// Show a 100x100px box in the center of the image (does not modify the actual image, only the view)
procedure TForm1.ImageEnView1DrawCanvas(Sender: TObject; ACanvas: TCanvas; ARect: TRect);
const
  Box_Width  = 100;
  Box_Height = 100;
var
  boxL, boxT, boxR, boxB: Integer;
begin
  if ImageEnView1.IsEmpty2 or
     ( ImageEnView1.IEBitmap.Width < Box_Width ) or
     ( ImageEnView1.IEBitmap.Height < Box_Height ) or
     not chkDrawBox.Checked then
    exit;

  boxL := ImageEnView1.XBmp2Scr( ImageEnView1.IEBitmap.Width  div 2 - Box_Width  div 2 );
  boxT := ImageEnView1.YBmp2Scr( ImageEnView1.IEBitmap.Height div 2 - Box_Height div 2 );
  boxR := ImageEnView1.XBmp2Scr( ImageEnView1.IEBitmap.Width  div 2 + Box_Width  div 2 );
  boxB := ImageEnView1.YBmp2Scr( ImageEnView1.IEBitmap.Height div 2 + Box_Height div 2 );

  ACanvas.Pen.Color := clRed;
  ACanvas.Brush.Style := bsClear;
  ACanvas.Rectangle( boxL, boxT, boxR, boxB );
end;

// Draw "thirds" within the selection box
procedure TForm1.ImageEnView1DrawCanvas(Sender: TObject; ACanvas: TCanvas; ARect: TRect);
var
  selRect: TIERectangle;
  {}
  procedure _DrawLine(x1, y1, x2, y2: Integer); // bmp values
  begin
    ACanvas.Pen.Color := clGray;
    ACanvas.MoveTo( ImageEnView1.XBmp2Scr( x1 ), ImageEnView1.YBmp2Scr( y1 ) );
    ACanvas.LineTo( ImageEnView1.XBmp2Scr( x2 ), ImageEnView1.YBmp2Scr( y2 ) );
  end;
  {}
begin
  if not ( ImageEnView1.Selected and chkDrawBox.Checked ) then
    exit;

  selRect := ImageEnView1.SelectedRect;

  // Horz Thirds
  _DrawLine( selRect.x, selRect.y + MulDiv( selRect.height, 1, 3 ),
             selRect.x + selRect.Width, selRect.y + MulDiv( selRect.height, 1, 3 ) );
  _DrawLine( selRect.x, selRect.y + MulDiv( selRect.height, 2, 3 ),
             selRect.x + selRect.Width, selRect.y + MulDiv( selRect.height, 2, 3 ) );

  // Vert Thirds
  _DrawLine( selRect.x + MulDiv( selRect.Width, 1, 3 ), selRect.y,
             selRect.x + MulDiv( selRect.Width, 1, 3 ), selRect.y + selRect.height );
  _DrawLine( selRect.x + MulDiv( selRect.Width, 2, 3 ), selRect.y,
             selRect.x + MulDiv( selRect.Width, 2, 3 ), selRect.y + selRect.height );
end;