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Problem with Background Subtraction KNN in android studio

I am trying to do background subtraction using android phone camera with android studio. I installed OpenCV4Android 3.4.7 and try to use Backgorund Subtraction KNN. I give the camera permission in manifests and can launch the camera correctly. However, I only got white(or blank) screen after run this App on my mobilephone. Here is my part of code:

import org.opencv.video.BackgroundSubtractorKNN;
@Override
    public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
        Mat frame = inputFrame.rgba();
        Mat fgMask = new Mat();
        BackgroundSubtractorKNN backSub = Video.createBackgroundSubtractorKNN();
        Imgproc.cvtColor(frame, frame, Imgproc.COLOR_RGBA2GRAY);
        backSub.apply(frame, fgMask,-1);
        return fgMask;}

Anyone can help me ? Many thanks !