Apparatus and method for processing a sequence of images of a scene, the method including: tracking a region of interest in the sequence of images (e.g. using a Self Adaptive Discriminant filter); selecting a particular image in the sequence; selecting a set of images from the sequence, the set having one or more images that precede the particular image in the sequence of images; for each pixel in the region of interest in the particular image, determining a value for a parameter; for each pixel in the region of interest of each image in the set of images, determining a value for the parameters; and comparing a function of the determined values for the region of interest in the particular image to a further function of the determined values for the regions of interest in the images in the set of images.