The flatten background filter analyzes all the images in a tile set searching for the brightest pixel in each position. It uses this information to build an adjustment image that is applied to each image in an effort to flatten the background portion of each image.
This filter is especially useful for removing small illumination gradient effects that, while not apparent on any individual image, are visible when viewing a complete tile set or deep zoom image.
This filter works better for large image sets (more than 100 images). Smaller image sets may show signs of strange artifacts.