Converting an OpenCV Image to Black and White

Question

How do you convert a grayscale OpenCV image to black and white? I see a similar question has already been asked, but I\'m using OpenCV 2.3, and the proposed solution no long

Answer 1

Approach 1

While converting a gray scale image to a binary image, we usually use cv2.threshold() and set a threshold value manually. Sometimes to get a decent result we opt for Otsu's binarization.

I have a small hack I came across while reading some blog posts.

1. Convert your color (RGB) image to gray scale.
2. Obtain the median of the gray scale image.
3. Choose a threshold value either 33% above the median

Why 33%?

This is because 33% works for most of the images/data-set.

You can also work out the same approach by replacing median with the mean.

Approach 2

Another approach would be to take an x number of standard deviations (std) from the mean, either on the positive or negative side; and set a threshold. So it could be one of the following:

• th1 = mean - (x * std)
• th2 = mean + (x * std)

Note: Before applying threshold it is advisable to enhance the contrast of the gray scale image locally (See CLAHE).