20160127 - Temporal AA Neighborhood Clamp


Thoughts too long for the twitter thread on this, back from my memory of working on this problem...

In TAA the purpose of the neighborhood clamp is to remove ghosting. Using the surrounding local neighborhood of the current frame to bound the color of the reprojection of the prior frame. Many techniques have been tried here, including using the min and max of the neighborhood as an AABB to clip to. Clipping to two AABBs one in RGB and the other in YCoCg, for a tigher bound. Etc. It is possible to continue down this path adding more complex and tigher bounds shapes, and doing better clipping for when colors are out of bounds. But ultimately it seemed like this exploration had passed the optimum !/$ point, and settled on the following low-tech solution for the fast path,

Why?
Lets reduce the problem to strictly monochrome. The complex 3D bound shape is now reduced to a 1D segment defined by the min and max of the neighborhood. When the neighborhood is high frequency and high contrast, which is common say with a field of grass, the min can be a shadow tone, and the max can be a bright highlight. Then just about any reprojection passes the neighborhood clamp without adjustment, and ghosting results. Or even if the reprojection gets clampped, it will be at the extreme ends of the large segment, resulting in distortion to the image.

Ultimately this kind of approach cannot solve the problem at hand, so I stopped searching in this specific solution space...