur (I have a separate Motion Blur node in my scene downstream of my RenderSettings node; this blocky artifact is present whether I use this Motion Blur node or not).

What am I doing wrong here?

Thanks!
Edited by dhemberg - 2022年11月18日 15:59:24

Attachments:
good.gif (1.7 MB)
bad.gif (1.6 MB)

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Hmm, can you please file a bug and attach your scene? Thank you!
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Sure, here is a complete setup:
https://www.dropbox.com/sh/baahmqh16tk3c9q/AABweKLaE1N08Sbo8OW_XQ3Ha?dl=0 [www.dropbox.com]

Though, I don't believe it's a bug (or, at least, I strongly suspect that the bug is me). My intuition makes me think I'm doing something wrong with how I'm sublayering the animation onto the static trees; it feels as though there might be an issue where the geometry moves but the extents/bounding boxes of the geo doesn't...or something.

I've seen several references to this notion of exporting static geo USD + animation as a separate USD layer; the tone always suggests this is a widely-accepted, common technique. I find it very compelling and want to do it this way, but have yet to find a complete guide demonstrating how to do it, and the various threads I've read about it on the forum don't make me feel like I have a complete understanding in my head just yet. It seems very easy to get wrong, which I suspect is what's causing this issue.
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Bumping on this for visibility.

A friend from Pixar with extensive USD experience seemed puzzled by the pictures I showed him (from above), and was skeptical it was an animated bounding box issue. He suggested I verify that my point counts (onto which my leaves are being instanced) are not changing frame to frame, as that would likely cause problems, but I stepped through the timeline of the file I shared above and confirmed that the point count for the leaves does stay the same (which is to say: the arrays containing animated orientation/scale/etc. data all seem to be the same length frame to frame). I'd love any advice about how to troubleshoot further, but the aforementioned friend worried this might indeed be a render issue rather than something wrong with my setup.
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