1. A system for tracking changes in an area of real space, comprising: a plurality of cameras, cameras in the plurality of cameras producing respective sequences of images of corresponding fields of view in the real space, the field of view of each camera overlapping with the field of view of at least one other camera in the plurality of cameras; a processing system coupled to the plurality of cameras, the processing system including: first image processors, including subject image recognition engines, receiving corresponding sequences of images from the plurality of cameras, which process images to identify subjects represented in the images in the corresponding sequences of images; second image processors, including background image recognition engines, receiving corresponding sequences of images from the plurality of cameras, which mask the identified subjects to generate masked images, process the masked images to identify and classify background changes represented in the images in the corresponding sequences of images, wherein the second image processors include a background image store to store background images for corresponding sequences of images; and mask logic to process images in the sequences of images to replace foreground image data representing the identified subjects with background image data from the background images for the corresponding sequences of images to provide the masked images, wherein the mask logic combines sets of N masked images in the sequences of images to generate sequences of factored images for each camera, and the second image processors identify and classify background changes by processing the sequence of factored images.