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DanielWichterich
10-19-2005, 03:31 PM
Hi.
I have a general compositing question:
for several reasons it's useful to render background and characters in different passes, but I don't know how to do so excactly.

Here my example:
I have background consisting of a bright floor and a dark wall in the front (first picture)
The character is hiding behind the wall. For that reason I assigned a matte/shadow material to the wall (second picture)
When compositing those two simple passes the following problem occurs: There is a white rim on the edge between wall and character (third picture). Ok, the reason for that is clear to me - in the background render the Anti-Aliasing blurs the edge of the dark wall and the bright floor. Since the character is not as bright as the floor the undesired rim is created.

Has someone a hint for me?
Hugh
10-19-2005, 04:45 PM
For a start, render the floor, wall and character seperately.

When rendering the character, render the whole character, not just the bit visible above the wall (or make a fake wall that's shorter than the real wall and use that to block the character's render)

Render each pass against black with an alpha (sorry if this is stating the obvious) and pre-multiplied.

If you're colour-correcting any of them, unpremultiply it first, colour correct, then multiply by the alpha again.

Then just do Wall OVER Character OVER floor
JoshKirk
10-20-2005, 01:34 AM
Many 3d packages have a "render multipass" option. Look into that.
DanielWichterich
10-20-2005, 08:02 PM
Thanks Hugh. Generally that sounds good to me.

But doesn't it make the process more complex?
Imagine the character would jump over the wall.
Ok that could be fixed in the post easily.

But what about a case in which he would bend over the wall?
Parts of the body are covered by the wall while other parts cover the wall.

Apart from that: how do I unpremultiply images?
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