想要首先找到焦距，然后使用opencv android实时检测到人脸的距离

在您添加的图像中，您在手机周围绘制了一个正方形，因此您已经拥有正方形的宽度。我从你的问题中了解到，你想在手机周围得到真正的矩形。

为了实现这一点，可以有几个解决方案，但通过处理轮廓来完成的一个解决方案类似于以下代码：

// localImage would be the cropped image of the square you have drawn,
// the global image is the original image and phoneSquare is the Rect you 
// have drawn
localImage = new Mat(globalImage, phoneSqure).clone();

// make the phone black and surroundings white
Imgproc.threshold(localImage, localImage, 127, 255, Imgproc.THRESH_OTSU + Imgproc.THRESH_BINARY_INV);

// get contours
ArrayList<MatOfPoint> contours = new ArrayList<>();
Mat hierarchy = new Mat();
Imgproc.findContours(canny, contours, hierarchy, Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_NONE);

// sort contours by size and get the biggest which is assumed to be the outer contour of the phone
contours.sort(new Comparator<MatOfPoint>() {
                                 @Override
                                 public int compare(MatOfPoint o1, MatOfPoint o2) {
                                     return (int) Math.signum(o2.size().area() - o1.size().area());
                                 }
                             });
MatOfPoints biggestContour = contours.get(contours.size() - 1);
// get the bounding rectangle of the phone, the you can get the width
Rect whatYouWant = Imgproc.boundingRect(biggestContour);