Particle aim direction geometry nodes

Question

My goal is import crowd particle data from Maya to Blender, but I'm stuck at how to find aim direction from each particle, because all points not pointing at the velocity direction.

I try this workflow:

• export particle cache (nparticle) via ABC
• in Blender import that Alembic
• then using geometry node point instance to add some object instance

Answer 1

Currently a velocity atribute is not part of .abc file. Velocity is stored only for mesh sequence like fluid, because there is not any other way to calculate velocity due vertex amount change.
For more info read here.

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For particles exported as vertices is kept vertex ID and continuity from frame to frame. It means you can calculate velocity from previous/next vertex position.

As workaround you would have to duplicate object and under Constraint Properties > Transform Cache > set Frame Offset 1 (don't forget to make this data block unique).

Then you can use Geometry Nodes to calculate velocity from previous vertex position by Transfering Attribute from the duplicated object and Subtract vector from original object.

With particles it works in the same way ...

Answer 2

Edit: In 2020 there was an implementation of velocity for .abc file ... you could use velocity attribute directly (see answer), but not anymore for 2.93 and above ... there was a unification of handling velocity data. Now Alembic stores only velocity for objects changing point count (like fluid).

I'm thinking to delete this answer

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Velocity Attribute

• connect Vector input socket of Align Euler to Vector node directly to empty socket of Group Input, it will automatically set socket type as Vector.

• go to Modifiers and under Geometry Nodes > Input click on icon and then click into empty field and select Point>Velocity

Note: Tested one of the developer's .abc file used as proof of implementation.