Hi,

While continuing to explore Houdini particles, I've run into yet another annoying problem. All I want to do is have some particles run zig zaggedly down an object, but I can't get it to work! In Maya, all you do is set the object you want the particles to stick to as the goal, set a few attributes, and you're all set. How would I do this with POPs? Here's where I am now…I'm emitting particles from the surface of the object I want to stick to, so all the particles that are created are on the object. Now how do I make the particles slide along the object? I've tried creating a Collision POP, but don't you need the particles to actually run into the object? I've tried changing the state…everything I can think of, but I still can't get it to work. Somebody please help me! And thank you all very much for all the help you've given so far…much appreciated.

Later,

fixxorion

ah…what you want is the new Particle Sliding (TM)
check out this video > top of this page [vislab.usyd.edu.au]

Thank you for your efforts, but…

I've watched those videos, but they are just throwing particles onto an object and having them slide off. How are you supposed to have them stick to an object if they're already on the object and have no velocity?

I want to generate a particle from the surface having it stick to it and then control its movement along the surface. This is VERY easy to do in Maya: simply generate the particle and have the surface as a goal. Then you can manipulate the particle's goalU and goalV properties to control the particles flow over the surface.

Thanks,

fixxorion

try the following:
source : your emitter object,
set impulse activation to 1,
impulse birth rate to 100 (or whatever)
at frame 1 then key to 0 at frame 2,
set const. activation and rate to 0.

wind: negative point something on Y,
and some noise with a high frequency,
experiment with other settings.

collision: put an xform on your emitter
node and offset it by a fraction in -Y
and use this for the collision node.
set behavior to bounce,
set gain tangent and normal to something
like 0.5 (more experimentation for these).

you should see some particles stuck to your
emitter at first and then sliding off the
emitter as time progresses.

for zig-zagging you can experiment with
various pop nodes and also animating parameters.

-cpb

If you want the particles only to move along the surface, and not inside or outside, why not trying to use a Ray OP to project your particles onto your surface?
RayOP is a surface OP so you have to apply it to a PopMerge/PopNetwork SOP that you link to your POP.

In rayOP, use “minimum distance” instead of “projecing rays”.

It works quite well in many cases.

Hope it helps, or at lesat, gives ideas!