>Psychtoolbox>PsychRects

rects = CenterRectArrayOnPoint(rects, x, y)

Centers a group of rectangles ‘rects’ on a given
coordinate (x,y). The center of the bounding box
of all given ‘rects’ is placed onto position (x,y)
by shifting the whole set of ‘rects’ accordingly.

Also see PsychRects.