Silly particle questions

[lorduk]

first of all, is there a simple way of making particles collide?

If no, then do I have make the sop and then use the sop as collision object again?

I'm not quite sure what the drag POP does, maybe little explenation - it just slows down the particle?

I quess this is it for the first round,

coming from maya background it is quite hard to get used to the Houdini workflow

-Kaspar

[betty]

hey lorduk

not exactly sure of what you mean, but have you tried the collisionPOP? just specify the SOP you want the particles to collide against and adjust parameters.

re: dragPOP, in the parameters pane just LMB on the 'dragPOP' icon at the top left hand corner for a simple explanation.

b

[anakin78z]

maybe also check out the interact pop. It causes particles to avoid each other.

as far as drag goes, here's a more mathematical explaination (and I'm making this up, so correct me if I'm wrong):

drag modifies acceleration like this:

acceleration = (original acceleration) - velocity * drag

so, as a simple example. say you have a location pop emitting a single particle on frame one, with no initial velocity on it. Append a force pop and put -10 into the y field. Bring up the geometry spreadsheet (right click pop and select spreadsheet) and check out the velocity and acceleration attributes. If you hit play you'll notice that the acceleration will stay at a constant -10, and the velocity will continuously increase ( v = v + accel * (1/$FPS) ).

Now, append a drag sop, and bring up the spreadsheet again. You'll notice that the acelleration will decrease as time increases. At frame 2 (at 30 fps) the velocity will be .333333 (v = 0 - 10 * .033333) and acceleration will be 9.66666 ( accel = -10 + .33333 * 1). Frame 3 will have a velocity of -.655555 ( v = -.33333 -9.66666 * .0333333) , and acceleration of -9.34444( accel = -10 + 0.65555* 1), and so on and so forth.

All clear?

Zig

[Jason]

You might have some luck with the Interact POP. That POP performs particle interactions for things like collision avoidance and so on.

[lorduk]

Thanks for all the help,

I got my particle system to look ok, at this point I think it is good to explain that the animation is 50f (2sec in PAL) of one person blowing smoke on the ham that another person is holding up for that purpose (don't ask me what the director was thinking!?) anyhow now they want some fire the early age of the particle and also variation of colors dependent of age.

I used imag3D volumetric shader for the smoke - so I got a constant color of the smoke, is there any way to assign different color values per particle so it would actually render using image3D volumetric shader?

If it requires some coding to be done, then could you point to that spot in documentation.

BTW: the version of it, that was used in the AD was done in Maya but I think it is just hard enugh to try do achive the better outcome in houdini + the extra fire effect

followup:

I've got my marcher in VOPS set up (extremly nice system, especialy like the while and for VOP - theese seem to be missing in pixar slim for example, at least in RAT 4.5)

When looking through the "fog3d.vfl" there is line

ps = p0 + dir*(float(nrandom())-.5);

where ps is the current shading point and dir is the stepsize, why there is random multiplyer added - for antilaiasing or what?