Help! Quickly please. Render only objects alpha channel

[nicknitro]

Max 2012 with Mental Ray.

 

All I want to do is render an objects alpha not the object itself because I need to comp in the objects behind it that are being passed over by the other objects. So I want the stuff in front to not render in the Diff chanel but only render out their alphas.

 

I tried the Matte/Shadow/Reflective material for MR but I cant seem to get it working. Any help would be awesome!

[Francisco Penaloza]

can you post an image to explain much better?

mate shadow material should work fine, sometimes I just paint everything back and the object of interest in white and render with no lights, but all depend what you are looking for.

[nicknitro]

I cant post an image because of work. Its sensative material, sorry.

 

I have an object in the background of an animation that I need to re render so instead of rendering everything again Ill just re render that one object then in post comp it into the original animation.

 

Because I need it to pass behind all objects that are infront of it I need to render out an Alpha pass for those objects. Is that worded a little better? I cant see to find a way to just render an objects alpha and have it invisible to the Diffuse part of the render.

[Francisco Penaloza]

Matte/shadows should work, it will render black all the areas where the other objects are in front of your background, if still does not work, just put black color to all the foreground object again you'll get your background render fine with black areas where the foreground cover it then in post you can select that black areas and use as mask.

 

What is the problem that you are getting?? Matte/shadows is not rendering?