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I have found some rule about this problem, this wrong line connects two point and do not cross any others contour points.

It seams that drawContours prefer convex shape, but I am not very sure about it.

std::vector<std::vector<Point>> collision_for_mask2_contours;

test for this variable, I have put it in bmp image in accessory file , you can get points from it for drawContours. Thank you very much!

G:\test.bmp

I have found some rule about this problem, this wrong line connects two point and do not cross any others contour points.

It seams that drawContours prefer convex shape, but I am not very sure about it.

std::vector<std::vector<Point>> collision_for_mask2_contours;

test for this variable, I have put it in bmp image in accessory file , you can get points from it for drawContours. Thank you very much!

G:\test.bmp

I have found some rule about this problem, this wrong line connects two point and do not cross any others contour points.

It seams that drawContours prefer convex shape, but I am not very sure about it.

std::vector<std::vector<Point>> collision_for_mask2_contours;

test for this variable, I have put it in bmp image in accessory file , you can get points from it for drawContours. Thank you very much!

G:\test.bmp

int  main(int argc, char* argv[])

// test code for drawContours

std::vector<std::vector<Point>> collision_for_mask2_contours;
std::vector<Point> constours;
Mat_<uchar> testbmp = imread("G:/test.bmp", WINDOW_AUTOSIZE);
namedWindow("testbmp");
imshow("testbmp", testbmp);
waitKey(0); // need to press any key to continue
for (int y = 0; y < testbmp.rows; ++y) {
    for (int x = 0; x < testbmp.cols; ++x) {
        if (testbmp(y, x)) {
            constours.push_back(Point(x, y));
        }
    }
}
collision_for_mask2_contours.push_back(constours);
Mat_<uchar> testForDrawContours = Mat_<uchar>::zeros(testbmp.size());
// one case, can not get concave shape
drawContours(testForDrawContours, collision_for_mask2_contours, -1, Scalar(255));
namedWindow("testForDrawContours", WINDOW_AUTOSIZE);
imshow("testForDrawContours", testForDrawContours);
waitKey(0);
// one another case, also can not get concave shape
drawContours(testForDrawContours, collision_for_mask2_contours, -1, Scalar(255), -1, 8);
namedWindow("testForDrawContours2", WINDOW_AUTOSIZE);
imshow("testForDrawContours2", testForDrawContours);
waitKey(0);

I have found some rule about this problem, this wrong line connects two point and do not cross any others contour points.

It seams that drawContours prefer convex shape, but I am not very sure about it.

std::vector<std::vector<Point>> collision_for_mask2_contours;

test for this variable, I have put it in bmp image in accessory file , you can get points from it for drawContours. Thank you very much!

G:\test.bmp

int  main(int argc, char* argv[])

{

argv[]){

// test code for drawContours

std::vector<std::vector<Point>> collision_for_mask2_contours;
std::vector<Point> constours;
Mat_<uchar> testbmp = imread("G:/test.bmp", WINDOW_AUTOSIZE);
namedWindow("testbmp");
imshow("testbmp", testbmp);
waitKey(0); // need to press any key to continue
for (int y = 0; y < testbmp.rows; ++y) {
    for (int x = 0; x < testbmp.cols; ++x) {
        if (testbmp(y, x)) {
            constours.push_back(Point(x, y));
        }
    }
}
collision_for_mask2_contours.push_back(constours);
Mat_<uchar> testForDrawContours = Mat_<uchar>::zeros(testbmp.size());
// one case, can not get concave shape
drawContours(testForDrawContours, collision_for_mask2_contours, -1, Scalar(255));
namedWindow("testForDrawContours", WINDOW_AUTOSIZE);
imshow("testForDrawContours", testForDrawContours);
waitKey(0);
// one another case, also can not get concave shape
drawContours(testForDrawContours, collision_for_mask2_contours, -1, Scalar(255), -1, 8);
namedWindow("testForDrawContours2", WINDOW_AUTOSIZE);
imshow("testForDrawContours2", testForDrawContours);
waitKey(0);
waitKey(0);}