code: function(image, options) {
          var imageKey = this.stampImage(image);
          var maskIndex = this._getImageMaskIndex(image, options);
          if (!maskIndex || !imageKey) {
              return this._buildImageMask(image, options);
          }
          var mask = maskIndex[imageKey];
          if (!mask) {
              mask = this._buildImageMask(image, options);
              maskIndex[imageKey] = mask;
          }
          return mask;
      }
docs: ------------------------------------------------------------------------- 
  Returns a mask corresponding to the specified image.