A method for processing video data, comprising: receiving a stream of input video data representative of a number of successive frames generated by an imaging device's image sensor; selecting at least some of the frames; for each selected frame: determining, using an image signature algorithm, a signature for each region of the given selected frame; and, based on such signatures, classifying each region in that frame as either a changing or a static region; and generating an output video data stream that is a compressed version of the input video data, with a greater average data reduction rate for static region data than for changing region data, of the selected frames. The signature algorithm is such that a region's signature has substantially smaller size than the input data representative of that region, and such that signatures for visually similar regions are the same or similar.