1. A method for processing digital video, comprising: computing a motion variation representation of the video; temporally segmenting the motion variation representation into sub-segments; detecting triangles in the sub-segments of the motion variation representation; and extracting key frames from the video based on the detected triangles, wherein extracting key frames further comprises using selection rules to extract the key frames, wherein the selection rules include selecting a key frame at an apex of each detected triangle where perceived motion energy is changing.