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Cristina Heredia

Outlier y graduada en Ing. Informática en la ugr. Computer science.

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If you are wishing to compile OpenCV 3.2 by your own with Java support, possibly this will help. Also it is for you if, done that, you want to use it in IntelliJ IDEA. Let’s start!

For compiling OPENCV 3.2 with Java support you must make sure you have the following installed:

Prerequisites



1. Download OpenCV 3.2

Go to opencv documentation and download the last stable version for Linux/Mac.

2. Unzip where you want to install and create build directory

Extract the content folder opencv-3.2.0 where you want it to be placed. Then, move to that directory and create a build empty folder, where compilation result will be stored:

    cris@cris ~ $ cd opencv-3.2.0/
    cris@cris ~ $ mkdir build

3. Export JAVA_HOME variable and generate a makefile

You must make sure $JAVA_HOME variable is set to JDK’s path and visible to child processes. For that when doing: echo $JAVA_HOME the JDK path must be displayed. If not, in terminal, set de variable value to JDK path and export it, for example:

cris@cris ~$ export JAVA_HOME=/home/jdk1.8.0_111/

then move to build directory and generate makefile:

cris@cris ~/opencv-3.2.0/build $ cmake -DBUILD_SHARED_LIBS=OFF ..

Notice that when generating the makefile, the output in the Java field looks like this:

building makefiles

That is, there is a specified ant and JNI path, and Java wrappers is set to YES.

4. Build

run make to build openCV with Java and create a jar:

cris@cris ~/opencv-3.2.0/build $ make -j8

when finished, make sure the files opencv-320.jar and libopencv_java320.so (.so or .dll) are created:

    cris@cris ~/opencv-3.2.0/build $ ls -R | grep opencv-320.jar
    opencv-320.jar
    opencv-320.jar.dephelper

    cris@cris ~/opencv-3.2.0/build $ ls -R | grep libopencv_java320.so
    libopencv_java320.so

Now that it is build, the next step would be create an example in Java to check if it works. I am using IntelliJ IDEA Ultimate 2016 IDE, so in next section I am going to explain how to create a Java OpenCV proyect in INTELLIJ.

Creating a Java OpenCV proyect in INTELLIJ 16

The easiest way to create a Java and Scala openCV proyect is using SBT. So we open IntelliJ and create a new SBT proyect. Once it is created, we select File->Project Structure (Make sure here Module JDK appears set with our SDK) and choose Dependencies tab. Here, we click to add and select the opencv-320.jar and the library, that is in opencv3.2.0/build/lib :

adding .jar file
adding .so file

finally, we take the face detection example from opencv documentation , correct it (because it is for OpenCV version 2.4.4) and store it in a file called helloOpenCV.java inside src->main->java folder. The example code adapted to OpenCV version 3.2 is :

    import org.opencv.core.*;
    import org.opencv.core.Mat;
    import org.opencv.core.MatOfRect;
    import org.opencv.core.Point;
    import org.opencv.core.Rect;
    import org.opencv.core.Scalar;
    import org.opencv.imgcodecs.Imgcodecs;
    import org.opencv.objdetect.CascadeClassifier;

    import static org.opencv.imgproc.Imgproc.rectangle;

    //
    // Detects faces in an image, draws boxes around them, and writes the results
    // to "faceDetection.png".
    //
    class DetectFaceDemo {
      public void run() {
        System.out.println("\nRunning DetectFaceDemo");

        // Create a face detector from the cascade file in the resources
        // directory.
        CascadeClassifier faceDetector = new CascadeClassifier(getClass().getResource("/lbpcascade_frontalface.xml").getPath());
        Mat image = Imgcodecs.imread(getClass().getResource("/lena.png").getPath());

        // Detect faces in the image.
        // MatOfRect is a special container class for Rect.
        MatOfRect faceDetections = new MatOfRect();
        faceDetector.detectMultiScale(image, faceDetections);

        System.out.println(String.format("Detected %s faces", faceDetections.toArray().length));

        // Draw a bounding box around each face.
        for (Rect rect : faceDetections.toArray()) {
            rectangle(image, new Point(rect.x, rect.y), new Point(rect.x + rect.width, rect.y + rect.height), new Scalar(0, 255, 0));
        }

        // Save the visualized detection.
        String filename = "faceDetection.png";
        System.out.println(String.format("Writing %s", filename));
        Imgcodecs.imwrite(filename, image);
      }
    }

    public class HelloOpenCV {
      public static void main(String[] args) {
        System.out.println("Hello, OpenCV");

        // Load the native library.
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
        new DetectFaceDemo().run();
     }
    }

Finally we run HelloOpenCV example and see it works! printing lenna’s face detection to faceDetection.png image.

example face detection
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