code: function (sector, levelZeroDelta, numLevels, imageFormat, cachePath,
                                                  tileWidth, tileHeight) {
              TiledImageLayer.call(this,
                  sector, levelZeroDelta, numLevels, imageFormat, cachePath, tileWidth, tileHeight);
  
              this.detectBlankImages = false;
  
              // These pixels are tested in retrieved images to determine whether the image is blank.
              this.testPixels = [
                  new Vec2(20, 20),
                  new Vec2(235, 20),
                  new Vec2(20, 235),
                  new Vec2(235, 235)
              ];
  
              // Create a canvas we can use when unprojecting retrieved images.
              this.destCanvas = document.createElement("canvas");
              this.destContext = this.destCanvas.getContext("2d");
          }
docs: Constructs a layer supporting Mercator imagery.
  @alias MercatorTiledImageLayer
  @constructor
  @augments TiledImageLayer
  @classdesc Provides an abstract layer to support Mercator layers.
  
  @param {Sector} sector The sector this layer covers.
  @param {Location} levelZeroDelta The size in latitude and longitude of level zero (lowest resolution) tiles.
  @param {Number} numLevels The number of levels to define for the layer. Each level is successively one power
  of two higher resolution than the next lower-numbered level. (0 is the lowest resolution level, 1 is twice
  that resolution, etc.)
  Each level contains four times as many tiles as the next lower-numbered level, each 1/4 the geographic size.
  @param {String} imageFormat The mime type of the image format for the layer's tiles, e.g., <em>image/png</em>.
  @param {String} cachePath A string uniquely identifying this layer relative to other layers.
  @param {Number} tileWidth The horizontal size of image tiles in pixels.
  @param {Number} tileHeight The vertical size of image tiles in pixels.
  @throws {ArgumentError} If any of the specified sector, level-zero delta, cache path or image format arguments are
  null or undefined, or if the specified number of levels, tile width or tile height is less than 1.