Methods having corresponding apparatus and computer-readable media comprise: capturing an image of a shape projected upon a display surface; and determining a first rectangle that is the largest inscribed rectangle for the shape, comprising generating a rectangular bounding box containing the shape, dividing the rectangular bounding box vertically into first and second sections, determining a second rectangle that is the largest inscribed rectangle for the shape in the first section of the bounding box, determining a third rectangle that is the largest inscribed rectangle for the shape in the second section of the bounding box, dividing the rectangular bounding box horizontally into third and fourth sections, determining a fourth rectangle that is the largest inscribed rectangle for the shape in a third section of the bounding box, and determining a fifth rectangle that is the largest inscribed rectangle for the shape in the fourth section of the bounding box.