Unset particle goalU/goalV after creation?

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  07 July 2013
Unset particle goalU/goalV after creation?

So, the Maya newbie might as well start with a bang, and ask for help on multiple fronts at once. My posts will probably appear in Jeopardy order, so I guess I ought to repeat my introduction here:

Standard fare - newbie to the forum, comes begging with a question...

I'll preface this by saying that I'm relatively new to Maya, but have been programming renderers under the hood for a couple decades. My SIGGRAPH geek card goes back to 1992, so by most of your standards, I'm some kind of fossil...

If I could just write what I need in C and be done with it, I'd have been done with this project weeks ago. However, today's project says it must be done in Maya, so here I am trying to figure out this pointy-clicky thing. You can talk science and math to me and assume I'll understand, but you won't insult me by assuming I'm clueless about Maya. Geezer that I am, I need help...

Issue #2 du-jour, is control of particles.

I need particles moving on a surface. Ok, that's not so hard, surface flow - only I need them moving on a convoluted polymesh surface, so surface flow seems to be out. Still not so bad, emit from surface, need ParentUVs, set goalU and goalV on creation. I can do that, attach it to a ramp in V with an appropriate UV map on the mesh, and I get particles emitted where I want them.

Then the fun starts. I need the particles constrained to the polymesh (set the polymesh as a goal, no problem), and affected by a handful of fields. Easy enough to set up, but the particles won't obey -- or rather, since the particles already have a goalU/goalV set, they attempt to obey both the goalU/goalV, the surface goal, and the fields, all at once. I need goalU/goalV obeyed for the initial position, but then I need them gone, so that the other goals and fields can take over.

Maya gives us a way to shut off the influence of the polymesh goal, either by creating the goal weight # PP parameter from the goal weights and objects pop-open, or, by shutting off the goal-active attribute for the goal.

However, it doesn't appear that there's any way to shut off the goalU/goalV influence, once these have values set. There's the goalPP parameter, but this appears to modify the influence of _all_ the goals that exist for a particle, not just the goalU/goalV parameters.

So, am I stuck? Is there an "undefine" expression method that will unset goalU/goalV? (setting them to zero, of course, doesn't help). Do I need to avoid using goalU/goalV for initial positioning? Scrap the goal mechanism altogether and just run the particles around with pure expression math, constraining them to the surface with nearestPointOnMesh, and setting the position manually?

Again, thanks in advance for any insight.
  07 July 2013
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