Convert RGB to Black & White in OpenCV

Question

I would like to know how to convert an RGB image into a black & white (binary) image.

After conversion, how can I save the modified image to disk?

Answer 1

AFAIK, you have to convert it to grayscale and then threshold it to binary.

1. Read the image as a grayscale image If you're reading the RGB image from disk, then you can directly read it as a grayscale image, like this:

// C IplImage* im_gray = cvLoadImage("image.jpg",CV_LOAD_IMAGE_GRAYSCALE);  // C++ (OpenCV 2.0) Mat im_gray = imread("image.jpg",CV_LOAD_IMAGE_GRAYSCALE); 

2. Convert an RGB image im_rgb into a grayscale image: Otherwise, you'll have to convert the previously obtained RGB image into a grayscale image

// C IplImage *im_rgb  = cvLoadImage("image.jpg"); IplImage *im_gray = cvCreateImage(cvGetSize(im_rgb),IPL_DEPTH_8U,1); cvCvtColor(im_rgb,im_gray,CV_RGB2GRAY);  // C++ Mat im_rgb  = imread("image.jpg"); Mat im_gray; cvtColor(im_rgb,im_gray,CV_RGB2GRAY); 

3. Convert to binary You can use adaptive thresholding or fixed-level thresholding to convert your grayscale image to a binary image.

E.g. in C you can do the following (you can also do the same in C++ with Mat and the corresponding functions):

// C IplImage* im_bw = cvCreateImage(cvGetSize(im_gray),IPL_DEPTH_8U,1); cvThreshold(im_gray, im_bw, 128, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);  // C++ Mat img_bw = im_gray > 128; 

In the above example, 128 is the threshold.

4. Save to disk

// C cvSaveImage("image_bw.jpg",img_bw);  // C++ imwrite("image_bw.jpg", img_bw); 

Answer 2

This seemed to have worked for me!

Mat a_image = imread(argv[1]);  cvtColor(a_image, a_image, CV_BGR2GRAY); GaussianBlur(a_image, a_image, Size(7,7), 1.5, 1.5); threshold(a_image, a_image, 100, 255, CV_THRESH_BINARY);