Hi Izzy!

Thank you for your kind welcome.
Nice and very useful to know the terms. I really didn't know it was called a Holdout Matte. Now I can google this up and study furthermore!

I don't know if I did quite get it, because I consider myself very noobie when dealing with compositing itself.
The way I understand, I should be creating this:
for example:
big tree pass - create another file in wich I convert grass and little tree to polys and apply Black Hole setting to them. My big tree will still remain as a PFX object, so it will preserve nice and correct shading.
Did I get it right?
Well... I think it really makes sense.

I'd better off testing it!

Thank you for your help Izzy!

Cheers

Mário