Add Java demo for YuNet face detectionFeat java yunet demo

models/face_detection_yunet/demo/java/demo.java

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+import org.opencv.core.*;
+import org.opencv.objdetect.FaceDetectorYN;
+import org.opencv.imgcodecs.Imgcodecs;
+import org.opencv.imgproc.Imgproc;
+import org.opencv.highgui.HighGui;
+import org.opencv.videoio.VideoCapture;
+
+public class demo {
+    public static void main(String[] args) {
+        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
+        String modelPath = "../../face_detection_yunet_2023mar.onnx";
+        String imagePath = "../../example_outputs/largest_selfie.jpg";
+        FaceDetectorYN detector = FaceDetectorYN.create(modelPath, "", new Size(320, 320), 0.9f, 0.3f, 5000);
+        Mat img = Imgcodecs.imread(imagePath);
+        if (img.empty()) {
+            System.out.println("Could not read the image, opening camera...");
+            runCamera(detector);
+        } else {
+            runImage(detector, img);
+        }
+    }
+
+    private static void runImage(FaceDetectorYN detector, Mat img) {
+        detector.setInputSize(img.size());
+        Mat faces = new Mat();
+        detector.detect(img, faces);
+
+        visualize(img, faces);
+        HighGui.imshow("Yunet Java Demo - Image", img);
+        HighGui.waitKey(0);
+        System.exit(0);
+    }
+
+    private static void runCamera(FaceDetectorYN detector) {
+        VideoCapture cap = new VideoCapture(0);
+        Mat frame = new Mat();
+        if (!cap.isOpened()) {
+            System.out.println("Could not open camera");
+            return;
+        }
+        while (true) {
+            cap.read(frame);
+            if (frame.empty()) {
+                System.out.println("No captured frame -- Break!");
+                break;
+            }
+            detector.setInputSize(frame.size());
+            Mat faces = new Mat();
+            detector.detect(frame, faces);
+
+            visualize(frame, faces);
+            HighGui.imshow("Yunet Java Demo - Camera", frame);
+            if (HighGui.waitKey(30) >= 0) {
+                break;
+            }
+        }
+        cap.release();
+    }
+
+    private static void visualize(Mat img, Mat faces) {
+        for (int i = 0; i < faces.rows(); i++) {
+            float[] data = new float[15];
+            faces.get(i, 0, data);
+
+            Imgproc.rectangle(img, new Rect((int) data[0], (int) data[1], (int) data[2], (int) data[3]),
+                    new Scalar(0, 255, 0), 2);
+            for (int j = 0; j < 5; j++) {
+                Point p = new Point(data[4 + j * 2], data[5 + j * 2]);
+                Imgproc.circle(img, p, 2, new Scalar(255, 0, 0), 2);
+            }
+        }
+    }
+}