// Draw a box around all selected layers
// Note: To prevent the default selection box being drawn, create an empty OnDrawLayerBox event
procedure Tfmain.ImageEnView1DrawBackBuffer(Sender: TObject);
var
  cab: TRect;
  i, minX, minY, maxX, maxY : Integer;
begin
  minX := MAXINT;
  minY := MAXINT;
  maxX := 0;
  maxY := 0;

  for I := 0 to ImageEnView1.LayersCount - 1 do
    if ImageEnView1.Layers[ i ].Selected then
    begin
      cab := ImageEnView1.Layers[i].ClientAreaBox;
      minX := imin( minX, cab.Left );
      minY := imin( minY, cab.Top );
      maxX := imax( maxX, cab.Right );
      maxY := imax( maxY, cab.Bottom );
    end;

  // a green line
  if minX < MAXINT then
    with ImageEnView1.BackBuffer.Canvas do
    begin
      Pen.Style := psSolid;
      Pen.Width := 2;
      Pen.mode := pmCopy;
      Pen.Color := clGreen;
      Brush.Style := bsClear;
      Rectangle( minX - 1, minY - 1, maxX + 1, maxY + 1 );
    end;
end;