1. A method of identifying a region within a chosen image that is part of an image sequence, the method comprising: forming difference-image pixel values from the chosen image and at least one other image in the image sequence; forming first directionally-accumulated difference-image pixel values by accumulating difference-image pixel values in a first image direction; forming second directionally-accumulated difference-image pixel values by accumulating difference-image pixel values in a second image direction; combining the respective co-located first directionally-accumulated difference-image pixel values and the second directionally-accumulated difference-image pixel values to form combined directionally-accumulated difference-image pixel values; and utilizing the combined directionally-accumulated difference-image pixel values in identifying said region; further comprising: forming third directionally-accumulated difference-image pixel values by accumulating difference-image pixel values in a third image direction; and forming fourth directionally-accumulated difference-image pixel values by accumulating difference-image pixel values in a fourth image direction, wherein the third and fourth directions are opposing.