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 utilising the combined directionally-accumulated difference-image pixel values in identifying said region.