render pass and matte shadow question

[Moose]

calling all render pass gurus

i have this arch model and some card people [flat and ugly] and i would like to strip the people out into a seperate pass...speed up the main mod rendering and if the client says 'move that punter from here to there' it wont mean a rerender. you know the logic.

question is: can i get vray to cast shadows from the matte onto the visible objects but not onto itself all in one pass? [please review image example]
i can get the desired results by swapping out object casting properties but that would mean two people passes rather than one. would be sweet if vray could simply get the matte to ignore shadows from the matte and only receive them from the visible objects in the alpha?

am i missing something? still using 1.09n cos of the farm for this job...do the newer builds offer a solution?

any help appreciated

[Dynedain]

Have you tried selecting all the matte objects and setting them to not cast shadows? or exclude from shadowcasting in the light?

[RErender]

There is no way to clean it up directly, to do it right you need to render a gi shadow exclusion pass. A quick and dirty method is to render the scene with just the mattes and then use that alpha to subtract from the alpha of your "matte receiving shadows from matte" pass. In combustion a use a "compound alpha arithmetic" operator.

[Moose]

thanks guys.

Dynedain - the render on the right is what you are suggesting. It yields the clean alpha but the visble objects dont receive shadows so they look to bright when comped in.

RErender - cheers man, thanks for your help . I was hoping that i wouldnt have to render the extra pass and that i was missing a method so is this a reasonable item for the wishlist u think? is it within the realms for the matte system to ignore shadows from itself and only receive shadows from visible objects?

[RErender]

Yeah, It would make life much simpler if vray would do it :

From what I hear (over and over again :P) from a fellow artist here that used to use lightwave, lightwave can!

[Moose]

bummer!

k will add it to the wishlist

[vlado]

There is an option "No GI on other mattes" in the Object Settings dialog, which does exactly this; I'm not sure if it worked properly in the 1.09 versions though.

Best regards,
Vlado