Spatial Activity of an Image

I worked a lot with visual stimuli, and this function computes a measure for image activity, that is derived from the amount of edges in an image.
It was proposed in ITU-T Document P.910 for single images (see pages 12, 15, 22-24 in that document). Please change the code to your own needs.
Part of the results derived from this and other measures can be found in "Barkowsky, Eskofier: Influence of the presentation time on subjective votings of coded still images", ICIP 2006