Home How to draw a rectangle containing an object in Android (Java, OpenCV)
 Basically, I want to achieve this, and so far, I've written the following Java code...  // Display the camera frame public Mat onCameraFrame(CvCameraViewFrame inputFrame) { // The object's width and height are set to 0 objectWidth = objectHeight = 0; // frame is captured as a coloured image frame = inputFrame.rgba(); /** Since the Canny algorithm only works on greyscale images and the captured image is * coloured, we transform the captured cam image into a greyscale one */ Imgproc.cvtColor(frame, grey, Imgproc.COLOR_RGB2GRAY); // Calculating borders of image using the Canny algorithm Imgproc.Canny(grey, canny, 180, 210); /** To avoid background noise (given by the camera) that makes the system too sensitive * small variations, the image is blurred to a small extent. Blurring is one of the * required steps before any image transformation because this eliminates small details * that are of no use. Blur is a low-pass filter. */ Imgproc.GaussianBlur(canny, canny, new Size(5, 5), 5); // Calculate the contours Imgproc.findContours(canny, contours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE); /** The contours come in different sequences * 1 sequence for each connected component. * Taking the assumption only 1 object is in view, if we have more than 1 connected * component, this'll be considered part of the details of the object. * * For this, we put all contours together in a single sequence * If there is at least 1 contour, I can continue processing */ for (MatOfPoint mat : contours) { // Retrieve and store all contours in one giant map mat.copyTo(allContours); } MatOfPoint2f allCon = new MatOfPoint2f(allContours.toArray()); // Calculating the minimal rectangle to contain the contours RotatedRect box = Imgproc.minAreaRect(allCon); // Getting the vertices of the rectangle Point[] vertices = initialiseWithDefaultPointInstances(4); box.points(vertices); // Now the vertices are in possession, temporal smoothing can be performed. for (int i = 0; i < 4; i++) { // Smooth coordinate x of the vertex vertices[i].x = alpha * lastVertices[i].x + (1.0 - alpha) * vertices[i].x; // Smooth coordinate y of the vertex vertices[i].y = alpha * lastVertices[i].y + (1.0 - alpha) * vertices[i].y; // Assign the present smoothed values as lastVertices for the next smooth lastVertices[i] = vertices[i]; } /** With the vertices, the object size is calculated. * The object size is calculated through pythagoras theorm. In addition, it gives * the distance between 2 points in a bi-dimensional space. * * For a rectangle, considering any vertex V, its two sizes (width and height) can * be calculated by calculating the distance of V from the previous vertex and * calculating the distance of V from the next vertex. This is the reason why I * calculate the distance between vertici[0]/vertici[3] and vertici[0]/vertici[1] */ objectWidth = (int) (conversionFactor * Math.sqrt((vertices[0].x - vertices[3].x) * (vertices[0].x - vertices[3].x) + (vertices[0].y - vertices[3].y) * (vertices[0].y - vertices[3].y))); objectHeight = (int) (conversionFactor * Math.sqrt((vertices[0].x - vertices[1].x) * (vertices[0].x - vertices[1].x) + (vertices[0].y - vertices[1].y) * (vertices[0].y - vertices[1].y))); /** Draw the rectangle containing the contours. The line method draws a line from 1 * point to the next, and accepts only integer coordinates; for this reason, 2 * temporary Points have been created and why I used Math.round method. */ Point pt1 = new Point(); Point pt2 = new Point(); for (int i = 0; i < 4; i++) { pt1.x = Math.round(vertices[i].x); pt1.y = Math.round(vertices[i].y); pt2.x = Math.round(vertices[(i + 1) % 4].x); pt2.y = Math.round(vertices[(i + 1) % 4].y); Imgproc.line(frame, pt1, pt2, red, 3); } //If the width and height are non-zero, then print the object size on-screen if (objectWidth != 0 && objectHeight != 0) { String text; text = String.format("%d x %d", objectWidth, objectHeight); widthValue.setText(text); } // This function must return return frame; } // Initialising an array of points public static Point[] initialiseWithDefaultPointInstances(int length) { Point[] array = new Point[length]; for (int i = 0; i < length; i++) { array[i] = new Point(); } return array; }  What I want to achieve is drawing a rectangle on-screen that contains the object's contours (edges). If anyone knows the answer to my question, please feel free to comment below, as I have been stuck on this for a couple of hours