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afaik, bisk, brief, orb descriptors are 32 byte uchar arrays, so your desc Mat will have n rows and 32 cols ( SURF and SIFT are 128 element float arrays )

but why don't you go, and look for yourself ?

cerr << desc.rows << " " << desc.cols << " " << desc.type() << endl;

if the type() is CV_8U (0), you'd access it like:

 uchar d = desc.at<uchar>(row,col);
 desc.at<uchar>(row,col) = 19;

if the type() is CV_32F, that's :

float d = desc.at<float>(row,col);
desc.at<float>(row,col) = 19.9f;