1. A method comprising: (a) specifying at least two sites in a frame, the sites being arranged in a particular spatial distribution, each site corresponding with the pixel locations of a block of pixels, each block having at least two pixels; (b) calculating a block parameter for each block of first and second frames in a sequence of frames, each block parameter being calculated from the pixels of a respective one of the blocks; (c) generating a block-pair similarity determination for each pair of spatially-corresponding pixel blocks of the first and second frames by determining whether there is a difference between the respective block parameters which is greater than a particular block-level threshold; and (d) generating a frame similarity determination by combining the block-pair similarity determinations.