An apparatus and method are providing for generating a hash in video in which a sample series of temporal difference are sampled in an image order. A temporal averaging is performed and a rate of change is detected to identify as distinctive events regions of high rate of change. Images having a distinctive event are labelled as distinctive images. For each image, the temporal spacing in images is calculated between that image and other distinctive images to provide a set of temporal spacings for that image; and a hash is derived for that image from that set of temporal spacings.