let pixel_of_point po sz p = (int_of_float((p.x-.po.x)/.sz.x*.float_of_int(size_x())), int_of_float((p.y-.po.y)/.sz.y*.float_of_int(size_x())));;

let pixel_of_point po sz p = (int_of_float((p.x-.po.x)/.sz.x*.float_of_int(size_x())), int_of_float((p.y-.po.y)/.sz.y*.float_of_int(size_x())));; Shouldn't it be `size_y' in the last line?