OpenCV bitwise_or just adds up two images' pixel value - opencv

Question

I tried to use bitwise_or on two CV_8UC1 images, but the result is not what I expected.
In my case, for image_1, every pixel value is set to 2, and for image_2, every pixel value is set to 3, the output of bitwise_or is an image with every pixel value set to 5, while what I expected is every pixel value is 2|3, which should be 2.
Can someone tell me why?

Answer 1

The result of the bitwise or operation between 2 and 3 is 3. You can check it like this:
cout<<(2|3); // The result will be 3
Also, if you do a bitwise_or on two matrices that have all pixels 2 and 3 respectively, you should get a matrix with all its pixels set to 3, like in this example:
Mat m1 = Mat(3, 3, CV_8UC1, Scalar(2));
Mat m2 = Mat(3, 3, CV_8UC1, Scalar(3));
Mat r;
bitwise_or(m1, m2, r);
cout<<r;
Result:
[3, 3, 3;
3, 3, 3;
3, 3, 3]
Do you want to add the two images? If this is the case, you can simply use the + operator, like this:
Mat m1 = Mat(3, 3, CV_8UC1, Scalar(2));
Mat m2 = Mat(3, 3, CV_8UC1, Scalar(3));
Mat r = m1+m2;
cout<<r;
Result:
[5, 5, 5;
5, 5, 5;
5, 5, 5]
In decimal system, the equivalent of the or operation is the maximum operation. (Also, the equivalent of the and operation is the minimum operation).
If this is what you want, OpenCV provides a cv::max() function that calculates the elementwise maximum from two matrices of the same size. Here is an example:
Mat a = Mat::ones(3, 3, CV_8UC1) * 2;
Mat b = Mat::ones(3, 3, CV_8UC1) * 100;
cout<<a<<endl<<b<<endl;
Mat max = cv::max(a, b);
cout<<max;
The result is:
a=[2, 2, 2;
2, 2, 2;
2, 2, 2]
b=[100, 100, 100;
100, 100, 100;
100, 100, 100]
max=[100, 100, 100;
100, 100, 100;
100, 100, 100]