A method of forming a time-varying signal representative of at least variations in a value based on pixel values from a sequence of images, the signal corresponding in length to the sequence of images, the method includes acts of obtaining the sequence of images. A plurality of groups of sub-sets of pixel values are formed by selecting a sub-set of at least one pixel value from each of at least two images defining an interval to form a group of associated sub-sets. For any pair of groups with only sub-sets from the same images, the sub-sets from more than one image are different. Groups of sub-sets are selected to form the signal in accordance with at least one selection criterion. The selected groups of sub-sets cover different intervals, shorter than a length of the sequence.