Let's say you've downloaded or purchased some foliage meshes, like these Xfrog freebies from the Autodesk Seek site. These meshes use cutout/opacity maps for the leaves. When you use these with 3ds Max & mental ray and calculate your final gather solution it can become quite slow as it runs into these cutout type materials.
Example (click to enlarge):
For this scene I first converted the materials from standard materials (Scanline renderer) over to A&D materials. I then made a simple ground plane below the tree & enabled FG using 2 bounces on the "medium" preset. Render time as you can see on the enlarged image was four minutes and sixteen seconds. Most of that time was spent calculating the final gather.
However, if I simply disable the opacity/cutout map on the leaf material the render time (including FG calculation) dropped to one minute, thirty eight seconds. Here's that render (click to enlarge):
You'll notice that I changed the diffuse color as well for this render..why? This particular diffuse map used black for the area where the cutout happened. So if I calculated the FG on that, I wouldn't get much bounced light in the foliage at all. So I disabled the diffuse map & instead used a solid color that was similar to the overall color of the diffuse map and saved my FG calculation using this solid color.
Click to enlarge:
Afterwards, I froze the FG calculation and re-enabled the original diffuse map & cutout map and rendered the following image (click to enlarge) in one minute, fifty two seconds.
IMO it looks better than the original image as there's more bounced light in the foliage area, and most importantly it rendered much faster than the original setup.
Some may ask, why not use the local FG controls in the objects/properties settings? Or even use a raytype switcher to do this? The answer is speed & results. If I used the "Return Black" or "Pass through" modes in the FG settings (see image below) it wouldn't have the same bounced light as using a solid color. That would make the tree much darker.
As for using a raytype switcher instead...well, if I have to modify the materials it just seems quicker to enable/disable the cutout/diffuse maps rather than having to load/enable/disable/tune a raytype switcher for this.
So give this a try and hopefully you can shave some time off your renders with this method. Who knows, maybe someone will come along and even write a script to quickly do this process. :)