Unable to add overlay object on tracked face in openCV android

Question

Below is the code to draw rectangle on tracked face. I want to replace this rectangle with a custom overlay like hat image.

This code is executed inside onCameraFrame method of openCV and mRgba is the mat object of camera preview.

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);
        }

    }

    MatOfRect faces = new MatOfRect();

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

    Rect[] facesArray = faces.toArray();
    for (int i = 0; i < facesArray.length; i++) {
       Imgproc.rectangle(mRgba, facesArray[i].tl(), facesArray[i].br(),
                FACE_RECT_COLOR, 3);

    }
    return mRgba;
}