let circ ~x ~y ~radius ?(r=0) ?(g=0) ?(b=0) screen =
    let t1 = ref (radius / 16) in
    let t2 = ref !t1 in
    let cx, cy = ref radius, ref 0 in
    while !cx >= !cy do
        set_pixel screen (     !cx + x) (     !cy + y) ~r ~g ~b ;
        set_pixel screen (     !cx + x) (-1 * !cy + y) ~r ~g ~b ;
        set_pixel screen (-1 * !cx + x) (     !cy + y) ~r ~g ~b ;
        set_pixel screen (-1 * !cx + x) (-1 * !cy + y) ~r ~g ~b ;
        set_pixel screen (     !cy + x) (     !cx + y) ~r ~g ~b ;
        set_pixel screen (     !cy + x) (-1 * !cx + y) ~r ~g ~b ;
        set_pixel screen (-1 * !cy + x) (     !cx + y) ~r ~g ~b ;
        set_pixel screen (-1 * !cy + x) (-1 * !cx + y) ~r ~g ~b ;

        cy := !cy + 1 ;
        t1 := !t1 + !cy ;
        t2 := !t1 - !cx ;
        if !t2 >= 0 then begin
            t1 := !t2 ;
            cx := !cx - 1 ;
        end
    done