code: function(alphaThreshold) {
              var threshold = alphaThreshold || 0,
                  cachedCanvas = this._cache.canvas,
                  sceneCanvas = this._getCachedSceneCanvas(),
                  sceneContext = sceneCanvas.getContext(),
                  hitCanvas = cachedCanvas.hit,
                  hitContext = hitCanvas.getContext(),
                  width = sceneCanvas.getWidth(),
                  height = sceneCanvas.getHeight(),
                  sceneImageData, sceneData, hitImageData, hitData, len, rgbColorKey, i, alpha;
  
              hitContext.clear();
  
              try {
                  sceneImageData = sceneContext.getImageData(0, 0, width, height);
                  sceneData = sceneImageData.data;
                  hitImageData = hitContext.getImageData(0, 0, width, height);
                  hitData = hitImageData.data;
                  len = sceneData.length;
                  rgbColorKey = Kinetic.Util._hexToRgb(this.colorKey);
  
                  // replace non transparent pixels with color key
                  for(i = 0; i < len; i += 4) {
                      alpha = sceneData[i + 3];
                      if (alpha > threshold) {
                          hitData[i] = rgbColorKey.r;
                          hitData[i + 1] = rgbColorKey.g;
                          hitData[i + 2] = rgbColorKey.b;
                          hitData[i + 3] = 255;
                      }
                  }
  
                  hitContext.putImageData(hitImageData, 0, 0);
              }
              catch(e) {
                  Kinetic.Util.warn('Unable to draw hit graph from cached scene canvas. ' + e.message);
              }
  
              return this;
          }
docs: draw hit graph using the cached scene canvas
  @method
  @memberof Kinetic.Shape.prototype
  @param {Integer} alphaThreshold alpha channel threshold that determines whether or not
  a pixel should be drawn onto the hit graph.  Must be a value between 0 and 255.
  The default is 0
  @returns {Kinetic.Shape}
  @example
  shape.cache();
  shape.drawHitFromCache();