I’d like to render a matte pass for the hair in my scene (using Shave & a Haircut). Normally I would assign an object ID, and add that to a multimatte. It appears that you cannot assign object IDs to Shave hair. Is there another approach to creating a matte pass for Shave hair?
I’d love to know the answer to his question as well.
My solution has been to render everything with a VrayMaterialWrapper and setting it to a mattesurface with an alpha contribution of -1 (except for the hair) and doing a separate render for just the hair. The obvious downside is having 2 sets of frames for the render.
I don’t see a way right now other than to modify the V-Ray shader for Shave&Haircut. It would be best to let Joe Alter know about this so it can be implemented.
Best regards,
Vlado
you can make matte to use shave&haircut. now see, shave global open. and Hair render Mode change “Geometry”.
render now. u can see matte and velocity. 
but, it’s converting geometry. so very slow and. too many use memory.
maybe u see some different output image. 