anchor.diff:            if colors.outlineColor<>0 then
             begin
               aBitmap.Canvas.Stroke.Color := colors.outlineColor;
  -            //aBitmap.Canvas.StrokeThickness := 1; // todo: default width?
               aBitmap.Canvas.Stroke.Thickness := 1; // todo: default width?
               aBitmap.Canvas.DrawEllipse(rect, 1);
             end;
  
positive.code:   status: TGenerateTileStatus;
  begin
    Result := NaN; // sentinel
    //status  := gtsRestart; // sentinel
    repeat
      // pre calc phase, potentially write lock of tile -> only when not avoidable
      if doGenerateTilePreCalc then
  
negative.code:   inherited;
  end;
  
  function TDiffSlice.GenerateTile(const aExtent: TExtent; var aStream: TStream; aWidthPixels, aHeightPixels: Integer;
    const aFileName, aCacheFolder: string; aThreadPool: TMyThreadPool): Integer;
  var
    status: TGenerateTileStatus;
  begin
    repeat
      // pre calc phase, potentially write lock of tile -> only when not avoidable
      if fCurrentSlice.doGenerateTilePreCalc then
  