Compositing - shadow receivers

[Eternius]

In a number of situations involving compositing or pre-rendering for games, I need to render the shadows cast by an object on an imaginary 'ground' without actually rendering the plane itself (or somehow making it pure white outside the shadow regions without trying to hack it with light settings). Is there some sort of typical approach to this? I also use FPrime so multiple pass rendering is pretty much ruled out.

[Sekhar]

Lots of ways. You could set your bg image in LW and use front projection for the plane using that bg image - the plane will show the bg image (blending seamlessly with the bg), but with the shadow.

If you don't like using LW for compositing (e.g., want to use AE), you could capture the shadow on the plane as an alpha by using shadow density for alpha under advanced in surface editor. You then render out for the alpha and use it with a solid shadow color (say black) to generate the shadow for compositing.

You could also render out the shadow as a white image with everything else black (make light color black and shadow color white) and use the rendered image as the alpha.