1. An apparatus comprising: a plurality of interconnected processors operative to carry out coding functions for coding respective parts of a picture of a video stream in parallel, each part including blocks of the picture, the processors being further operative to determine in parallel, for at least some of the blocks in each processor's respective part, block-level temporal difference features, wherein each part includes a plurality of blocks of the picture; and block-level processing logic operative to receive the block-level temporal difference features and to carry out processing to determine which blocks in the picture are likely to be that of a face, wherein the block-level temporal difference features provide a measure such that for the picture, blocks that are motionless compared to blocks are in the same position in a selected previous picture in the video stream have no or negligible temporal difference features.