请教：
procedure Tpic_Form.ZoomPicture(Zoom:single);
var
   DRect:TRect;
   w,h  :integer;
   bitmap:TBitMap;
begin
   w:=Round(Image1.Width*Zoom);
   h:=Round(Image1.Height*Zoom);
   DRect:=Rect(0,0,w,h);
   Bitmap:=TBitmap.Create;
   try
       bitmap.Width:=w;
       bitmap.Height:=h;
       bitmap.Canvas.StretchDraw(DRect,Image1.Picture.Graphic);
       Image2.Parent:=ScrollBox1;
       image2.Width:=w;
       image2.Height:=h;
       image2.BringToFront;
       image2.Visible:=true;
       image1.Visible:=false;
       Image2.Picture.Assign(bitmap);
   finally
       BitMap.Free;
   end;
end;
请帮我看看这段代码的功能！谢谢！！