OpenCV Android - How to play sound when object detection occur?

Question

How to play sound when object detection occur? In which function I can put sound code?

Sound snippet

final MediaPlayer mp = MediaPlayer.create(this, R.raw.soho);
mp.stop();
mp.start();

CameraActivity.java

public class CameraActivity extends Activity implements CvCameraViewListener2 {

    private static final String    TAG                 = "OCVSample::Activity";
    private static final Scalar FACE_RECT_COLOR     = new Scalar(0, 255, 0, 255);
    public static final int        JAVA_DETECTOR       = 0;
    public static final int        NATIVE_DETECTOR     = 1;

    private MenuItem               mItemFace50;
    private MenuItem               mItemFace40;
    private MenuItem               mItemFace30;
    private MenuItem               mItemFace20;
    private MenuItem               mItemType;

    private Mat                    mRgba;
    private Mat                    mGray;
    private File                   mCascadeFile;
    private CascadeClassifier      mJavaDetector;
    private DetectionBasedTracker  mNativeDetector;

    private int                    mDetectorType       = JAVA_DETECTOR;
    private String[]               mDetectorName;

    private float                  mRelativeFaceSize   = 0.2f;
    private int                    mAbsoluteFaceSize   = 0;

    private CameraBridgeViewBase   mOpenCvCameraView;

    private BaseLoaderCallback  mLoaderCallback = new BaseLoaderCallback(this) {
        @Override
        public void onManagerConnected(int status) {
            switch (status) {
                case LoaderCallbackInterface.SUCCESS:
                {
                    Log.i(TAG, "OpenCV loaded successfully");

                    // Load native library after(!) OpenCV initialization
                    System.loadLibrary("detection_based_tracker");

                    try {
                        // load cascade file from application resources
                        InputStream is = getResources().openRawResource(R.raw.cascade);
                        File cascadeDir = getDir("cascade", Context.MODE_PRIVATE);
                        mCascadeFile = new File(cascadeDir, "cascade.xml");
                        FileOutputStream os = new FileOutputStream(mCascadeFile);

                        byte[] buffer = new byte[4096];
                        int bytesRead;
                        while ((bytesRead = is.read(buffer)) != -1) {
                            os.write(buffer, 0, bytesRead);
                        }
                        is.close();
                        os.close();

                        mJavaDetector = new CascadeClassifier(mCascadeFile.getAbsolutePath());
                        mJavaDetector.load(mCascadeFile.getAbsolutePath());
                        if (mJavaDetector.empty()) {
                            Log.e(TAG, "Failed to load cascade classifier");
                            mJavaDetector = null;
                        } else
                            Log.i(TAG, "Loaded cascade classifier from " + mCascadeFile.getAbsolutePath());

                        mNativeDetector = new DetectionBasedTracker(mCascadeFile.getAbsolutePath(), 0);

                        cascadeDir.delete();

                    } catch (IOException e) {
                        e.printStackTrace();
                        Log.e(TAG, "Failed to load cascade. Exception thrown: " + e);
                    }

                    mOpenCvCameraView.enableView();
                } break;
                default:
                {
                    super.onManagerConnected(status);
                } break;
            }
        }
    };

    public CameraActivity() {
        mDetectorName = new String[2];
        mDetectorName[JAVA_DETECTOR] = "Java";
        mDetectorName[NATIVE_DETECTOR] = "Native (tracking)";

        Log.i(TAG, "Instantiated new " + this.getClass());
    }

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.i(TAG, "called onCreate");
        super.onCreate(savedInstanceState);
        //getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.activity_camera);

        mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.show_camera_activity_java_surface_view);
        mOpenCvCameraView.setCvCameraViewListener(this);
    }

    @Override
    public void onPause()
    {
        super.onPause();
        if (mOpenCvCameraView != null)
            mOpenCvCameraView.disableView();
    }

    @Override
    public void onResume()
    {
        super.onResume();
        if (!OpenCVLoader.initDebug()) {
            Log.d(TAG, "Internal OpenCV library not found. Using OpenCV Manager for initialization");
            OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_3_0_0, this, mLoaderCallback);
        } else {
            Log.d(TAG, "OpenCV library found inside package. Using it!");
            mLoaderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS);
        }
    }

    public void onDestroy() {
        super.onDestroy();
        mOpenCvCameraView.disableView();
    }

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

    public void onCameraViewStopped() {
        mGray.release();
        mRgba.release();
    }

    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, // TODO: objdetect.CV_HAAR_SCALE_IMAGE
                        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++)
            Imgproc.rectangle(mRgba, facesArray[i].tl(), facesArray[i].br(), FACE_RECT_COLOR, 3);

        return mRgba;
    }

    private void setMinFaceSize(float faceSize) {
        mRelativeFaceSize = faceSize;
        mAbsoluteFaceSize = 0;
    }

    private void setDetectorType(int type) {
        if (mDetectorType != type) {
            mDetectorType = type;

            if (type == NATIVE_DETECTOR) {
                Log.i(TAG, "Detection Based Tracker enabled");
                mNativeDetector.start();
            } else {
                Log.i(TAG, "Cascade detector enabled");
                mNativeDetector.stop();
            }
        }
    }
}

Answer 1

You're calling

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 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++)
        Imgproc.rectangle(mRgba, facesArray[i].tl(), facesArray[i].br(), FACE_RECT_COLOR, 3);

In both cases you're passing a MatOfRect, which the function will fill up with rectangles corresponding to face outlines(or empty if there are none).

To play a sound then you'll simply have to

if(facesArray.Size()>0){
playAudio();
}

after the conversion from MatOfRect to Rect[]