hello everyone,

I am having issues with trying to extract a shadow pass on my ground plane. The shadowmatte shop that I use in normal renders doesnt pick up any shadows because the only light that I have in my scene is a GI light. I have it set to no shadows, but I wouldn't think this would make a difference because it is illuminated by the an ambient occlusion map anyway.

Shadows generated from other objects are created if I render the ground with its texture on it, but that doesn't help very much because I want the shadow as a separate render pass.

Is this even possible using GI or ambient occlusion?

Any one have any ideas?

waibeans

waibeans
hello everyone,

I am having issues with trying to extract a shadow pass on my ground plane. The shadowmatte shop that I use in normal renders doesnt pick up any shadows because the only light that I have in my scene is a GI light. I have it set to no shadows, but I wouldn't think this would make a difference because it is illuminated by the an ambient occlusion map anyway.

Shadows generated from other objects are created if I render the ground with its texture on it, but that doesn't help very much because I want the shadow as a separate render pass.

Is this even possible using GI or ambient occlusion?

Any one have any ideas?

waibeans

The way the shadowmatte shader works is that it first computes the illumination from the light source. It then runs the shadow shader on that illumination and by comparing the un-shadowed and shadowed illumination is able to determine how much shadow there is on the surface.

With global-illlumination, the shadowing is baked into the light shader and there is no pre/post shadowed illumination possible.

If you only have a constant color in the background (i.e. for ambient occlusion), you could probably fake this by assuming that the difference between the background color (or some fraction of that) and the color returned by the gi light was the amount of shadow.

Hello again,

If anyone is wondering the workaround that I found is to turn the shadow into a luminance pass that I can then use as an alpha channel for the shadow itself.

I removed all of my shaders from the ground objects and replaced them with a lambert white. The variation in the lumination is enough to bring it into shake and create the matte for the shadow.

Thanks for the help in figuring out the shadowmatte.

Waibeans