CameraStuff.java

public void init() {
//...
  for (int i = 0; i < numberOfCameras; i++) {
    Camera.getCameraInfo(i, ci);
    if (ci.facing == Camera.CameraInfo.CAMERA_FACING_FRONT) {
      isFrontFacing = true;
      break;
    }
  }
  //...
  FaceDetector detector =
      new FaceDetector
          .Builder(context)
          .setLandmarkType(FaceDetector.ALL_LANDMARKS)
          .setClassificationType(FaceDetector.ALL_CLASSIFICATIONS)
          .setTrackingEnabled(true)
          .setMode(FaceDetector.FAST_MODE)
          .setProminentFaceOnly(isFrontFacing)
          .setMinFaceSize(isFrontFacing ? 0.35f : 0.15f)
          .build();

  Detector.Processor<Face> processor;
  if (isFrontFacing) {
    Tracker<Face> tracker = new FaceTracker(overlay);
    processor = 
        new LargestFaceFocusingProcessor.Builder(detector, tracker).build();
  } else {
    MultiProcessor.Factory<Face> factory = face -> new FaceTracker(overlay);
    processor = new MultiProcessor.Builder<>(factory).build();
  }
  detector.setProcessor(processor);

  cameraSource = 
      new CameraSource
      .Builder(context, detector)
      .setFacing(facing)
      .setRequestedPreviewSize(defaultWidth, defaultHeight)
      .setRequestedFps(60.0f)
      .setAutoFocusEnabled(true)
      .build();
}