Framework for measurement of the intensity of motion activity of video segments

We present a psychophysical and analytical framework for the comparison of the performance of different analytical measures of motion activity in video segments with respect to a subjective ground truth. We first construct a test-set of video segments and conduct a psychophysical experiment to obtain a ground truth for the motion activity. Then we present several low-complexity motion activity descriptors computed from compressed domain block motion vectors. In the first analysis, we quantize the descriptors and show that they perform well against the ground truth. We also show that the MPEG-7 motion activity descriptor is among the best. In the second analysis, we find the pairs of video segments for which the human subjects unanimously rate one as higher activity than the other. Then we examine the specific cases where each descriptor fail to give the correct ordering. We show that the distance from camera, and strong camera motion are main cases where motion vector based descriptors tend to overestimate or underestimate the intensity of motion activity. We finally discuss the experimental methodology and analysis methods we used and possible alternatives. We review the applications of motion activity and how the results presented here relate to those applications.