That's what I first thought about doing.
But since I'm using Paint Effects Objects (they're are not converted to polygons), as far as I know, I don't have that option.
That's what I was wondering: if there's no Matte Opacity control in Attribute Editor for Paint Effects nor a way to assing a normal Maya shader to a PFX object, how can I create a MATTE for it?
Of course I can convert the Paint Effects Object to Polygons and easily assign a surface shader (or any other Maya shader) so I can set MATTE opacity the way I want. The fact is that I don't want to convert them to polys.
I know that's a whole new issue: "to convert or not to convert" PFX to Polys, but the main reason I chose not to convert them to polys is that the conversion seems to completely change the way the objects are shaded. You can see what I mean with the picture attached below.
I just used a grass preset out of the box from the visor. As a PFX it renders quite nice. As soon as I convert it to polys, the shading turns out to be a lot less realistic and very flattened and washed out. I tried to tweak those polys shading a lot, but I really can't get even close to the result of the PFX object. I also tried tweaking lighting and shadows to no avail.
Thanks again for help! Hope we can figure out some solution for this.