How to run deep networks on Android device

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Original author	Dmitry Kurtaev
Compatibility	OpenCV >= 3.3

Introduction

In this tutorial you'll know how to run deep learning networks on Android device using OpenCV deep learning module.

Tutorial was written for the following versions of corresponding software:

• Android Studio 2.3.3
• OpenCV 3.3.0+

Requirements

• Download and install Android Studio from https://developer.android.com/studio.
• Get the latest pre-built OpenCV for Android release from https://github.com/opencv/opencv/releases and unpack it (for example, opencv-4.X.Y-android-sdk.zip).
• Download MobileNet object detection model from https://github.com/chuanqi305/MobileNet-SSD. We need a configuration file MobileNetSSD_deploy.prototxt and weights MobileNetSSD_deploy.caffemodel.

Create an empty Android Studio project

• Open Android Studio. Start a new project. Let's call it opencv_mobilenet.
  
• Keep default target settings.
  

• Use "Empty Activity" template. Name activity as MainActivity with a corresponding layout activity_main.

  
  
• Wait until a project was created. Go to Run->Edit Configurations. Choose USB Device as target device for runs.
  
  Plug in your device and run the project. It should be installed and launched successfully before we'll go next.

  Note

  Read Introduction into Android Development in case of problems.

  

Add OpenCV dependency

• Go to File->New->Import module and provide a path to unpacked_OpenCV_package/sdk/java. The name of module detects automatically. Disable all features that Android Studio will suggest you on the next window.

  
  

• Open two files:

  1. AndroidStudioProjects/opencv_mobilenet/app/build.gradle
  2. AndroidStudioProjects/opencv_mobilenet/openCVLibrary330/build.gradle

  Copy both compileSdkVersion and buildToolsVersion from the first file to the second one.

  compileSdkVersion 14 -> compileSdkVersion 26

  buildToolsVersion "25.0.0" -> buildToolsVersion "26.0.1"

• Make the project. There is no errors should be at this point.

• Go to File->Project Structure. Add OpenCV module dependency.

  
  
• Install once an appropriate OpenCV manager from unpacked_OpenCV_package/apk to target device.

  adb install OpenCV_3.3.0_Manager_3.30_armeabi-v7a.apk

• Congratulations! We're ready now to make a sample using OpenCV.

Make a sample

Our sample will takes pictures from a camera, forwards it into a deep network and receives a set of rectangles, class identifiers and confidence values in [0, 1] range.

• First of all, we need to add a necessary widget which displays processed frames. Modify app/src/main/res/layout/activity_main.xml:

  <?xml version="1.0" encoding="utf-8"?>
  <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context="org.opencv.samples.opencv_mobilenet.MainActivity">
  
      <org.opencv.android.JavaCameraView
          android:id="@+id/CameraView"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:visibility="visible" />
  </FrameLayout>

• Put downloaded MobileNetSSD_deploy.prototxt and MobileNetSSD_deploy.caffemodel into app/build/intermediates/assets/debug folder.
• Modify /app/src/main/AndroidManifest.xml to enable full-screen mode, set up a correct screen orientation and allow to use a camera.

  <?xml version="1.0" encoding="utf-8"?>
  <manifest xmlns:android="http://schemas.android.com/apk/res/android"
            package="org.opencv.samples.opencv_mobilenet"
  >
  
      <application
          android:allowBackup="true"
          android:icon="@mipmap/ic_launcher"
          android:label="@string/app_name"
          android:roundIcon="@mipmap/ic_launcher_round"
          android:supportsRtl="true"
          android:theme="@style/Theme.AppCompat.NoActionBar">  <!--Full screen mode-->
          <activity android:name=".MainActivity"
              android:screenOrientation="landscape">  <!--Screen orientation-->
              <intent-filter>
                  <action android:name="android.intent.action.MAIN" />
                  <category android:name="android.intent.category.LAUNCHER" />
              </intent-filter>
          </activity>
      </application>
  
      <!--Allow to use a camera-->
      <uses-permission android:name="android.permission.CAMERA"/>
      <uses-feature android:name="android.hardware.camera" android:required="false"/>
      <uses-feature android:name="android.hardware.camera.autofocus" android:required="false"/>
      <uses-feature android:name="android.hardware.camera.front" android:required="false"/>
      <uses-feature android:name="android.hardware.camera.front.autofocus" android:required="false"/>
  
  </manifest>

• Replace content of app/src/main/java/org/opencv/samples/opencv_mobilenet/MainActivity.java:

  package org.opencv.samples.opencv_mobilenet;
  
  import android.content.Context;
  import android.content.res.AssetManager;
  import android.os.Bundle;
  import android.support.v7.app.AppCompatActivity;
  import android.util.Log;
  
  import org.opencv.android.BaseLoaderCallback;
  import org.opencv.android.CameraBridgeViewBase;
  import org.opencv.android.CameraBridgeViewBase.CvCameraViewFrame;
  import org.opencv.android.CameraBridgeViewBase.CvCameraViewListener2;
  import org.opencv.android.LoaderCallbackInterface;
  import org.opencv.android.OpenCVLoader;
  import org.opencv.core.Core;
  import org.opencv.core.Mat;
  import org.opencv.core.Point;
  import org.opencv.core.Scalar;
  import org.opencv.core.Size;
  import org.opencv.dnn.Net;
  import org.opencv.dnn.Dnn;
  import org.opencv.imgproc.Imgproc;
  
  import java.io.BufferedInputStream;
  import java.io.File;
  import java.io.FileOutputStream;
  import java.io.IOException;
  
  public class MainActivity extends AppCompatActivity implements CvCameraViewListener2 {
  
      // Initialize OpenCV manager.
      private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this) {
          @Override
          public void onManagerConnected(int status) {
              switch (status) {
                  case LoaderCallbackInterface.SUCCESS: {
                      Log.i(TAG, "OpenCV loaded successfully");
                      mOpenCvCameraView.enableView();
                      break;
                  }
                  default: {
                      super.onManagerConnected(status);
                      break;
                  }
              }
          }
      };
  
      @Override
      public void onResume() {
          super.onResume();
          OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION, this, mLoaderCallback);
      }
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          // Set up camera listener.
          mOpenCvCameraView = (CameraBridgeViewBase)findViewById(R.id.CameraView);
          mOpenCvCameraView.setVisibility(CameraBridgeViewBase.VISIBLE);
          mOpenCvCameraView.setCvCameraViewListener(this);
      }
  
      // Load a network.
      public void onCameraViewStarted(int width, int height) {
          String proto = getPath("MobileNetSSD_deploy.prototxt", this);
          String weights = getPath("MobileNetSSD_deploy.caffemodel", this);
          net = Dnn.readNetFromCaffe(proto, weights);
          Log.i(TAG, "Network loaded successfully");
      }
  
      public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
          final int IN_WIDTH = 300;
          final int IN_HEIGHT = 300;
          final float WH_RATIO = (float)IN_WIDTH / IN_HEIGHT;
          final double IN_SCALE_FACTOR = 0.007843;
          final double MEAN_VAL = 127.5;
          final double THRESHOLD = 0.2;
  
          // Get a new frame
          Mat frame = inputFrame.rgba();
          Imgproc.cvtColor(frame, frame, Imgproc.COLOR_RGBA2RGB);
  
          // Forward image through network.
          Mat blob = Dnn.blobFromImage(frame, IN_SCALE_FACTOR,
                  new Size(IN_WIDTH, IN_HEIGHT),
                  new Scalar(MEAN_VAL, MEAN_VAL, MEAN_VAL), /*swapRB*/false, /*crop*/false);
          net.setInput(blob);
          Mat detections = net.forward();
  
          int cols = frame.cols();
          int rows = frame.rows();
  
          detections = detections.reshape(1, (int)detections.total() / 7);
  
          for (int i = 0; i < detections.rows(); ++i) {
              double confidence = detections.get(i, 2)[0];
              if (confidence > THRESHOLD) {
                  int classId = (int)detections.get(i, 1)[0];
  
                  int left   = (int)(detections.get(i, 3)[0] * cols);
                  int top    = (int)(detections.get(i, 4)[0] * rows);
                  int right  = (int)(detections.get(i, 5)[0] * cols);
                  int bottom = (int)(detections.get(i, 6)[0] * rows);
  
                  // Draw rectangle around detected object.
                  Imgproc.rectangle(frame, new Point(left, top), new Point(right, bottom),
                                    new Scalar(0, 255, 0));
                  String label = classNames[classId] + ": " + confidence;
                  int[] baseLine = new int[1];
                  Size labelSize = Imgproc.getTextSize(label, Imgproc.FONT_HERSHEY_SIMPLEX, 0.5, 1, baseLine);
  
                  // Draw background for label.
                  Imgproc.rectangle(frame, new Point(left, top - labelSize.height),
                                    new Point(left + labelSize.width, top + baseLine[0]),
                                    new Scalar(255, 255, 255), Imgproc.FILLED);
                  // Write class name and confidence.
                  Imgproc.putText(frame, label, new Point(left, top),
                          Imgproc.FONT_HERSHEY_SIMPLEX, 0.5, new Scalar(0, 0, 0));
              }
          }
          return frame;
      }
  
      public void onCameraViewStopped() {}
  
      // Upload file to storage and return a path.
      private static String getPath(String file, Context context) {
          AssetManager assetManager = context.getAssets();
  
          BufferedInputStream inputStream = null;
          try {
              // Read data from assets.
              inputStream = new BufferedInputStream(assetManager.open(file));
              byte[] data = new byte[inputStream.available()];
              inputStream.read(data);
              inputStream.close();
  
              // Create copy file in storage.
              File outFile = new File(context.getFilesDir(), file);
              FileOutputStream os = new FileOutputStream(outFile);
              os.write(data);
              os.close();
              // Return a path to file which may be read in common way.
              return outFile.getAbsolutePath();
          } catch (IOException ex) {
              Log.i(TAG, "Failed to upload a file");
          }
          return "";
      }
  
      private static final String TAG = "OpenCV/Sample/MobileNet";
      private static final String[] classNames = {"background",
              "aeroplane", "bicycle", "bird", "boat",
              "bottle", "bus", "car", "cat", "chair",
              "cow", "diningtable", "dog", "horse",
              "motorbike", "person", "pottedplant",
              "sheep", "sofa", "train", "tvmonitor"};
  
      private Net net;
      private CameraBridgeViewBase mOpenCvCameraView;
  }

• Launch an application and make a fun!