View Full Version : Rendering shadows

07 July 2007, 11:08 AM
I'm trying to render an object WITH shadow, but without the object that shadow is cast upon.
Let's say there's a Cube on a Grid. I want to render Cube and its shadow, but don't want the Grid.
I tried using Passes, and FX Tree, but this doesn't work if I apply Amient Occlusion to the Grid object.
Is there another way of doing this?

07 July 2007, 01:31 AM
I think the problem is that ambient occlusion is a render time effect which occurs as geometry is being rendered out. Thus if you do a pass which doesn't have the grid in it, then the ambient occlusion rays won't strike the grid and give you the effect.

HOwever, can't you do one pass for your shadow, another pass for just the cube animation and then an ambient occlusion pass separately? Then use FX to bring them all back together.

07 July 2007, 03:36 AM
i think if you go to the grids shader in the render tree and take the conections out of the shaders 'surface' it should just render the shadow.

07 July 2007, 02:22 PM
Just turn off Primary Rays in the Visibility PPG. Turn it off on the object that you don't want to be visible but still cast shadows.

07 July 2007, 02:34 PM
I think I misunderstood the question.

Try plugging a shadow node into the surface port. Turn on RGB in the PPG. Then in a comp, invert that and comp the shadow in.

08 August 2007, 05:22 PM
If I'm getting this right, it seems to be the same functionality Maya has with the useBackground shader.

Essentially, you create a surface (poly, Nurb, whatever) and use the useBackground shader on it. You then place your shadow casting object on it, and turn off the primary visibility.

When you render it out, the RGB channels will show nothing, but the alpha channel will have the shadow pass.

Of course, there's a few tweaks more that you can do on it.

Question is, does XSI have something similar to that? if not, is it possible to script/create?

(p.s. prefer using XSI for my personal work, but unfortunately, stuck with Maya for job)

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08 August 2007, 05:22 PM
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