1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:

   function Constrain(min, max, value: longint): longint; 
    begin
       result := value;
       if value < min then result := min;
       if value > max then result := max;
    end;

    function EmptyIntersection (Intersection: TRect): Boolean;
    begin
      result := ((Intersection.Left = Intersection.Right)
              or (Intersection.Top  = Intersection.Bottom));
    end;

    function IntersectionRect(inner, outer: TRect): TRect;
    begin;
      result.Left   := Constrain(outer.left,outer.Right, inner.Left);
      result.right  := Constrain(outer.left,outer.Right, inner.right);
      result.Top    := Constrain(outer.Top,outer.Bottom, inner.Top);
      result.Bottom := Constrain(outer.Top,outer.Bottom, inner.Bottom);
    end;                    

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:

var rDest, r1, r2: TRect;
begin
  r1 := Rect(0,0,100,100);
  Canvas.Brush.Color := clRed;
  Canvas.FillRect(r1);
  r2 := Rect(90,90,200,200);
  Canvas.Brush.Color := clBlue;
  Canvas.FillRect(r2);
  if IntersectRect(rDest, r1, r2) then
    begin
    showmessage('r2 überlappt r1');
    Canvas.Brush.Color := clBlack;
    Canvas.FillRect(rDest);
    end
    else
      showmessage('Keine Überlappung');