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

cfo · Accepted Answer · 2013-04-08T12:43:19.137

17

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);

edited Apr 08 '13 at 12:43

answered Nov 18 '12 at 21:25

cfo

442
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welcome! can the down-voter give a reason or improvement to my answer? – cfo Nov 19 '12 at 11:51
Your final statement is wrong. And your answer isn't significantly different from my answer. – ArtemStorozhuk Nov 19 '12 at 13:29
@Astor I have found cfo's answer more helpful. There is just one thing that I don't undersand: When I'm trying to draw a line or a contour using cvDrawContour or cv::Line on the image img_rgb I can only use red, green or blue. If I use CV_RGB(255,0,0) it will be red, CV_RGB(0,255,0) it will be green... but if I want to use orange (CV_RGB(253,152,0) for example) it draws in white (or black depending on the values of R, G, B). – alvinleetya Nov 19 '12 at 15:41

score 0 · Answer 2 · answered Nov 18 '12 at 16:17

0

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.

answered Nov 18 '12 at 16:17

ArtemStorozhuk

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openCV - draw color contours on greyscale image

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