trace -> extrude -> emit smoke -> ...question?

[johnLIC]

I'm trying to emit smoke off some hair that's whipping around in slow motion. I brought in the live action plate and ran a trace node, and then an extrude node. Now I'd like to isolate a couple of small sections to emit smoke from. Will that work? How can I do it? Is there a better way? It seems like a nice approach because the extruded geo immediately has the motion I need. How can I mask out emission from most of the volume? I saw someone us a paint node with some vopsops. That looked like a reasonable way to go. Any other ideas?

thanks

[WesleyE]

If I remember correctly, the trace sop does not create velocity or update its geometry. It creates new geometry on every frame, so you won't be able to get the velocity of the hair that you want.

You could animate some strands to the general motion of the hair and emit smoke from that.

[anim]

I imagine you are having the hair movement in 2D plane if you are going for trace approach

in that case you can deform a simple grid with bones and do a rotomation of the hair so that the grid deforms roughly the same but it always cover all the hair you need (it can be bigger since you will mask it afterwards)

then you can generate velocity attribute and paint the areas you want to emit from ecause the grid will have constant topology

afterwards you can mask it by your traced outline to get final emitter

[johnLIC]

I just did the trace and extrude and it worked very well! I just masked the hair in Nuke and ran out those frames to plug into the trace in Houdini. Thanks all!

Yes, the volume was essentially emitted in "2D", but the camera is locked off and the shot is not in stereo, so that's fine. I think it made it easier to read the shapes of the fluid.