getting a value in binary image - c++

Question

i'm trying to get a set of values in a binary image for inverting it.. but i'm having troubles to index the matrix, the first lines of my code are.
std::string path = "img/lena.jpg";
//Our color image
cv::Mat imageMat = cv::imread(path, CV_LOAD_IMAGE_GRAYSCALE);
if(imageMat.empty())
{
std::cerr << "ERROR: Could not read image " << argv[1] << std::endl;
return 1;
}
//Grayscale matrix
cv::Mat grayscaleMat (imageMat.size(), CV_8U);
//Convert BGR to Gray
cv::cvtColor( imageMat, grayscaleMat, CV_BGR2GRAY );
//Binary image
cv::Mat binaryMat(grayscaleMat.size(), grayscaleMat.type());
//Apply thresholding
cv::threshold(grayscaleMat, binaryMat, 100, 255, cv::THRESH_BINARY);
Now i need to work with the values in binaryMat, but i don't know how get it...

Answer 1

1: with opencv's c++ api, you don't need to allocate output/result Mat's. just leave them empty.
//Convert BGR to Gray
cv::Mat grayscaleMat;
cv::cvtColor( imageMat, grayscaleMat, CV_BGR2GRAY );
//Apply thresholding
cv::Mat binaryMat;
cv::threshold(grayscaleMat, binaryMat, 100, 255, cv::THRESH_BINARY);
2: now access the pixels:
uchar p = binaryMat.at<uchar>(y,x); // row,col world !
binaryMat.at<uchar>(5,5) = 17;