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 lt cv  KeyPoint gt  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

User · Accepted Answer

Using the C   API  the function name has slightly changed and it writes now     include  lt opencv2 imgproc imgproc hpp gt   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  lt opencv2 imgproc imgproc hpp gt   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

User · Answer

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  lt  lt   Usage  display Image ImageToLoadandDisplay   lt  lt  endl      return -1   else      Mat image      Mat grayImage       image   imread argv 1   IMREAD COLOR       if   image data            cout  lt  lt   Could not open the image file   lt  lt  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

[c++] How can I convert a cv::Mat to a gray scale in OpenCv?

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