How can I convert a cv::Mat to a gray scale in OpenCv?

Question

How can I convert a cv::Mat to a gray scale?

I am trying to run drawKeyPoints func from opencv, however I have been getting an Assertion Filed error. My guess is that it needs to receive a gray scale image rather than a color image in the parameter.

void SurfDetector(cv::Mat img){ vector<cv::KeyPoint> keypoints; cv::Mat featureImage;  cv::drawKeypoints(img, keypoints, featureImage, cv::Scalar(255,255,255) ,cv::DrawMatchesFlags::DRAW_RICH_KEYPOINTS);  cv::namedWindow("Picture"); cv::imshow("Picture", featureImage); 

}

Answer 1

Using the C++ API, the function name has slightly changed and it writes now:

#include <opencv2/imgproc/imgproc.hpp>  cv::Mat greyMat, colorMat; cv::cvtColor(colorMat, greyMat, CV_BGR2GRAY); 

The main difficulties are that the function is in the imgproc module (not in the core), and by default cv::Mat are in the Blue Green Red (BGR) order instead of the more common RGB.

OpenCV 3

Starting with OpenCV 3.0, there is yet another convention. Conversion codes are embedded in the namespace cv:: and are prefixed with COLOR. So, the example becomes then:

#include <opencv2/imgproc/imgproc.hpp>  cv::Mat greyMat, colorMat; cv::cvtColor(colorMat, greyMat, cv::COLOR_BGR2GRAY); 

As far as I have seen, the included file path hasn't changed (this is not a typo).

Answer 2

May be helpful for late comers.

#include "stdafx.h" #include "cv.h" #include "highgui.h"  using namespace cv; using namespace std;  int main(int argc, char *argv[]) {   if (argc != 2) {     cout << "Usage: display_Image ImageToLoadandDisplay" << endl;     return -1; }else{     Mat image;     Mat grayImage;      image = imread(argv[1], IMREAD_COLOR);     if (!image.data) {         cout << "Could not open the image file" << endl;         return -1;     }     else {         int height = image.rows;         int width = image.cols;          cvtColor(image, grayImage, CV_BGR2GRAY);           namedWindow("Display window", WINDOW_AUTOSIZE);         imshow("Display window", image);          namedWindow("Gray Image", WINDOW_AUTOSIZE);         imshow("Gray Image", grayImage);         cvWaitKey(0);         image.release();         grayImage.release();         return 0;     }    }  }