OpenCV - get grey value from an image - c++

Question

Hello everybody right now I'm trying to getting grey value for every pixel in an image
what I mean with grey value is the white or black level from an image let's say 0 for white and 1 for black. for an example for this image
the value I want will be like
0 0 0 0 0 0
0 1 1 1 0 0
0 0 1 1 0 0
0 0 1 1 0 0
0 0 1 1 0 0
0 0 1 1 0 0
0 0 1 1 0 0
0 0 0 0 0 0
is this possible? if yes how to do it with OpenCV in C? or if it's impossible with OpenCV is there any other library that can do this?

Answer 1

What you ask is certainly possible but how it can be done depends on a lot of things. If you use C++, on SO we generally expect you to use the C++ interface which means you have a cv::Mat object and loaded the image with something like this: (using namespace cv)
#include <opencv2/core/core.hpp>
Mat mat_gray = imread(path, CV_LOAD_IMAGE_GRAYSCALE);
or by
Mat mat = imread(path); // and assuming it was originally a color image...
Mat mat_gray;
cvtColor(mat, mat_gray, CV_BGR2GRAY); //...convert it to grayscale.
Now, if you just want to access pixel values one-by-one, you use _Tp& mat.at<_Tp>(int x,int y);. That is:
for(int x=0; x<mat_gray.rows; ++x)
for(int y=0; y<mat_gray.cols; ++y)
mat_gray.at<uchar>(x,y); // if mat.type == CV_8U
You can look up your type here, which you should use in place of uchar if the mat.type is other than CV_8U.
As for the pure C interface, you can check this answer. But if you use C++, you should definitely use the C++ interface.