Simple applyAlphaMask for haxe/openfl...

applyAlphaMask.hx

	/**
	 * Masks the `source` BitmapData (in place) using alpha data from `alphaMask`.
	 * 
	 * Note: `source` and `alphaMask` must be of the same size.
	 * 
	 * @param	source		BitmapData to be masked.
	 * @param	alphaMask	BitmapData to be used as mask.
	 * @param	copyAlpha	If true the alpha value of `alphaMask` will be copied over to `source`.
	 */
	public static function applyAlphaMask(source:BitmapData, alphaMask:BitmapData, copyAlpha:Bool = false):Void 
	{
		var sourceRect = source.rect;
		var sourceBytes:ByteArray = source.getPixels(sourceRect);
		var alphaMaskBytes:ByteArray = alphaMask.getPixels(sourceRect);
		
		sourceBytes.position = 0;
		alphaMaskBytes.position = 0;
		
		var nPixels:Int = Std.int(sourceRect.width * sourceRect.height);
		var sourceAlpha:Int = 0;
		var maskAlpha:Int = 0;
		var alphaIdx:Int = 0;
		
		for (idx in 0...nPixels) {
			alphaIdx = idx << 2;
			maskAlpha = alphaMaskBytes[alphaIdx];
			sourceAlpha = sourceBytes[alphaIdx];
			sourceBytes[alphaIdx] = copyAlpha ? maskAlpha : sourceAlpha * (maskAlpha > 0 ? 1 : 0);
		}
		
		source.setPixels(sourceRect, sourceBytes);
	}

Wouldn't copyChannel do the same job?

targetBitmap.copyChannel(sourceBitmap, sourceBitmap.rect, new Point(), BitmapDataChannel.ALPHA, BitmapDataChannel.ALPHA);

@musanek: Yes, this behaves like copyChannel if you set copyAlpha = true, but I needed to replicate the behaviour of copyPixels with alphaBitmapData (which is done with this snippet and copyAlpha = false).

Basically if the mask has alpha > 0 outside opaque pixels of the source don't copy it over.

Is there a way to have something faster than that ?
for (idx in 0...nPixels) {
alphaIdx = idx << 2;
maskAlpha = alphaMaskBytes[alphaIdx];
sourceAlpha = sourceBytes[alphaIdx];
sourceBytes[alphaIdx] = copyAlpha ? maskAlpha : sourceAlpha * (maskAlpha > 0 ? 1 : 0);
}