How to find Image Contours using Java OpenCV library?

Contours are nothing but the line joining all the points along the boundary of a particular shape. Using this you can −

• Find the shape of an object.

• Calculate the area of an object.

• Detect an object.

• Recognize an object.

You can find the contours of various shapes, objects in an image using the findContours() method. This method accepts the following parameters −

• A binary image.

• An empty list object of type MatOfPoint to store the contours.

• An empty Mat object to store the image topology.

• Two integer variables to specify the mode and method to find the contours of the given image.

Example

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.MatOfPoint;
import org.opencv.core.Scalar;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
public class FindingContours {
   public static void main(String args[]) throws Exception {
      //Loading the OpenCV core library
      System.loadLibrary( Core.NATIVE_LIBRARY_NAME );
      String file ="D:\\Images\\shapes.jpg";
      Mat src = Imgcodecs.imread(file);
      //Converting the source image to binary
      Mat gray = new Mat(src.rows(), src.cols(), src.type());
      Imgproc.cvtColor(src, gray, Imgproc.COLOR_BGR2GRAY);
      Mat binary = new Mat(src.rows(), src.cols(), src.type(), new Scalar(0));
      Imgproc.threshold(gray, binary, 100, 255, Imgproc.THRESH_BINARY_INV);
      //Finding Contours
      List<MatOfPoint> contours = new ArrayList<>();
      Mat hierarchey = new Mat();
      Imgproc.findContours(binary, contours, hierarchey, Imgproc.RETR_TREE,
      Imgproc.CHAIN_APPROX_SIMPLE);
      Iterator<MatOfPoint> it = contours.iterator();
      while(it.hasNext()) {
         System.out.println(it.next());
      }
      /*
      Mat draw = Mat.zeros(binary.size(), CvType.CV_8UC3);
      for (int i = 0; i < contours.size(); i++) {
         System.out.println(contours);
         Scalar color = new Scalar(0, 0, 255);
         //Drawing Contours
         Imgproc.drawContours(draw, contours, i, color, 2, Imgproc.LINE_8, hierarchey, 2, new Point() ) ;
      }
      HighGui.imshow("Contours operation", draw);
      HighGui.waitKey();
      */
   }
}

Output

Mat [ 29*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829510,
dataAddr=0x19826dc0 ]
Mat [ 58*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829580,
dataAddr=0x19826f00 ]
Mat [ 35*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19828be0,
dataAddr=0x19827100 ]
Mat [ 117*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829190,
dataAddr=0x19827280 ]
Mat [ 1*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x198292e0,
dataAddr=0xba8280 ]
Mat [ 78*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829350,
dataAddr=0x19827680 ]
Mat [ 63*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x198289b0,
dataAddr=0x19827940 ]
Mat [ 120*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19828e80,
dataAddr=0x19827b80 ]
Mat [ 4*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829430,
dataAddr=0xb84580 ]
Mat [ 4*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19829120,
dataAddr=0xb84440 ]
Mat [ 136*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19828ef0,
dataAddr=0x19827f80 ]
Mat [ 120*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x19828b00,
dataAddr=0x19828440 ]