Correct Size of bits in brisk descriptors? opencv

Question

I have a little problem of comprehension.

The standard type of a Brisk descriptor Mat value in opencv is an unsigned char value with a standard size of 8 bit. A complete descriptor of a KeyPoint has a size of 16 unsigned char values. 8bitx16 = 128bit although in papers you just find out that you have a 512 bit descriptor.

Can anybody help me out of that mismatch? I have not found out in the OpenCV documentation if the bitsize of the descriptors is scalable (yet?).

Answer 1

Where did you find that 16 figure? It seems to be wrong.

Mat img = imread( "lena.png", cv::IMREAD_GRAYSCALE );

BRISK brisk;
vector<KeyPoint> kp;
brisk.detect(img,kp);

Mat desc;
brisk.compute( img, kp, desc) ;
cerr << desc.rows << " " << desc.cols << " " << desc.type() << endl;

167 64 0

So, 167 BRISK descriptors a 64 bytes found (that's 512 bits per descriptor).