Android Development with OpenCV?

This tutorial has been created to help you use OpenCV library within your Android project.

This guide was written with Windows 7 in mind, though it should work with any other OS supported byOpenCV4Android SDK.

This tutorial assumes you have the following installed and configured:

• JDK

• Android SDK and NDK

• Eclipse IDE

• ADT and CDT plugins for Eclipse

If you need help with anything of the above, you may refer to our Introduction into Android Development guide.

This tutorial also assumes you have OpenCV4Android SDK already installed on your developmentmachine and OpenCV Manager on your testing device correspondingly. If you need help with any ofthese, you may consult our OpenCV4Android SDK tutorial.

If you encounter any error after thoroughly following these steps, feel free to contact us viaOpenCV4Android discussion group or OpenCVQ&A forum . We’ll do our best to help you out.

Using OpenCV Library Within Your Android Project?

In this section we will explain how to make some existing project to use OpenCV.Starting with 2.4.2 release for Android, OpenCV Manager is used to provide apps with the bestavailable version of OpenCV.You can get more information here: Android OpenCV Manager and in theseslides.

Java?

Application Development with Async Initialization?

Using async initialization is a recommended way for application development. It uses the OpenCVManager to access OpenCV libraries externally installed in the target system.

1. Add OpenCV library project to your workspace. Use menuFile -> Import -> Existing project in your workspace.

   Press Browse button and locate OpenCV4Android SDK(OpenCV-2.4.9-android-sdk/sdk).

   

2. In application project add a reference to the OpenCV Java SDK inProject -> Properties -> Android -> Library -> Add select OpenCV Library - 2.4.9.

   

In most cases OpenCV Manager may be installed automatically from Google Play. For the case, whenGoogle Play is not available, i.e. emulator, developer board, etc, you can install it manuallyusing adb tool. See How to select the proper version of OpenCV Manager for details.

There is a very base code snippet implementing the async initialization. It shows basic principles.See the “15-puzzle” OpenCV sample for details.

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public class Sample1Java extends Activity implements CvCameraViewListener { 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_2_4_6, this, mLoaderCallback); } ...}

It this case application works with OpenCV Manager in asynchronous fashion. OnManagerConnectedcallback will be called in UI thread, when initialization finishes. Please note, that it is notallowed to use OpenCV calls or load OpenCV-dependent native libs before invoking this callback.Load your own native libraries that depend on OpenCV after the successful OpenCV initialization.Default BaseLoaderCallback implementation treat application context as Activity and callsActivity.finish() method to exit in case of initialization failure. To override this behavioryou need to override finish() method of BaseLoaderCallback class and implement your ownfinalization method.

Application Development with Static Initialization?

According to this approach all OpenCV binaries are included into your application package. It isdesigned mostly for development purposes. This approach is deprecated for the production code,release package is recommended to communicate with OpenCV Manager via the async initializationdescribed above.

1. Add the OpenCV library project to your workspace the same way as for the async initializationabove. Use menu File -> Import -> Existing project in your workspace,press Browse button and select OpenCV SDK path(OpenCV-2.4.9-android-sdk/sdk).

   

2. In the application project add a reference to the OpenCV4Android SDK inProject -> Properties -> Android -> Library -> Add select OpenCV Library - 2.4.9;

   

3. If your application project doesn’t have a JNI part, just copy the corresponding OpenCVnative libs from <OpenCV-2.4.9-android-sdk>/sdk/native/libs/<target_arch> to yourproject directory to folder libs/<target_arch>.

   In case of the application project with a JNI part, instead of manual libraries copying youneed to modify your Android.mk file:add the following two code lines after the "include $(CLEAR_VARS)" and before"include path_to_OpenCV-2.4.9-android-sdk/sdk/native/jni/OpenCV.mk"

   12	OPENCV_CAMERA_MODULES:=onOPENCV_INSTALL_MODULES:=on

   The result should look like the following:

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   include $(CLEAR_VARS)# OpenCVOPENCV_CAMERA_MODULES:=onOPENCV_INSTALL_MODULES:=oninclude ../../sdk/native/jni/OpenCV.mk

   After that the OpenCV libraries will be copied to your application libs folder duringthe JNI build.v

   Eclipse will automatically include all the libraries from the libs folder to theapplication package (APK).

4. The last step of enabling OpenCV in your application is Java initialization code before callingOpenCV API. It can be done, for example, in the static section of the Activity class:

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   static { if (!OpenCVLoader.initDebug()) { // Handle initialization error }}

   If you application includes other OpenCV-dependent native libraries you should load themafter OpenCV initialization:

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   static { if (!OpenCVLoader.initDebug()) { // Handle initialization error } else { System.loadLibrary("my_jni_lib1"); System.loadLibrary("my_jni_lib2"); }}

Native/C++?

To build your own Android application, using OpenCV as native part, the following steps should betaken:

1. You can use an environment variable to specify the location of OpenCV package or just hardcodeabsolute or relative path in the jni/Android.mk of your projects.

2. The file jni/Android.mk should be written for the current application using the commonrules for this file.

   For detailed information see the Android NDK documentation from the Android NDK archive, in thefile <path_where_NDK_is_placed>/docs/ANDROID-MK.html.

3. The following line:

   include C:\Work\OpenCV4Android\OpenCV-2.4.9-android-sdk\sdk\native\jni\OpenCV.mk

   Should be inserted into the jni/Android.mk file after this line:

   include $(CLEAR_VARS)

4. Several variables can be used to customize OpenCV stuff, but you don’t need to use them whenyour application uses the async initialization via the OpenCV Manager API.

   Note

   These variables should be set before the "include .../OpenCV.mk" line:

   OPENCV_INSTALL_MODULES:=on

   Copies necessary OpenCV dynamic libs to the project libs folder in order to include theminto the APK.

   OPENCV_CAMERA_MODULES:=off

   Skip native OpenCV camera related libs copying to the project libs folder.

   OPENCV_LIB_TYPE:=STATIC

   Perform static linking with OpenCV. By default dynamic link is used and the project JNI libdepends on libopencv_java.so.

5. The file Application.mk should exist and should contain lines:

   APP_STL := gnustl_staticAPP_CPPFLAGS := -frtti -fexceptions

   Also, the line like this one:

   APP_ABI := armeabi-v7a

   Should specify the application target platforms.

   In some cases a linkage error (like "In function 'cv::toUtf16(std::basic_string<...>...undefined reference to 'mbstowcs'") happens when building an application JNI library,depending on OpenCV. The following line in the Application.mk usually fixes it:

   APP_PLATFORM := android-9

6. Either use manual ndk-build invocation orsetup Eclipse CDT Builder to build native JNI lib before (re)building the Javapart and creating an APK.

Hello OpenCV Sample?

Here are basic steps to guide you trough the process of creating a simple OpenCV-centricapplication. It will be capable of accessing camera output, processing it and displaying theresult.

1. Open Eclipse IDE, create a new clean workspace, create a new Android projectFile ? New ? Android Project

2. Set name, target, package and minSDKVersion accordingly. The minimal SDK version for buildwith OpenCV4Android SDK is 11. Minimal device API Level (for application manifest) is 8.

3. Allow Eclipse to create default activity. Lets name the activity HelloOpenCvActivity.

4. Choose Blank Activity with full screen layout. Lets name the layout HelloOpenCvLayout.

5. Import OpenCV library project to your workspace.

6. Reference OpenCV library within your project properties.

   

7. Edit your layout file as xml file and pass the following layout there:

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   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:opencv="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent" > <org.opencv.android.JavaCameraView android:layout_width="fill_parent" android:layout_height="fill_parent" android:visibility="gone" android:id="@+id/HelloOpenCvView" opencv:show_fps="true" opencv:camera_id="any" /></LinearLayout>

8. Add the following permissions to the AndroidManifest.xml file:

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   </application><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"/>

9. Set application theme in AndroidManifest.xml to hide title and system buttons.

   1234	<application android:icon="@drawable/icon" android:label="@string/app_name" android:theme="@android:style/Theme.NoTitleBar.Fullscreen" >

10. Add OpenCV library initialization to your activity. Fix errors by adding requited imports.

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    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; } }};@Overridepublic void onResume(){ super.onResume(); OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_2_4_6, this, mLoaderCallback);}

11. Defines that your activity implements CvCameraViewListener2 interface and fix activity relatederrors by defining missed methods. For this activity define onCreate, onDestroy andonPause and implement them according code snippet bellow. Fix errors by adding requitedimports.

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    private CameraBridgeViewBase mOpenCvCameraView; @Override public void onCreate(Bundle savedInstanceState) { Log.i(TAG, "called onCreate"); super.onCreate(savedInstanceState); getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON); setContentView(R.layout.HelloOpenCvLayout); mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.HelloOpenCvView); mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE); mOpenCvCameraView.setCvCameraViewListener(this); } @Override public void onPause() { super.onPause(); if (mOpenCvCameraView != null) mOpenCvCameraView.disableView(); } public void onDestroy() { super.onDestroy(); if (mOpenCvCameraView != null) mOpenCvCameraView.disableView(); } public void onCameraViewStarted(int width, int height) { } public void onCameraViewStopped() { } public Mat onCameraFrame(CvCameraViewFrame inputFrame) { return inputFrame.rgba(); }

12. Run your application on device or emulator.

Lets discuss some most important steps. Every Android application with UI must implement Activityand View. By the first steps we create blank activity and default view layout. The simplestOpenCV-centric application must implement OpenCV initialization, create its own view to showpreview from camera and implements CvCameraViewListener2 interface to get frames from camera andprocess it.

First of all we create our application view using xml layout. Our layout consists of the onlyone full screen component of class org.opencv.android.JavaCameraView. This class isimplemented inside OpenCV library. It is inherited from CameraBridgeViewBase, that extendsSurfaceView and uses standard Android camera API. Alternatively you can useorg.opencv.android.NativeCameraView class, that implements the same interface, but usesVideoCapture class as camera access back-end. opencv:show_fps="true" andopencv:camera_id="any" options enable FPS message and allow to use any camera on device.Application tries to use back camera first.

After creating layout we need to implement Activity class. OpenCV initialization process hasbeen already discussed above. In this sample we use asynchronous initialization. Implementation ofCvCameraViewListener interface allows you to add processing steps after frame grabbing fromcamera and before its rendering on screen. The most important function is onCameraFrame. It iscallback function and it is called on retrieving frame from camera. The callback input is objectof CvCameraViewFrame class that represents frame from camera.

Note

Do not save or use CvCameraViewFrame object out of onCameraFrame callback. This objectdoes not have its own state and its behavior out of callback is unpredictable!

It has rgba() and gray() methods that allows to get frame as RGBA and one channel gray scaleMat respectively. It expects that onCameraFrame function returns RGBA frame that will bedrawn on the screen.