At least two sites in a frame of pixels are specified. The sites are arranged in a particular spatial distribution and correspond with the pixel locations of a block of pixels. Block parameters are calculated for each pixel block of first and second frames. The block parameters may be calculated using fewer than all of the bits of each pixel. A block-pair similarity determination for each pair of spatially-corresponding pixel blocks of the first and second frames is generated by determining whether there is a difference between the respective block parameters which is greater than a particular block-level threshold. A frame similarity determination is generated by combining the block-pair similarity determinations. A user-interface indication may be provided, or a frame may be stored, as a result of the frame similarity determination.