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I tried it and I couldn’t get the Direct Light to affect the alpha. Not sure how it’s supposed to work in a post compositing pipeline. As you say, it works when rendering directly onto the background image, but rendering on to alpha doesn’t work.
When I do a final render with MSR for comp, I render onto plain black alpha, with a switcher in the MSR’s background for the camera mapping. But if the background is black, direct light has no effect on the background or the alpha.
I really like the MSR mat, along with the other production shaders, I’ve had great results from them, but I’ve never actually needed to use direct light on an MSR material before, so haven’t noticed this problem before, or come up with a workaround, I’m sure there will be one, there is always a way to do anything. It will probably involve rendering a few passes with different settings, and more messing around in the compositor than you bargined for.
I suppose in reality you wouldn’t really want all these things in one pass, the CGI geom, shadows, reflections and direct illumination, because you would probably want to use different apply modes for them in the compositor. Eg, for the direct light, I would probably want it as a B&W mask on it’s own, then put a colour correcter through it in the compositor, hmm, unless there is more tan one colour light.
Ideally what we want is Render Elements for MSR the material. The regular REs don’t work with it, nor do the new mr A&D REs. One for the Wishlist me thinks. Come on mental ray, let’s have some MSR Render Elements, or have the A&D REs include MSR mats, you listening Zap? :)
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