Drawing Polygons in OpenCV? - c++

Question

What am I doing wrong here?
vector <vector<Point> > contourElement;
for (int counter = 0; counter < contours -> size (); counter ++)
{
contourElement.push_back (contours -> at (counter));
const Point *elementPoints [1] = {contourElement.at (0)};
int numberOfPoints [] = {contourElement.at (0).size ()};
fillPoly (contourMask, elementPoints, numberOfPoints, 1, Scalar (0, 0, 0), 8);
I keep getting an error on the const Point part. The compiler says
error: cannot convert 'std::vector<cv::Point_<int>, std::allocator<cv::Point_<int> > >' to 'const cv::Point*' in initialization
What am I doing wrong? (PS: Obviously ignore the missing bracket at the end of the for loop due to this being only part of my code)

Answer 1

Just for the record (and because the opencv docu is very sparse here) a more reduced snippet using the c++ API:
std::vector<cv::Point> fillContSingle;
[...]
//add all points of the contour to the vector
fillContSingle.push_back(cv::Point(x_coord,y_coord));
[...]
std::vector<std::vector<cv::Point> > fillContAll;
//fill the single contour
//(one could add multiple other similar contours to the vector)
fillContAll.push_back(fillContSingle);
cv::fillPoly( image, fillContAll, cv::Scalar(128));

Answer 2

Let's analyse the offending line:
const Point *elementPoints [1] = { contourElement.at(0) };
You declared contourElement as vector <vector<Point> >, which means that contourElement.at(0) returns a vector<Point> and not a const cv::Point*. So that's the first error.
In the end, you need to do something like:
vector<Point> tmp = contourElement.at(0);
const Point* elementPoints[1] = { &tmp[0] };
int numberOfPoints = (int)tmp.size();
Later, call it as:
fillPoly (contourMask, elementPoints, &numberOfPoints, 1, Scalar (0, 0, 0), 8);

Answer 3

contourElement is vector of vector<Point> and not Point :)
so instead of:
const Point *elementPoints
put
const vector<Point> *elementPoints

Answer 4

Some people may arrive here due to an apparently bug in the samples/cpp/create_mask.cpp from the OpenCV. This way, considering the above explained I edited the "if (event == EVENT_RBUTTONUP)" branch para to:
...
mask = Mat::zeros(src.size(), CV_8UC1);
vector<Point> tmp = pts;
const Point* elementPoints[1] = { &tmp[0] };
int npts = (int) pts.size();
cout << "elementsPoints=" << elementPoints << endl;
fillPoly(mask, elementPoints, &npts, 1, Scalar(255, 255, 255), 8);
bitwise_and(src, src, final, mask);
...
Hope it may help someone.