Passing OpenCV contours from a JNI C++ function to Java in Android

Question

What is the best way to pass OpenCV vector< std::vector<Point > > contours from a JNI C++ function to Java in Android? My current approach is to use arrays of doubles but this is very inefficient. Is there a way to use pointers maybe?

There are no pointers in java. You can store a pointer value into a java long but that only helps to keep the reference. — Fildor, Dec 02 '14 at 06:14
Maybe this answer helps you http://stackoverflow.com/a/23116736/2455351 (It's done for passing a vector) — migue02, Dec 02 '14 at 07:51
FYI, we can directly access that type in Java using [JavaCPP](https://github.com/bytedeco/javacpp/): http://bytedeco.org/javacpp-presets/opencv/apidocs/org/bytedeco/javacpp/class-use/opencv_core.PointVectorVector.html — Samuel Audet, Dec 06 '14 at 08:13
@SamuelAudet your comment is closer to what I was looking for, but I've never used JavaCPP. If you post an answer where you elaborate more on this, I will select it. — Jaime Ivan Cervantes, Dec 07 '14 at 18:53

score 2 · Answer 1 · answered Dec 02 '14 at 07:56

Well, you can create a class at Java side equivalent to vector< std::vector<Point > > in C++ side. Then, write a serializer function in C++ and deserailizer method in Java.

Your serializer can be a String composed of double values which are seperated by , and ;, which can be deserialized easliy in Java.

In this way, instead of sending multiple arrays you can sen just one string. You need to be careful with the precision of double when converting it to double and converting it back.

Samuel Audet · Accepted Answer · 2014-12-29T13:53:25.747

Here's an efficient way to access contours with the wrapper from the JavaCPP Presets for OpenCV:

import org.bytedeco.javacpp.indexer.*;
import static org.bytedeco.javacpp.opencv_core.*;
import static org.bytedeco.javacpp.opencv_imgproc.*;
import static org.bytedeco.javacpp.opencv_highgui.*;

public class Contours {
    public static void main(String[] args) {
        Mat grayscaleImage = imread("lena.png", CV_LOAD_IMAGE_GRAYSCALE);
        Mat binarizedImage = new Mat();
        MatVector contours = new MatVector();
        threshold(grayscaleImage, binarizedImage, 128, 255, THRESH_BINARY);
        findContours(binarizedImage, contours, RETR_LIST, CHAIN_APPROX_NONE);
        int contoursSize = (int)contours.size();
        System.out.println("size = " + contoursSize);
        for (int contourIdx = 0; contourIdx < contoursSize; contourIdx++) {
            // compute center
            float x = 0, y = 0;
            Mat contour = contours.get(contourIdx);
            IntIndexer points = contour.createIndexer(false);
            int pointsSize = contour.rows();
            for (int pointIdx = 0; pointIdx < pointsSize; pointIdx++) {
                x += points.get(pointIdx, 0);
                y += points.get(pointIdx, 1);
            }
            System.out.println("center = (" + x / pointsSize + ", " + y / pointsSize + ")");
        }
    }
}

For simplicity, I'm using Java SE, but we can do the same thing on Android.

Passing OpenCV contours from a JNI C++ function to Java in Android

2 Answers2