I have an animated (deforming) alembic file, and I want to snap a Xform to a vertex on the surface and have it track with with the deforming geo. I haven't gotten into this side of Houdini/Solaris yet so I was curious how this is done. (In Katana I would have created a facese, which defines a child prim of the mesh, given it the index of the face/vert I wanted to stick to, and then constrained my item to that faceset prim).

It's not clear to me how to achieve a similar constraint in Solaris. Any pointers?

The Surface Constraint LOP should work for that; in Point Group mode you can give it a point index, and it'll use that.

Thanks, I was trying that but didn't fully understand the point group mode. How can I find/see/identify vert indices? Is there a way I can select them directly in the viewport? Haven't quite gotten the hang of this yet.

There's actually a Point Constraint LOP, with a tool that puts the target prim into a sop state, so you can select points (see the gif). The Surface Constraint can do a similar thing, to let you select Prim+UV or UV positions, but Point Constraint is simple enough too.

Oh that's perfect! Thanks for explaining this!