don’t think he used the mix8layer on the plants… if he doesn’t modeled 'em… I mean get out from the paintFX the leaves are generally grids matted with transparecy… well… under maya… if any of the last shaders are not a build in material shaders… the transparency is not preserved… I mean you have to enable raytrace transparency… under the material node… you do have to check ‘refraction’… this is not a mix8layer ‘feature’… under xsi it works without a glich also with scanline-based transparencies… under maya none of the custom shaders work and also none of the build.in color correction nodes.
when you do not need silhuoette ghosting… I mean simple layering like when you add speculars on the top of a diffuse shader (or like here probably for the wall)… matte (turn black) the material and then add or ‘screen’ that layer (alpha transparency)… instead to plug the alpha into the material transparency and then add only the visible part… its faster and actually safer… but take care that in this case (when are the mats to be transp and the obj stays fully opaque…) works also the 2nd way.
another pretty thing is… that when you plug any shader into the mentalray material node in the SG… well all the filtering settings are not preserved (those that come from the ‘file’ node and also those from the bump one)… if you never touched them… well they’re by default at 1… if you plug any shader into the mr mat node… it is like if they’re set to 0… (and do not try to plug a build.in material node into that custom mat slot… coz you’ll be trashed to the desktop !!)… so in the case of mix8layer plug always into the surfaceshader slot… by default the bitch try to plug it into the mr material slot via the ‘message’ attribute.