opencv find perimeter of a connected component - c++

Question

I'm using opencv 2.4.13
I'm trying to find the perimeter of a connected component, I was thinking of using ConnectedComponentWithStats but it doesn't return the perimeter, only the area, width, etc...
There is a method to find the area with the contour but not the opposite (with one component i mean, not the entire image).
The method arcLength doesn't work as well beause i have all the points of the component, not only the contour.
I know there is a BF way to find it by iterating through each pixel of the component and see if he has neighbors which aren't in the same component. But I'd like a function which costs less.
Otherwise, if you know a way to link a component with the contours found by the method findContours, it suits me as well.
Thanks

Answer 1

Adding to #Miki's answer, This is a faster way to find the perimeter of the connected component
//getting the connected components with statistics
cv::Mat1i labels, stats;
cv::Mat centroids;
int lab = connectedComponentsWithStats(img, labels, stats, centroids);
for (int i = 1; i < lab; ++i)
{
//Rectangle around the connected component
cv::Rect rect(stats(i, 0), stats(i, 1), stats(i, 2), stats(i, 3));
// Get the mask for the i-th contour
Mat1b mask_i = labels(rect) == i;
// Compute the contour
vector<vector<Point>> contours;
findContours(mask_i, contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);
if(contours.size() <= 0)
continue;
//Finding the perimeter
double perimeter = contours[0].size();
//you can use this as well for measuring perimeter
//double perimeter = arcLength(contours[0], true);
}

Answer 2

The easiest thing is probably to use findContours.
You can compute the contour on the i-th component computed by connectedComponents(WithStats) , so they are aligned with your labels. Using CHAIN_APPROX_NONE you'll get all the points in the contour, so the size() of the vector is already a measure of the perimeter. You can eventually use arcLength(...) to get a more accurate result:
Mat1i labels;
int n_labels = connectedComponents(img, labels);
for (int i = 1; i < n_labels; ++i)
{
// Get the mask for the i-th contour
Mat1b mask_i = labels == i;
// Compute the contour
vector<vector<Point>> contours;
findContours(mask_i.clone(), contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);
if (!contours.empty())
{
// The first contour (and probably the only one)
// is the one you're looking for
// Compute the perimeter
double perimeter_i = contours[0].size();
}
}