Way to have shadows affect alpha but without Matte Object?

[dlparisi]

I'm trying to have an object (a road in this case) have the shadows from other objects affect the alpha channel but also have the texture of the road show up in the RGB channel. I can get the shadows to affect the alpha via the properties Matte Properties section (Matte checked, alpha = -1, Shadows and Affect Alpha checked) but this only works if Matte Object is checked. In this setup the material for the object does not show in the rgb channel (as expected). I'd like to set it up so the road material shows up in the RGB but the alpha is only opaque where shadows fall on it. Is this possible?

[hermit.crab]

It is, with a second render. Just render a separate alpha channel with the road as a matte.

[dlparisi]

Thanks. That's what I've been doing but I was trying to get it all done with one render.