The first step in detecting splits and merges is to eliminate images in which no split or merge occurred. We then apply the same procedure to find a mapping between cells in the successor image with those in the predecessor image. The mapping step may fail because cells move large distances, reposition in unexpected ways, split or merge. The approach presented in this paper assumes continuity of motion, and we do not consider unexpected repositioning. Several factors must be considered in determining the correspondence of cells in the current image to their correct ancestors in the previous image during a split or a merge.
Distance is still a primary factor in completing the mapping process.
Another important factor is the amount of pixel overlap present between the cells in subsequent images. Pixel overlap is roughly the number of pixels a cell in one image has in common with a cell in another image if the two images are overlaid. An efficient method for approximating this overlap is to compare bounding boxes of each of the cells in the two images as shown in Figure 2 . The overlap of the bounding boxes can be expressed directly in terms of the coordinates of the rectangle corners. In the case where a is zero, it is adjusted to be a small positive value so that is defined.