A method comprising: parsing a stream of video compressed under a codec; obtaining respective macroblock size information for each one of a respective plurality of macroblocks in two successive i-frames and all p-frames present between the two successive i-frames under the codec from the stream of video as parsed, the respective macroblock size information documenting a respective number of bits for each one of the plurality of macroblocks in the two successive i-frames and all of the p-frames present between the two successive i-frames; storing the respective macroblock size information for each one of the respective plurality of macroblocks in the two successive i-frames; characterised by computing a first respective ratio between the respective macroblock size information for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames and the respective macroblock size information of first corresponding ones of each one of the respective plurality of macroblocks in a first one of the two successive i-frames; computing a second respective ratio between the respective macroblock size information for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames and the respective macroblock size information of second corresponding ones of each one of the respective plurality of macroblocks in a second one of the two successive i-frames; comparing the first respective ratio and the second respective ratio for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames with a ratio threshold to identify a first subset of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames that are probably affected by motion, the first subset containing the macroblocks for which at least one of the first respective ratio and the second respective ratio is greater than the ratio threshold; designating as motion macroblocks each macroblock of the first subset of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames where the respective macroblock size information is greater than a bit threshold; and detecting motion when a designated number of the motion macroblocks is greater than a threshold value. parsing a stream of video compressed under a codec; obtaining respective macroblock size information for each one of a respective plurality of macroblocks in two successive i-frames and all p-frames present between the two successive i-frames under the codec from the stream of video as parsed, the respective macroblock size information documenting a respective number of bits for each one of the plurality of macroblocks in the two successive i-frames and all of the p-frames present between the two successive i-frames; storing the respective macroblock size information for each one of the respective plurality of macroblocks in the two successive i-frames; characterised by computing a first respective ratio between the respective macroblock size information for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames and the respective macroblock size information of first corresponding ones of each one of the respective plurality of macroblocks in a first one of the two successive i-frames; computing a second respective ratio between the respective macroblock size information for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames and the respective macroblock size information of second corresponding ones of each one of the respective plurality of macroblocks in a second one of the two successive i-frames; comparing the first respective ratio and the second respective ratio for each one of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames with a ratio threshold to identify a first subset of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames that are probably affected by motion, the first subset containing the macroblocks for which at least one of the first respective ratio and the second respective ratio is greater than the ratio threshold; designating as motion macroblocks each macroblock of the first subset of the respective plurality of macroblocks for all of the p-frames present between the two successive i-frames where the respective macroblock size information is greater than a bit threshold; and detecting motion when a designated number of the motion macroblocks is greater than a threshold value.