Sample Lookup?

What is the difference between “Nearest quad-balanced” and “Precalc’d overlapping” in the sample lookup? Does one have a better quality over the other? Precalc’d seems to be faster.
Also, What does “Check sample visibility” do?

I cant find any of this in the documentation.
-k

2 - 3 posts underneath this completely covers this, but hey, heres the reference again.

http://www.vrayrender.com/stuff/ManualPreview/render\_params\_advancedimap.htm

Thanks