The render cache uses a fixed size cache of points and in the original version, a point would remain in the cache until it was overwritten by new sample point. Effects such as nondiffuse shading or scene editing can cause a point’s color to become incorrect, or stale. If the rate of new samples being computed per frame is very low, this stale data may remain in the point cache for a long time. We have added a new mechanism to allow points to be evicted from the cache even if there is no point available to overwrite it. Evicting points can actually speed up the image convergence by clearing out stale data more quickly.
Each point has an associated age which is stored in a byte (0-255). At the beginning of each frame, all the existing points are aged by some increment. This increment is chosen based on the number of new points added to the cache such that on average a point should reach the age of 128 before being overwritten. But several conditions can cause points to age at a faster rate such as if the point is not visible in the current frame or if color changes are detected in nearby points in the image plane. These can cause a point to reach the maximum age of 255 at which point it is automatically evicted from the cache. In the future, additional aging penalties may further improve stale data eviction.