how to use SIFT in opencv

Question

I am learning C++ and OpenCV these days. Given an image, I want to extract its SIFT features. From http://docs.opencv.org/modules/nonfree/doc/feature_detection.html, we can know

Answer 1

Update for OpenCV 4.2.0 (don’t forget to link opencv_xfeatures2d420.lib, of course)

#include 
#include 
#include 

int main(int argc, char** argv)
{
    const cv::Mat input = cv::imread("input.jpg", 0); //Load as grayscale

    cv::Ptr siftPtr = cv::xfeatures2d::SIFT::create();
    std::vector keypoints;
    siftPtr->detect(input, keypoints);

    // Add results to image and save.
    cv::Mat output;
    cv::drawKeypoints(input, keypoints, output);
    cv::imwrite("sift_result.jpg", output);it.

    return 0;
}