Matte/Shadow?

[devin]

I'm just getting into camera motion tracking and I'm having trouble exporting a separate shadow channel for compositing. I've tired using the Max Matte/Shadow material as well as saving out different render elements (VrayAlpha, VrayMatteShadow, and VrayRawShadow) but what I get in an alpha channel is the entire plane that the shadows are being projected on instead of the shadows by them selves. I'm absolutely aware that I don't know what I'm doing so please show me where I'm screwing up. I've tried to search the subject but there seems to be some debate on the best way to do this, can someone give me some direction?

Here

[jacksc02]

if you right click on the object you want to matte out and go to vray properties there are options there for controlling mattes

[devin]

So after adjusting the settings under the properties panel which channels should I be exporting?

[cheerioboy]

Check the alpha of those two shadow elements

I usually end up doing two renders, one for the object and another for the shadow with the object not visible to the camera. If you're going to do them in a single pass then you wont need to export any channels (except maybe some sort of object ID to separate the car from shadow), you're just going to have the shadow in the alpha.

Right click on the plane, V-Ray properties, check matte objects, Alpha contribution -1.0, and check shadows and affect alpha for the direct light. That will give you a nice alpha for the shadow.

[devin]

Thanks cheerioboy that did the trick, one other question I have is about glass, do you use a similar procedure to get an alpha for transparency?