openCV - draw color contours on greyscale image
28,513
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);
Author by
alvinleetya
Updated on April 09, 2020Comments
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alvinleetya about 4 years
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 over 11 yearswelcome! can the down-voter give a reason or improvement to my answer?
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ArtemStorozhuk over 11 yearsYour final statement is wrong. And your answer isn't significantly different from my answer.
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alvinleetya over 11 years@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).