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).