How to join png with alpha / transparency in a frame in realtime

edit

I'm working under the example of OpenCV android 2.4.11 which detects faces using the camera. Instead of drawing a rectangle on the face found, I'm trying to put a mask (png image) on the face. But to display the image on the face, the png image is coming with a black background where there was transparency.

FdActivity.java

public void onCameraViewStarted(int width, int height) {
        mGray = new Mat();
        mRgba = new Mat();

        //Load my mask png
        Bitmap image = BitmapFactory.decodeResource(getResources(), R.drawable.mask_1);

        mask = new Mat();

        Utils.bitmapToMat(image, mask);

}

public Mat onCameraFrame(CvCameraViewFrame inputFrame) {

        mRgba = inputFrame.rgba();
        mGray = inputFrame.gray();

        if (mAbsoluteFaceSize == 0) {
            int height = mGray.rows();
            if (Math.round(height * mRelativeFaceSize) > 0) {
                mAbsoluteFaceSize = Math.round(height * mRelativeFaceSize);
            }
            mNativeDetector.setMinFaceSize(mAbsoluteFaceSize);
        }

        MatOfRect faces = new MatOfRect();

        if (mDetectorType == JAVA_DETECTOR) {
            if (mJavaDetector != null)
                mJavaDetector.detectMultiScale(mGray, faces, 1.1, 2, 2,
                        new Size(mAbsoluteFaceSize, mAbsoluteFaceSize), new Size());
        }
        else if (mDetectorType == NATIVE_DETECTOR) {
            if (mNativeDetector != null)
                mNativeDetector.detect(mGray, faces);
        }
        else {
            Log.e(TAG, "Detection method is not selected!");
        }

        Rect[] facesArray = faces.toArray();


        for (int i = 0; i < facesArray.length; i++) {

              overlayImage(mRgba, mask, facesArray[i]);

        }

        return mRgba;
    }

    public Mat overlayImage(Mat background, Mat foregroundMask, Rect faceRect)
    {
        Mat mask = new Mat();

        Imgproc.resize(this.mask, mask, faceRect.size());

        Mat source = new Mat();
        Imgproc.resize(foregroundMask, source, background.size());

        mask.copyTo( background.submat( new Rect((int) faceRect.tl().x, (int) faceRect.tl().y, mask.cols(), mask.rows())) );

        source.release();
        mask.release();
        return background;
    }