Can someone please give me a technical explanation as to why the particle origin rotation changes depending on the incoming geometry having a velocity attribute and point normals ? . The only way round the issue of having incoming point normals I have found is to promote the N point attribute to a Prim attribute.
I would be interested in other approaches to control .see the attached file.


Rob

My try:

Primitives don't have N nor V by default.
So if you're emitting based on primitive centers, Houdini falls back to the point attributes.
Particles “point” in the direction of their velocity. If there is no velocity they align to the normals.

The promotion of point attributes to primitives isn't a way round the issue IMHO. Like the helps says:

If you want the particles to inherit the velocity of the source geometry they are emitting from, the geometry must have a velocity attribute (v) on the points or primitives (depending on where the particles are emitted from).

You can use a Trail surface node to add the velocity attribute to the geometry, with the Compute Velocities option. You can copy the velocity attribute to a different level of the source geometry (for example, points to primitives) with the Attrib Promote surface node.

It always depends on what you're after.
And as SOPs are rather about points it fair that you need to promote attributes to primitives. Especially as POPs are about points, too.

Rather nontechnical.