class AlphaMaskFilter extends BitmapFilter {

 final BitmapData _alphaBitmapData;
 final Matrix _matrix;

 AlphaMaskFilter(BitmapData alphaBitmapData, [Matrix matrix]) :
   _alphaBitmapData = alphaBitmapData,
   _matrix = (matrix != null) ? matrix : new Matrix.fromIdentity();

 Matrix get matrix => _matrix;

 BitmapFilter clone() => new AlphaMaskFilter(_alphaBitmapData, _matrix.clone());
 Rectangle get overlap => new Rectangle.zero();

 //-----------------------------------------------------------------------------------------------

 void apply(BitmapData bitmapData, [Rectangle rectangle]) {

   RenderTextureQuad renderTextureQuad = rectangle == null
       ? bitmapData.renderTextureQuad
       : bitmapData.renderTextureQuad.cut(rectangle);

   int width = renderTextureQuad.textureWidth;
   int height = renderTextureQuad.textureHeight;
   Matrix matrix = renderTextureQuad.drawMatrix;
   CanvasElement canvas = renderTextureQuad.renderTexture.canvas;
   RenderContextCanvas renderContext = new RenderContextCanvas(canvas, Color.Transparent);
   CanvasRenderingContext2D context = renderContext.rawContext;

   context.save();
   context.setTransform(matrix.a, matrix.b, matrix.c, matrix.d, matrix.tx, matrix.ty);
   context.rect(0, 0, width, height);
   context.clip();
   context.globalCompositeOperation = CompositeOperation.DESTINATION_IN;
   matrix.prepend(this.matrix);
   renderContext.renderQuad(_alphaBitmapData.renderTextureQuad, matrix, 1.0);
   context.restore();
 }

}