openCV - draw color contours on greyscale image

Question

I'm working on a segmentation algorithme for medical images and during the process it must display the evolving contour on the original image.
I'm working with greyscale JPEG. In order to display the contours I use the drawContours function but I can't manage to draw a color contour. I would like to draw a red contour on the greyscale image but it only appears black or white.
Here is the section of the code:

Mat_<uchar> matrix = imread(path,0);
int row = matrix.rows;
int col = matrix.cols;
Mat_<uchar> maskInter(row,col);
for (int i=0; i<row; i++)
    for (int j=0; j<col; j++)
    {
        if ((double)matrix(i,j) <0)
        {maskInter(i,j)=255;}
        else
        {maskInter(i,j)=0;}
    };

vector<vector<Point> > contours; 
vector<Vec4i> hierarchy;
Mat mat_out = maskInter.clone();
findContours( mat_out, contours, hierarchy, CV_RETR_TREE , CHAIN_APPROX_SIMPLE);

drawContours( img, contours, -1, RGB(255,0,0),1.5,8,hierarchy,2,Point());
namedWindow(title);
moveWindow(title, 100, 100);
imshow(title, img);
waitKey(0);

Is it possible to display a color contour on a greyscale image?
Thanks

Answer 1

You need a 3-channel (RGB) image in order to draw and display colors. Your Mat_<uchar> matrix is only one channel. You can convert your grayscale image to a color image as follows:

  // read image
  cv::Mat img_gray = imread(path,0);

  // create 8bit color image. IMPORTANT: initialize image otherwise it will result in 32F
  cv::Mat img_rgb(img_gray.size(), CV_8UC3);

  // convert grayscale to color image
  cv::cvtColor(img_gray, img_rgb, CV_GRAY2RGB);

Answer 2

Is it possible to display a color contour on a greyscale image?

If it's really grayscale (1 byte per pixel) than no, you can't. To draw color contours you should convert image to RGB (3 bytes per pixel) using cvtColor and after this draw color contour.