How do you compose an object?

[Crayox13]

I have this exterior scene (some problems with long caustics) where I'm using vrayfur for grass. Now vrayfur is slowing my scene down.

This is really a basic question for all you who've been doing this all the time.

How do I first render a scene with hidden vrayfur generator (so I only have a flat ground), and then render only that ground object AND the vrayfur generator but with everything else hidden? Not only that, but I need to have all the occlusions correct, so I can't just hide stuff since I won't be able to compose this back in PS again.

Thanks

[thomasfenger]

Set the various objects to be matte objects in vray properties.

[Crayox13]

Not sure what you mean. Where do I set them to matte and what does it do? Please, if you don't mind.

Also, do you mean that after I render everything, I set all objects except the one I need to matte and then compose?

[Morbid Angel]

For the grass pass, select everything else in scene and in vray properties assign (matte object) with alpha contribution to -1.
For the other objects pass, select the grass and make it not visible to the camera (in properties)
In photoshop lay the grass over the other objects image and you should get what you need.

[Crayox13]

Oh I see. I'll try that now! Thank you!

[gfa2]

This does a nice job of creating a mask you can use in PS. Just select the object(s) you want a mask of and hit the button. Works with animations too.

http://www.scriptspot.com/3ds-max/rendermask

[Crayox13]

Thanks! Looks interesting, will try!

[digitarch]

very helpfull tool indeed, but how can i add the shadow of the object? Would be wonderful to have the object as isolated blackandwhite and also its shadow on other objects....