1. A computer implemented method of, on a computer system having one or more processors and memory storing programs executed by the one or more processors, visualizing an activity represented in a video segment by manipulating a sequence of video frames from the video segment comprising the steps of: (a) classifying a plurality of pixels in the sequence of video frames as representing a foreground object or a background object; (b) assigning one or more alpha values for each classified pixel in each video frame, based at least in part on the classification of the classified pixel; (c) manipulating the plurality of alpha values in order to emphasize one or more of the pixels classified as representing a foreground object; and (d) applying the plurality of alpha values to each pixel to visualize the activity in the video segment within a single still image.