[Sidefx-houdini-list] alpha for particles in mantra?

Michael O'Brien mobrien at pixar.com
Sun Jul 20 18:26:38 EDT 2008


Following the directions on the sidefx website, I was able to get a disk
sprite displayed in the OpenGL viewport that correctly responds to color and
alpha in GL. I created a VEX decal and a Mantra Sprite Procedural that are
read by the sprite pop.

However, when I go to render them, alpha isn't used. I'm new to the VEX
language, so I might be just missing something obvious, but it looks like
alpha isn't accounted for in the decal code. Is that right? Sometimes I look
at VEX and try to read it as SL, which might be getting in the way.

The particles have an alpha property. I setup pscale and all the mantra
knobs I can find to accommodate rendering transparent disks. The color works
fine, but the alpha doesn't. Any help would be appreciated.

One thing I find interesting is that in the FX Tools both wispy smoke and
heavy smoke have rough geometric cages with shading applied to make it look
like smoke. At Pixar, we would render heavy smoke using a volumetric
approach and wispy smoke as using a ton of nearly transparent particles as
disks. Do people render large numbers of transparent disks in Mantra? In
other words, is it that my approach is wrong?


Michael K O'Brien                               Production Engineering
Pixar                                                mobrien at pixar.com

REMY: What is that?
EMILE: I don't really know.
REMY: You don't know... and you're eating it.

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