How I setup OpenCV4Android Java SDK with a blank Android project

Setting up an Android Studio project with OpenCV3 can be a little tricky. The official documentation is still using Eclipse and has not been updated with Android Studio. So setting up OpenCV with Android Studio required some trial and error experimentation. I managed to get a template Android Studio project to load the native OpenCV library with the following steps.

Download OpenCV4AndroidSdk

1. If you have not done so, download OpenCV4Android from this website https://sourceforge.net/projects/opencvlibrary/files/opencv-android/
2. Extract the files into a folder e.g. /path/to/Open-CV-android-sdk/

Create an Android Project with C++ Support

1.  Run Android Studio. Choose Create a New Project.
   
   
2. In the Create New Project dialog box, toggle on Include C++ support.
3. Choose appropriate options and click Next in the following screens.
   
4. Click Finish.
   
   The new project is created.
   

Import the OpenCV module into Gradle

1. In the Android Studio File menu, select File | New | Import Module.
   
   The Import Module from Source dialog box appears.
   
2. In the Source directory field, click the ... button and browse to the downloaded OpenCV4Android files and select the java folder e.g. /path/to/OpenCV-android-sdk/sdk/java/.
3. Optional. In the Module name field, type in a name e.g. :opencv.
4. Click Next and Finish.
   
   The module is imported (with most likely error messages)
   
5. Options. You can download the missing dependencies by clicking the Install missing platforms and sync project link. Or you can edit the OpenCV4Android's build.gradle file to enter the installed dependencies.

Fix OpenCV4Android configuration files

1. In Android Studio, open up the file build.gradle from the imported module opencv module e.g. /path/to/opencv/build.gradle. It may be necessary to change the Project pane to display as project view.
   
   
2. In the editor, changed the compiledSdkVersion to 27 (or any installed version), the targetSdkVersion to 27 (or any installed version).
3. Then sync the gradle project.
   
   The project is synced but now there is an error about the manifest file.
4. Open up the manifest file /path/to/opencv/src/main/AndroidManifest.xml in the editor.
   
   
5. Now, comment out the uses-sdk line as shown in the screenshot above.
6. Sync the project again.
   
   The OpenCV module is successfully configured.

Add the OpenCV module as a dependency to the Android App project

1. In Android Studio, select App in the Project pane.
2. Select File | Project Structure.
   
   The Project Structure dialog box appears.
   
3. Select app. Then click the Dependencies tab. Click +. Choose module dependency.
4. Choose the imported Opencv module.
   
   OpenCV is appended to the list.
   
5. Click OK.
   
   The module is added the Android App project.

Copy over the OpenCV native platform binaries to the Android App project

1.  Open up a File Explorer. Select all the native platform folders under the /path/to/OpenCV-android-sdk/sdk/native/libs/*.
   
   
2. Right click and select Copy.
3. In Android Studio, right click on the app folder (in project view). Choose Paste.
   
   The Copy dialog box appears.
   
4. In the To directory field, change the text to /path/to/myandroid/app/jniLibs/.
5. Click OK.
   
   The native binaries are copied to the project.
6. To enable the Android project to know where the native binaries are, open up the app's build.gradle file in the editor.
   
   
7. Add in the android class, add in the following sourceSets lines:
   
   Note: 'jniLibs' means the jniLibs directory relative to this build.gradle file location.
   

   sourceSets {
       main {
           jniLibs.srcDirs = ['jniLibs']
       }
   }

8. Sync the project.

Load the native library

1.  In the MainActivity Kotlin class, type in the following to load in the OpenCV library.
   
   System.loadLibrary("opencv_java3")
2. If everything is configured properly, then the Android app can be launched successfully.
   
   Now you can make any java/Kotlin calls to the built in OpenCV interfaces.