Need to render beauty pass of tress and their shadow on grass as separate layer

[RobH2]

I'm transiting from VRay to Corona and am finding it difficult to do the following simple scenario.

I have a building with Forest Pack grass in front. There are trees on the grass. I need to render the trees along with their shadow as a separate beauty pass with alpha so the trees can be turned off or on.

I can't use shadow catcher in a 'Blend' as Corona does not support multi/sub-object materials (the grass) in a blend or composite material.

When a render a 'CMasking_mask' I get an imperfect mask, especially against the black sky area where I eventually drop a sky in behind. The tips of the leaves don't antialias properly. Plus, that does not allow me to turn "off" the tress, just allows me to manipulate them.

Is there an accepted method to use so that I can achieve this need for layering elements? I eventually need 3 layers. 1: background with building, grass and asphalt. 2: Trees with their organic shadows on the grass and asphalt and 3: people and their shadows doing the same thing.

I need to be able to turn the two top layers on and off.

[maru]

I tried setting up a simple scene to reproduce this, but I am afraid for now the only solution would be re-rendering the image 3 times with no trees and no people, with trees, and with people, and then cutting out each of the "layers" separately.
"Render selected" option may be useful for this under the scene tab.

[RobH2]

Thanks maru. I was afraid of that. I couldn't figure out a way to do it other than what you suggested. Drat....