用这个代码Rect rect = DetectRectangle(canny_Image, out points);实现

这段代码的作用是利用Canny算法检测图像中的矩形，并返回四个点的坐标。其中，canny_Image是经过Canny算法处理后的图像。可以参考以下示例代码：

// 读取图像
Mat image = Cv2.ImRead("test.jpg");

// 灰度化
Mat gray = new Mat();
Cv2.CvtColor(image, gray, ColorConversionCodes.BGR2GRAY);

// 边缘检测
Mat edges = new Mat();
Cv2.Canny(gray, edges, 50, 200);

// 检测矩形
Point[] points;
Rect rect = DetectRectangle(edges, out points);

// 在图像上绘制矩形
Cv2.Polylines(image, new Point[][] { points }, true, Scalar.Red);

// 显示图像
Cv2.ImShow("result", image);
Cv2.WaitKey();

其中，DetectRectangle函数的实现可以参考以下代码：

public static Rect DetectRectangle(Mat image, out Point[] points)
{
    // 轮廓检测
    Point[][] contours;
    HierarchyIndex[] hierarchy;
    Cv2.FindContours(image, out contours, out hierarchy, RetrievalModes.External, ContourApproximationModes.ApproxSimple);

    // 筛选矩形
    List<Point[]> rectangles = new List<Point[]>();
    for (int i = 0; i < contours.Length; i++)
    {
        Point[] approxCurve = Cv2.ApproxPolyDP(contours[i], Cv2.ArcLength(contours[i], true) * 0.02, true);
        if (approxCurve.Length == 4 && Cv2.IsContourConvex(approxCurve))
        {
            double cos = Math.Abs(Cv2.ContourArea(approxCurve)) / (Cv2.ArcLength(approxCurve, true) * Cv2.ArcLength(approxCurve, true));
            if (cos < 0.1)
            {
                rectangles.Add(approxCurve);
            }
        }
    }

    // 计算四边形的重心
    Point[] centerPoints = new Point[rectangles.Count];
    for (int i = 0; i < rectangles.Count; i++)
    {
        centerPoints[i] = new Point((rectangles[i][0].X + rectangles[i][2].X) / 2, (rectangles[i][0].Y + rectangles[i][2].Y) / 2);
    }

    // 根据距离筛选最大的矩形
    Point centerPoint = new Point(image.Cols / 2, image.Rows / 2);
    double maxDistance = 0;
    Point[] maxRectangle = null;
    for (int i = 0; i < rectangles.Count; i++)
    {
        double distance = Math.Sqrt((centerPoints[i].X - centerPoint.X) * (centerPoints[i].X - centerPoint.X) + (centerPoints[i].Y - centerPoint.Y) * (centerPoints[i].Y - centerPoint.Y));
        if (distance > maxDistance)
        {
            maxDistance = distance;
            maxRectangle = rectangles[i];
        }
    }

    // 返回矩形四个点的坐标
    points = maxRectangle;
    return Cv2.BoundingRect(maxRectangle);
}

这个函数使用了轮廓检测和多边形拟合的方法，可以有效地检测图像中的矩形。