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Particle control and render passes.


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Hey Guys, 

My deadline is coming up and I have a few small problems that I quickly need to resolve. 



I have a snow covered car.

The snow falls off the car to reveal a new body-paint color. The source is a pre-constructed, scattered and point-replicated point cloud on top of the car geometry, revealed based on input particles from falling snow. (I used attribute transfer with a solver node for that) Everything is working fine, except of course that all my particles are dropping from the car, simultaneously. My question is; how do I start them off "frozen" and have them fall based on my input? I was imagining an animated noise pattern so control the activation, of sorts. But maybe an animated metaball setup? I am just not sure what particle attribute to use. The "active" one doesnt seem to do anything.



Ideally I would like this snow to be grains, so that I can have clumping and stacking, but since I am colliding with car geometry, the grains always explode. Is there a way to maintain constraints without everything exploding? I am using a vdb collision volume sample of the car. It looks like Houdini automatically adjusts the particles so that there are no intersections with the collision VDB, but I cant seem to get away from the explosion.



How the hell do I get geometries with a shadow matte shader assigned, to not cast shadows on one another?! but only from objects that are NOT a shadow matte?

I am basically just trying to render this snow out in two passes, and one is a shadow pass. I only want shadow from the snow, affecting my ground and the car. But no shadows from the car affecting the ground & vice versa. Much like Vray's material wrapper.



I have 6 licenses but have not yet set up HQ server. Could anyone briefly explain the process of getting this setup? 


Thanks so much for any help and input!

// Mats







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that's a lot of questions... I'll take a stab at the first one.


not eloquent, but this should work.


Wyvill force from animated metaball gives color change.

Delete points from whole until they are red.

Delete points from previous frames selection

Emit all incoming points, stop at point no particles available.


Should work because it gives you each of your original particles for emission one time only.

This would have to be modified if you used a lot of metaballs and the particles become "un- red".





P.S. this seems pretty straightforward, just go step by step. http://www.sidefx.com/docs/hqueue14.0/help/installation.html

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2 is probably gonna be a combination of the mass and max speeds. compute mass in the grain source and in the solver tab of grain update play with max acceleration and max speed. That SHOULD fix it , but would need file to know for sure? If i got 3 correctly you could use the light linker tab in the dropdown change to shadows and just turn off certain objects to cast shadows from your light source


Have you seen this for hqueue? 

i have windows mac and linux and couldnt get em all working together but if you are all linux that should be good 
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I'm not experienced enough with the grain sims to give good advice, but maybe you need to up the sub-steps on the solver to avoid the exploding.


For the shadow pass, have a look at the light linker tab.  You may have to do 2 different renders for what you are describing, (one using a holdout object).  Unfortunately I don't have access to houdini at the moment, so I can't look at your file.  But try reading the help card for the light linker, I think you'll find the answer.

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