gistfile1.as

function renderWithAlpha(source: DisplayObject, target: BitmapData, tranform: Matrix = null, currentStarling: Starling = null): Boolean {
    if (!tranform) {
        _helpMatrix.identity();
        tranform = _helpMatrix;
    }
    var rect: Rectangle = target.rect;
    if (!_render(source, target, rect, tranform, 0, 0, null, currentStarling)) {
        return false;
    }
    var alphaBitmap: BitmapData = new BitmapData(target.width, target.height, false);
    if (!_render(source, alphaBitmap, rect, tranform, 0xffffff, 1, BlendMode.ERASE)) {
        return false;
    }
    alphaBitmap.applyFilter(alphaBitmap, rect, ZERO_POINT, INVERT_COLOR);
    target.copyChannel(alphaBitmap, rect, ZERO_POINT, BitmapDataChannel.RED, BitmapDataChannel.ALPHA);
    return true;
}

function _render(source: DisplayObject, target: BitmapData, clip: Rectangle = null, transform: Matrix = null, bgColor: uint = 0, bgAlpha: Number = 0, blendMode: String = null, currentStarling: Starling = null): Boolean {

    var mStage: Stage = Starling.current.stage;
    var _support: Utils_RenderSupport = _getSupport();
    _support.nextFrame();
    _support.setOrthographicProjection(0, 0, mStage.stageWidth, mStage.stageHeight);
    try {
        _support.clear(bgColor, bgAlpha);
    } catch (err: Error) {
        // context is lost
        return false;
    }
    _support.setBlendMode(blendMode ? blendMode : BlendMode.NORMAL);
    if (transform) {
        _support.pushMatrix();
        _support.prependMatrix(transform);
    }
    if (clip) {
        _support.pushClipRect(clip);
    }
    source.render(_support, 1);
    _support.finishQuadBatch();
    if (clip) {
        _support.popClipRect();
    }
    Starling.context.drawToBitmapData(target);
    return true;
}

class Utils_RenderSupport extends RenderSupport {

    public function setBlendMode(mode: String): void {
        super.blendMode = mode;
    }

    public override function get blendMode(): String {
        return super.blendMode;
    }

    public override function set blendMode(value: String): void {
    }

}