i have a ball that disappears into some clouds and i want to render the ball and clouds separately for speed sake. How do i render the ball and cloud separately so that i can join them together in compositing while maintain the disappearing-into-the-clouds effect?

To have the compositing correct, you're going to have render the ball and clouds in the same shot at least once. The idea is to render a matte pass : simply put:
*) render the ball constant white, clouds black (turn off all lighting/shadowing for optimal speed)
*) now feel free to render the clouds and ball seperately - tweak your lighting, colour correct seperately etc.
*) now for the compositing. the theory is to use the matte pass as your new visibility for the ball. you basically want to multiply the fill pass of the ball by the matte pass and when you composite it over the clouds, it'll disappear nicely into there. but if you do this you'll notice that there is a black halo around the ball in the area of the antialiasing. this is because the image you rendered in “premultiplied” - it's RGB has already been scaled by it's Alpha. so in the compositing stage you're going to have to account for that by “unpremultiplying” the fill pass of the ball before you multiply it by the matte pass.
- so:
fill pass -> unpremult -> mult by matte pass -> comp over the clouds.

make sense?

i hope it helps
j

Hey all, I'm interested in getting this type of thing to work as well. I had made some clouds in a tutorial, so I threw a ball in there…..I rendered 2 passes: 1 with the ball by itself, and another, with the clouds but with a Vex matte shop on it, and alpha set to 0. Here's the render of clouds only and the Vex matte on the ball:

http://members.cox.net/content1/cloudsballmatte1.jpg [members.cox.net]

and here is the comp with the ball layer behind the cloud layer:

http://members.cox.net/content1/cloudsball1.jpg [members.cox.net]

but I still had to compensate for the premultiplying of the alpha on the edges.

Has anyone else here ever used the Vex matte sop for this type of thing, is it something that has it's limitations?

Jason, if you're out there, is it better to render a matte pass seperately to use in the comp, or to use the Vex matte, or does it matter?

Fairly new to Houdini here so if the newbie bells and sirens go off, please ignore them!!!!!

Karen