Convert RGB to Black & White in OpenCV


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?

1/18/2013 11:08:56 PM

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);

// C++
Mat im_rgb  = imread("image.jpg");
Mat im_gray;

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

// C++
imwrite("image_bw.jpg", img_bw);
2/27/2013 8:49:32 PM

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