Access to image values using a Mat of coordinates (OpenCV/C++)

Question

In openCV(C++) I have an image IMG and I'd like to extract the values of each pixel at positions store in coor . Is it possible to get this pixel values in an efficient way? Is a foor loop (using .at(i,j) ) efficient ?Is there a built-in function for doing this? This is my code:

cv::Mat IMG = cv::imread("image.png",CV_LOAD_IMAGE_GRAYSCALE);
cv::Mat coor = (cv::Mat_<float>(3,2) << 1, 0, 0, 1, 2, 2); // search coordinates
cv::Mat pixel_values;
// and I'd like to do something like this:
//This should result in a matrix with the same size as *coor*
pixel_values = IMG.at(coor); // similar to the matrix accesing method in Matlab. 

Answer 1

Threr are different ways to access the value of a "pixel" in a cv::Mat. Have you read the documentation of OpenCV?

I suggest you to start form here

I've corrected the code: this code works with msvc 2015 and opencv (3.1) but also > 2 is ok 我已经更正了代码：此代码适用于msvc 2015和opencv（3.1），但> 2也可以

#include <iostream>
#include <vector>
#include <opencv2/opencv.hpp>
#include <cstdint>
using namespace std;

#if defined(NDEBUG)
#pragma comment(lib, "opencv_world310.lib")
#else
#pragma comment(lib, "opencv_world310d.lib")
#endif // 


cv::Mat GetPixelsFromMat( const cv::Mat& I, const std::vector<cv::Point2f>& points )
{
    // some pre-condition:
    cv::Mat res( 1, (int)points.size( ), CV_8UC1 );

    int i = 0;
    for( const auto& point : points )
        res.ptr( 0 )[ i++ ] = I.at<uchar>( cvRound( point.y ), cvRound( point.x ) );

    return res;
}

int main( int argc, char* argv[] )
{
    cv::Mat testImg( 1, 10, CV_8UC1 );
    for( int i = 0; i < 10; ++i )
        testImg.ptr( 0 )[ i ] = i;

    std::vector<cv::Point2f> points;
    points.push_back( cv::Point2f( 1, 0 ) );
    points.push_back( cv::Point2f( 5, 0 ) );
    points.push_back( cv::Point2f( 9, 0 ) );

    cv::Mat pixelsMap = GetPixelsFromMat( testImg, points );

    if( pixelsMap.ptr( 0 )[ 0 ] == 1 )
        cout << "OK 0" << endl;
    else
        cout << "FAIL 0" << endl;

    if( pixelsMap.ptr( 0 )[ 1 ] == 5 )
        cout << "OK 1" << endl;
    else
        cout << "FAIL 1" << endl;

    if( pixelsMap.ptr( 0 )[ 2 ] == 9 )
        cout << "OK 2" << endl;
    else
        cout << "FAIL 2";


    return EXIT_SUCCESS;
}

This is a big semplification but I think it's a starting point.

Answer 2

If you already know the type of your matrix, you can easily go with @elvis.dukaj answer . You just need to iterate over your coordinates and retrieve corresponding pixel value.

But, if you need something more general that supports:

• an input matrix of any type and channels
• an input list of coordinates being either a cv::Mat or a std::vector , with coordinates of any type ( int , float , etc...)

you may find this getPixels function useful:

#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;

Mat getPixels(InputArray _src, InputArray _coords)
{
    // Get src matrix
    Mat src = _src.getMat();

    // Get coords matrix, eventually convert to integer type
    Mat coords = _coords.getMat();
    if (coords.type() != CV_32S)
    {
        coords.convertTo(coords, CV_32S);
    }
    // Check coordinates
    int n = coords.checkVector(2, CV_32S);
    CV_Assert(n > 0);

    // Create output matrix
    Mat dst(n, 1, src.type());

    // Get pixel values at given coordinates
    int esz = src.elemSize();
    Point* coords_ptr = (Point*)coords.data;
    uchar* dst_ptr = dst.data;

    for (int i = 0; i < n; i++, dst_ptr += esz)
    {
        memcpy(dst_ptr, src.ptr(coords_ptr[i].y) + coords_ptr[i].x * esz, esz);
    }

    return dst;
}


int main() 
{

    Mat img(5, 5, CV_8UC1);
    randu(img, Scalar(0, 0, 0), Scalar(10, 10, 10));

    vector<Point> coor{ Point(1, 0), Point(0, 1), Point(2, 2) };
    //cv::Mat coor = (cv::Mat_<float>(3, 2) << 1, 0, 0, 1, 2, 2);

    Mat out = getPixels(img, coor);

    cout << out;

    return 0;
}