function AlphaMapFilter(alphaMap) {
		this.Filter_constructor();
	
	// public properties:
		/**
		 * The greyscale image (or canvas) to use as the alpha value for the result. This should be exactly the same
		 * dimensions as the target.
		 * @property alphaMap
		 * @type HTMLImageElement|HTMLCanvasElement
		 **/
		this.alphaMap = alphaMap;
		
		
	// private properties:
		/**
		 * @property _alphaMap
		 * @protected
		 * @type HTMLImageElement|HTMLCanvasElement
		 **/
		this._alphaMap = null;
		
		/**
		 * @property _mapData
		 * @protected
		 * @type Uint8ClampedArray
		 **/
		this._mapData = null;
		this._mapTexture = null;

		this.FRAG_SHADER_BODY = (
			"uniform sampler2D uAlphaSampler;"+

			"void main(void) {" +
				"vec4 color = texture2D(uSampler, vRenderCoord);" +
				"vec4 alphaMap = texture2D(uAlphaSampler, vTextureCoord);" +

				// some image formats can have transparent white rgba(1,1,1, 0) when put on the GPU, this means we need a slight tweak
				// using ceil ensure that the colour will be used so long as it exists but pure transparency will be treated black
				"gl_FragColor = vec4(color.rgb, color.a * (alphaMap.r * ceil(alphaMap.a)));" +
			"}"
		);
	}
Applies a greyscale alpha map image (or canvas) to the target, such that the alpha channel of the result will
be copied from the red channel of the map, and the RGB channels will be copied from the target.

Generally, it is recommended that you use {{#crossLink "AlphaMaskFilter"}}{{/crossLink}}, because it has much
better performance.

<h4>Example</h4>
This example draws a red->blue box, caches it, and then uses the cache canvas as an alpha map on a 100x100 image.

      var box = new createjs.Shape();
      box.graphics.beginLinearGradientFill(["#ff0000", "#0000ff"], [0, 1], 0, 0, 0, 100)
      box.graphics.drawRect(0, 0, 100, 100);
      box.cache(0, 0, 100, 100);

      var bmp = new createjs.Bitmap("path/to/image.jpg");
      bmp.filters = [
          new createjs.AlphaMapFilter(box.cacheCanvas)
      ];
      bmp.cache(0, 0, 100, 100);
      stage.addChild(bmp);

See {{#crossLink "Filter"}}{{/crossLink}} for more information on applying filters.
@class AlphaMapFilter
@extends Filter
@constructor
@param {HTMLImageElement|HTMLCanvasElement} alphaMap The greyscale image (or canvas) to use as the alpha value for the
result. This should be exactly the same dimensions as the target.