OpenCV Java-如何裁剪图像中检测到的圆

Question

I am using the following algorithm from OpenCV.

1) Detect circles in the image.

2) Draw the detected circle on the image. (This is already done, the code is as follow).

3) Crop the detected region.

But, the question is how can i crop out this detected region on the image?

Answer 1

1. create a mask:

Mat mask = new Mat(src.rows(), src.cols(), CvType.CV_8U, Scalar.all(0));

2. Draw the circle on that mask (set thickness to -1 to fill the circle):

Imgproc.circle(mask, center, radius, new Scalar(255,255,255), -1, 8, 0 );

3. Copy the image using the mask:

Mat masked = new Mat();
src.copyTo( masked, mask );

4. Apply Threshold

Mat thresh = new Mat();
Imgproc.threshold( mask, thresh, 1, 255, Imgproc.THRESH_BINARY );

5. Find contour

List<MatOfPoint> contours = new ArrayList<>();
Imgproc.findContours(thresh, contours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);

6. Crop

Rect rect = Imgproc.boundingRect(contours.get(0));
Mat cropped = masked.submat(rect);

Full code example (OpenCV 3.4):

import org.opencv.core.*;
import org.opencv.core.Point;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

import java.util.ArrayList;
import java.util.List;

class HoughCirclesRun {
    public void run(String[] args) {
        String filename = "smartie.png";
        // Load an image
        Mat src = Imgcodecs.imread(filename, Imgcodecs.IMREAD_COLOR);
        // Check if image is loaded fine
        if( src.empty() ) {
            System.out.println("Error opening image!");
        }

        Mat gray = new Mat();
        Imgproc.cvtColor(src, gray, Imgproc.COLOR_BGR2GRAY);
        Imgproc.medianBlur(gray, gray, 5);
        Mat circles = new Mat();
        Imgproc.HoughCircles(gray, circles, Imgproc.HOUGH_GRADIENT, 1.0,
                (double)gray.rows()/16, // change this value to detect circles with different distances to each other
                100.0, 30.0, 1, 30); // change the last two parameters (min_radius & max_radius) to detect larger circles

        Mat mask = new Mat(src.rows(), src.cols(), CvType.CV_8U, Scalar.all(0));

        for (int x = 0; x < circles.cols(); x++) {
            double[] c = circles.get(0, x);
            Point center = new Point(Math.round(c[0]), Math.round(c[1]));
            // circle outline
            int radius = (int) Math.round(c[2]);
            Imgproc.circle(mask, center, radius, new Scalar(255,255,255), -1, 8, 0 );
        }

        Mat masked = new Mat();
        src.copyTo( masked, mask );

        // Apply Threshold
        Mat thresh = new Mat();
        Imgproc.threshold( mask, thresh, 1, 255, Imgproc.THRESH_BINARY );

        // Find Contour
        List<MatOfPoint> contours = new ArrayList<>();
        Imgproc.findContours(thresh, contours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);

        // Crop
        Rect rect = Imgproc.boundingRect(contours.get(0));
        Mat cropped = masked.submat(rect);

        HighGui.imshow("Cropped circle", cropped);
        HighGui.waitKey();
        System.exit(0);
    }
}
public class CropCircle {
    public static void main(String[] args) {
        // Load the native library.
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
        new HoughCirclesRun().run(args);
    }
}