What is the output vector of the convexHull function in OpenCV? - c++

Question

C++:
void
convexHull(InputArray points,
OutputArray hull,
bool clockwise=false,
bool returnPoints=true);
The description given on OutputArray hull is as follows:
hull – Output convex hull. It is either an integer vector of indices or vector of points. In the first case, the hull elements are 0-based indices of the convex hull points in the original array (since the set of convex hull points is a subset of the original point set). In the second case, hull elements are the convex hull points themselves.
So what is integer vector of indices ?
If I use the output array as a vector<vector<int>>, what do I get in it?
Can I print the results?

Answer 1

Answering to your specific questions:
Integer vector of indices (where index starts from 0) are the indices that indicates which points from InputArray points are in the set of convex hull points.
You can use either a vector of integer or vector of points. In the first case, you get the indices that allow to access to the actual point given the input array of points. In the second case, you can read directly the coordinates of the points from the output array.
This question is not entirely clear as you don't mention where you want to print the results. Assuming that you want to show it in an image, you can draw the convex hull with polylines. Specifically (look at cv::polylines for more information):
void cv::polylines (
InputOutputArray img,
InputArrayOfArrays pts,
bool isClosed,
const Scalar & color,
int thickness = 1,
int lineType = LINE_8,
int shift = 0
)
To print the coordinates of the points in the console, assuming that the output vector is a vector of integers, thus indices:
size_t hull_size = hull.size();
for (size_t i = 0; i < hull_size; i++)
{
std::cout << points[hull[i]] << std::endl;
}