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

Christopher Chapman chap at rhythm.com
Mon Jul 21 16:42:57 EDT 2008

Hi Michael,

I'm working with sprites currently to create a heat distortion fx and am 
using a material that combines vex_layered surface and the sprite mantra 
procedural and get my color and alpha no prob. Not sure which tutorial 
yer following from the sidefx site, but double chk that you have the 
sprite POP pointing to your material. I also always write the particles 
to disk and then read them back in to render them with a file SOP. The 
OBJ where I read them in I assign the material and it's all good to go.
FWIW, I've always used a raymarching volume render for any kinda smoke 
fx both here at R+H and at DD. Using sprites are great for the thin 
non-volumetric stuff like heat distortion, but I prefer the control I 
get with a volumetric approach and the flexibility to go from dense 
self-shadowed smoke to thin and wispy, not to mention lighting/shadowing 
and holding out other non-smoke objects. Feel free to shoot me a sample 
hip file if yer still stuck.



Michael O'Brien wrote:
> Hola~
> 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?
> MO
> --
> 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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