Symbols of Death and Time
At the next iteration, the classes unmixed would be "concrete," "metal," "water," and "vegetation." Once a class fails a threshold, it is not subdivided later. If there were a third level to the "man-made" class, and "vegetation" had an unmixed fraction of at the second iteration, a third unmixing would not include vegetation sub-classes. This makes the coarser unmixings more important, and allows the algorithm to find fewer endmembers in each pixel.