code: function () {
          this.cvsDiv = document.createElement("canvas");
          this.layer.div.appendChild(this.cvsDiv);
  
          this.canvasContext = this.cvsDiv.getContext("2d");
  
          this.className = "olTileMask";
  
          var style = this.cvsDiv.style;
          if (this.layer.opacity < 1) {
              style.filter = 'alpha(opacity=' +
                              (this.layer.opacity * 100) +
                              ')';
          }
          style.position = "absolute";
      }
docs: Method: createCanvas
  Creates and returns the tile canvas.