Hi Yukio, Hi Ben - interesting - I work on a comparable problem. A light hanging on a winch in a well (with a staircase) that moves up and down with the people climbing the stairs. As the camera (on top) will always monitor both light and people my workaround is to exclude any of the possible light positions (i.e. coordinates in pix_blob) from the calculation. Maybe that could do the trick as well in your case - if you cannot influence (design) the space that way Ben suggests. (the artistically intersting question being as to what extent we want the "interspace" to be really clean or whether we allow for "contextual noise" -) Cheers Achim