OpenCV drawContours() function

OpenCV drawContours() does not work properly

void showImage(Mat image) {
    namedWindow("outPut",WINDOW_NORMAL);
    Size size(960,720);
    Mat result;
    resize(image, result, size);
    resizeWindow("outPut",size);
    while (true)
    {
        imshow("outPut", result);
        int exit;
        exit = waitKey(24);
        if (27 == exit) {
            break;
        }
    }
    destroyAllWindows();
}

void m_findContours() {
    Mat image,gray,result, result_1;
    Mat soleColor(720,640, CV_8UC3, Scalar(0, 0, 0));
    double square,circumference;
    image = imread(hand);//0表示单通道
    cvtColor(image, gray, COLOR_BGR2GRAY);
    threshold(gray, result, 150, 255,THRESH_BINARY);
    vector<vector<Point>> contours;//储存查找出来的轮廓点
    vector<Vec4i> hierarchy;//储存层级，每一个vector数据存储四个int值
    findContours(result, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);//result_1是查找出来的轮廓
    vector<vector<Point>> poly(contours.size());
    vector<Point> hull;
    drawContours(soleColor,contours,-1,Scalar(255,255,255), 2);//绘制轮廓时会直接绘制在原图上，第一个drawContours
    for (int i = 0; i < contours.size();i++) {
    approxPolyDP(contours[i], poly[i], 10, false);
    }
    drawContours(soleColor, poly, -1,Scalar(255,0,0), 2);//第二个drawContours
    showImage(soleColor);
    convexHull(contours[0], hull);
    drawContours(soleColor, hull, -1, Scalar(0, 0, 255),2);//第三个drawContours，不能正常完成绘图
    showImage(soleColor);
}

The error is as follows:

But the hull works again by changing the second argument of drawContours to(Mat) in the second row from the bottom, but only displaying one or two points(the red dot in the empty palm). Looking at the description of drawContours' function, it's OK if it's a vector. We have proved that the first drawContours and the second drawcontours can run, but we don't quite understand why we can't draw all the lines if we are not converted to mat, and we can't run if we are not converted to mat.

The comparison of the two copies is as follows: