It may deceptively seem to work the same way as before, but it doesn't. One really important point is that each FG point stored isn't just a color, it's a whole set of colors - with directional information (for the tech heads: think spherical harmonics).
What does this mean? It means a lot for bump-mapping!!
In earlier version of mental ray, bumps basically had to be resolved by the adaptivity of the final gathering, causing an extreme density of final gather points due to the variation in the bumps.
But in mr 3.5 it works completely differently; the final gathering is actually calculated on the un-bumped normal vector, and the result is stored including directional data... and then at render time, data is looked up into this directional data.
The result is:
- Much lower density of final gather points needed, the existance of bump maps does not increase the density!
- Very high quality of directional effects of indirect light on the bump maps, at levels previously impossible.
This makes bumpy surfaces render faster, and resolve in much greater detail, causing details in the bump maps in "dark" areas, i.e. areas only lit by bounce lights or other forms of indirect light such as photons.
Some earlier shaders out in the wild intentionally tried to work around this problem. These workaround can potentially interfere with the "correctness" of the new method.
The good stuff:
The cool stuff is that due to this new FG storage format, it is possible to do directional lookups into the FG map. This is how the "Highlights Only" mode in the Arch&Design material works, which can give a very good "visual simulation" of extremely glossy reflections, without actually shooting a ton of rays and taking almost no extra render time!