Motion Blur on changing topology

[eloi_andaluz]

Is Velocity channel under vray properties broken? Or I am missunderstanding something?

I am trying to get motionblur working on changing topology object.
First I am storing the velocity information on a map channel using tyvertexvelocity.
Then I am trying to use velocity channel to use this stored velocity, after the topology changes. But somehow this looks broken.
Could be an icompatibility in how tyflow stores this information? But it looks good in other scenarios, so I am not sure if I am missunderstanding something.
Check the video to understand what I am trying to do: https://youtu.be/OborMu0_y-s
And the scene file if you want it is attached, you need tyflow free.

[hermit.crab]

Is Velocity channel under vray properties broken? Or I am missunderstanding something? I am trying to get motionblur working on changing topology object.
First I am storing the velocity information on a map channel using tyvertexvelocity.
Then I am trying to use velocity channel to use this stored velocity, after the topology changes. But somehow this looks broken.

There's an error in the log saying: "Warning: Can't use map channel 3 for velocity: map channel has different number of vertices (1544) than geometry (200)", although the Velocity source is set to "Velocity from faces", which does not seem to change because of the Slice modifier (check the Channel info utility in Utilities > More > Channel Info). Hence, I don't think the channel is used at all.

What seems to work, but rather oddly is to set a very low Motion Blur duration in the Settings (i.e. 0.01 f) and increase the tyVertexVelocity modifier's Velocity multiplier. I presume this has something to do with the limitation stated here (blue square).

Could be an icompatibility in how tyflow stores this information?

Maybe. We need to ask Tyson to clarify things.

EDIT: I wrote to Tyson about the matter. Will write back when he answers.

[hermit.crab]

Tyson replied and gave a nice solution. Since V-Ray seems to handle map-based velocity info based on vertex count only (will investigate this further), Tyson suggested using a second tyVertexVelocity modifier, which takes the exported velocity data from the first (prior to the Slice modifier, which introduces mesh deformation). This seems to work flawlessly (up to the mentioned 0,98 f limitation mentioned above).

Step by step:
1. Untick the Velocity channel checkbox in the object -- so as not to get warnings
2. Tick the "Assign to map channel" in the tyVertexVelocity and choose a channel -- you already did this
3. Add a new tyVertexVelocity modifier on top of the Slice modifier
4. Set it to "Velocity from UVWs" and choose Channel 3
5. Render
(6.) Don't forget the motion blur duration limit is 0,98 frames; to get anything above, you should use the velocity multiplier of the tyVertexVelocity