How to save to vector coordinates of white pixels? - c++

Question

I want to loop through a binarized cv::Mat and save all coordinates of pixels with a value of 255.
cv::Mat bin;
std::vector<cv::Point2i> binVec;
int h = 0;
int white = 254; //Just for comparison with pointer of Matrix value
for (int i = 0; i < bin.rows; i++, h++) {
for (int j = 0; j < bin.cols; j++, h++) {
int* p = bin.ptr<int>(h); //Pointer to bin Data, should loop through Matrix
if (p >= &white) //If a white pixel has been found, push i and j in binVec
binVec.push_back(cv::Point2i(i, j));
}
}
This snippet is not working, and I don't know why.
Exception thrown at 0x76C6C42D in example.exe: Microsoft C++ exception: cv::Exception at memory location 0x0019E4F4.
Unhandled exception at 0x76C6C42D in example.exe: Microsoft C++ exception: cv::Exception at memory location 0x0019E4F4.
So how can I count h and let the pointer work?

Answer 1

You can avoid to scan the image. To save the coordinates of all white pixels in a vector you can do like:
Mat bin;
// fill bin with some value
std::vector<Point> binVec;
findNonZero(bin == 255, binVec);
You can use Point instead of Point2i, since they are the same:
typedef Point2i Point;
If you really want to use a for loop, you should do like:
const uchar white = 255;
for (int r = 0; r < bin.rows; ++r)
{
uchar* ptr = bin.ptr<uchar>(r);
for(int c = 0; c < bin.cols; ++c)
{
if (ptr[c] == 255) {
binVec.push_back(Point(c,r));
}
}
}
Remember that:
you binary image is probably CV_8UC1, and not a CV_32SC1, so you should use uchar instead of int.
bin.ptr<...>(i) gives you a pointer to the start of the i-th row, so you should take it out of the inner loop.
you should compare the values, not the address.
Point take as parameters x (cols) and y (rows), while you are passing i (rows) and j (cols). So you need to swap them.
this loop can be further optimized, but for your task I strongly recommend the findNonZero approach, so I don't show it here.

Answer 2

You should only increment h in the inner loop
You should compare the value pointed at by p with h, not compare p with the address of h.
So
cv::Mat bin;
std::vector<cv::Point2i> binVec;
int h = 0;
int white = 254; //Just for comparison with pointer of Matrix value
for (int i = 0; i < bin.rows; i++) {
for (int j = 0; j < bin.cols; j++) {
int* p = bin.ptr<int>(h++); //Pointer to bin Data, should loop through Matrix
if (*p >= white) //If a white pixel has been found, push i and j in binVec
binVec.push_back(cv::Point2i(i, j));
}
}