Colored Shadow Pass

[LFX]

Hi, I'm using PBR and in the extra image plane for direct/indirect shadow are different RGB values. What's the deal with that? Is there sth. to consider in compositing? because I don't get the right result, if I use it as a mask to shadow the diffuse and reflection pass.

Edited February 7, 2011 by LFX

[TiMc]

I'm not 100% sure if this is correct so someone may want to double check me on this but I've been doing some work in mental ray for maya and have also been getting a coloured shadow pass.

The reason for this is to accomodate for the colour of the lights. I've posted a few images to help explain what I mean.

There's a white sphere on a white ground plane with a red light on to the left and a green light to the right both of which are casting shadows. If we were to multiple a direct diffuse

pass with a simple matte shadow we would just be darkening down the areas in which the shadows occur. However, the shadowed area created by the green light is still being illuminated by

the red light and vice versa. We therefore only want to remove the green light's contribution from the shadow it casts and the red light's contribution from its shadow. The coloured shadow

image plane allows for correct compositing of shadows. Rather than multiplying the shadows they are in fact subtracted from the direct diffuse pass. You can see from the shadowColour that

we would be subtracting the areas where there is no green and red illumination.

I hope this makes sense. If I'm wrong then please someone let me know but this is what I've found in mental ray and I'm assuming that something similar is going on with mantra.

Tim