I've got a particle sim I'm meshing with openVDB,

I would like to export this with motion blur to an alembic cache to render in maya/arnold.

simple example, not using particles, but yeilding the same results..

make a torus, and add rotation animation xform node.
convert to VDB using vdbFromPolygon
convert back to polygons with convertVDB node.
add a trail sop to compute velocity.
export using alembic rop.

If I skip the convert to vdb and back and just export the rotating torus, I get subframe motion based on vertex velocity in the alembic file without exporting extra motion blur steps in the alembic file… however with the openVDB stuff I end up with static geo inbetween frames. even though it seems to be creating proper vertex velocity, shown in the spreadsheet after the vdb to poly conversion and trail sop.

I can export this as a realflow mesh and it will pass the velocitiy correctly so is this a problem with the houdini alembic exporter? or user error on my part?

I'm writing out 20+ mil poly meshes so I'd rather not have to subsample them and write out the data 2-3x just to get motion blur.. at that point I might as well go back to realflow meshes instead.

any help would be appreciated.

thanks in advance

-johnc

redpawfx

hi redpaw,


Not sure realy what could be wrong without the file, but from reading through your post i came across 2 things

–> you are lucky that the trail-sop is working on the torus, i guess because the point count is not changing, so trail finds corresponding points on the previous & current frame, otherwise the velocities could be all over the place

–> i'm not sure if the velocity on your vdb-volume via-trail works, if the point count/topology is changing, … do a test and display the v-vectors just to check if they make sense,

if you load the alembic file back in do you get the same velocity vectors back in ?

Seb

hi redpaw,

I just think your workflow is problem, like Infernalspawn sad.
You can chk this scene, your workflow and working one. Just display v attribut.

I hope this help

Igor

Thank you Igor, for the response, and the example file.

I just gave it a shot and while it seems to work and your solution makes a ton of sense, I figured it might have to do some attr transfer magic somehow.

However, once baking out that alembic file from your example,
bringing it into maya still seems to result in the same thing as I was getting before, namely only getting motion in the mesh on integer frames.

I'm testing this by turning off timeline “snapping” and looking to see if the mesh moves on non integer frames or not. Currently your solution does not move on subframes.

hmm I wonder if this is something still related to alembic somehow.

-johnc

A fantastic file ……..

Rob

hi redpaw,

Before export to alembic, if you display WORKING null node, you have subframes motion, only after export to alembic look like you lose subframes and only have integer frames. So maybe something with alembic export.

Igor

Thanks for all the help on this,

Yah I have a feeling this is something I should try to report as a bug, or with siggraph coming up I should try to talk to somone from sidefx about it.


thanks again.

-johnc

I did not have a chance to ask about this at siggraph, so I'm back here, and I'll be also trying to find out who our contact with support is as well,

but if anyone else has any suggestions for getting alembic to export motion that will work as subframe changes in the Alembic file I'd welcome the ideas.

thanks

-john