Get List of Black Pixel of a cv::Mat - c++

Question

I' am actually working with a cv::Mat with B&W pixels.
I'am searching for a way to get a list of my black point in this Mat.
Does someone know how to do such thing ?
I want to do that because I want to detect the bounding rect of this points.
(The best is to get them back in a vector)
somekind of :
cv::Mat blackAndWhite;
std::vector<cv::Point> blackPixels = MAGIC_FUNCTION(blackAndWhite);
Thanks for your help.
Edit: I want to precise that I want the best practices, the more Opencv compliant as possible.

Answer 1

You can traverse the cv::Mat to check the pixels that are 0, and get the x and y coordinates from the linear index if the matrix is continuous in memory:
// assuming your matrix is CV_8U, and black is 0
std::vector<cv::Point> blackPixels;
unsigned int const *p = blackAndWhite.ptr<unsigned char>();
for(int i = 0; i < blackAndWhite.rows * blackAndWhite.cols; ++i, ++p)
{
if(*p == 0)
{
int x = i % blackAndWhite.cols;
int y = i / blackAndWhite.cols;
blackPixels.push_back(cv::Point(x, y));
}
}

Answer 2

This example from OpenCV shows how to do exactly what you want: Creating Bounding boxes and circles for contours. Basically, it this:
// ...
/// Find contours
findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
/// Approximate contours to polygons + get bounding rects and circles
vector<vector<Point> > contours_poly( contours.size() );
vector<Rect> boundRect( contours.size() );
vector<Point2f>center( contours.size() );
vector<float>radius( contours.size() );
for( int i = 0; i < contours.size(); i++ )
{
approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
boundRect[i] = boundingRect( Mat(contours_poly[i]) );
minEnclosingCircle( (Mat)contours_poly[i], center[i], radius[i] );
}