Over the past few months I've run into a number of bugs with the Pixar-supplied mtor/RAT occlusion shader "ratOcclusion". First of all, it has these crazy artifacts at small scales, and second, it gives me yet-to-be-understood white grid patches when the shading rate is too low, or the image is too big, or... well, that's the problem. I have no idea what these artifacts come from, I just know I hate them, and that I spent hours re-creasing the mtor subdivs in the film (yes, this is still about The Incident at Tower 37 project) when, in fact, the creases weren't the problem at all.
Anyway, I just put together my own occlusion shader that seems to fix these bugs. The hope is that it can be the show-wide occlusion shader, so I need to make sure it is fast. How my straightforward shader is different from what ratOcclusion is (under the hood) is a mystery.