"Add shell to surface" leads to no motion blur

[robertfreitag]

Hey, I think I've come across a bug in the current release 0.16038.

Whenever you have a cloth mesh and you turn on "Add shell to surface", V-Ray next will not render motion blur on this mesh anymore.

I've attached a file that demonstrates the behavior, if you render it as it is on frame 30 for example, it will not render motion blur. But if you turn "Add shell to surface" on, Tyflow will apparently not generate motion blur for this mesh anymore.


Thank you!
-Robert

[tyFlow]

It's not a bug, it's because when you add a shell to a cloth with tearing enabled, the number of faces in the mesh will change when a tear occurs, even though the number of faces of the underlying (non-shelled) cloth doesn't change. This is due to the fact that new tears create new open edges that must be bridged.

A unified mesh velocity framework is on the todo list, that will eventually support motion blur regardless of changing face counts, but it's probably a way's off. It doesn't help that max has no consistent mesh velocity framework so I have to manually wrangle all the different ways of calculating motion blur offered by each renderer myself.

An admittedly awful but temporary workaround would be to render your camera with multi-pass moblur enabled.

[robertfreitag]

It's not a bug, it's because when  you add a shell to a cloth with tearing enabled, the number of faces in the mesh will change when a tear occurs, even though the number of faces of the underlying (non-shelled) cloth doesn't change. This is due to the fact that new tears create new open edges that must be bridged.

A unified mesh velocity framework is on the todo list, that will eventually support motion blur regardless of changing face counts, but it's probably a way's off. It doesn't help that max has no consistent mesh velocity framework so I have to manually wrangle all the different ways of calculating motion blur offered by each renderer myself.

An admittedly awful but temporary workaround would be to render your camera with multi-pass moblur enabled.

Ah, I see. Thank you for the info. And thanks for doing all this by the way!  It's amazing already!

-Robert

---

Hey Tyson. Sorry for having to bother you again.
Should the number of faces stay the same if I disable the shell?

Because, I have a scene here I'm working on where I've disabled the shell and I still do not get Velocities/motion blur.

Vray Standalone sometimes gives me an error for this mesh about non matching face count. Even though the shell is disabled.
Can you have a quick look? I've stripped the whole scene down so it's fairly small but the behavior is reproducible (max 2019.3 / Vray next).

Just open the scene, go to frame 7 for example and render.

Again, thank you for doing all that!

http://fridayvfx.com/clipupload/tyFlowdebug.7z

-Robert