CODEWITHSUNDEEP
opencvimage-processingcomputer-vision
GilLevi
GilLevi

Reputation: 2137

What are the values of the SURF descriptor?

I'm extracting SURF descriptors from an image using the following simple lines:

Ptr<DescriptorExtractor> descriptor = DescriptorExtractor::create("SURF");
    descriptor->compute(im1, kp, desc1);

Now, when I "watch" the variable desc1.data, it contains integer values in the range 0 to 255.

However, when I investigate the values using the code:

for (int j=0;j<desc1.cols; j++){
            float a=desc1.at<float>(0,j);

it contains values between -1 and 1. How is that possible? SURF shouldn't return integer values like SIFT?

Upvotes: 1

Views: 1113

Answers (1)

Bharat
Bharat

Reputation: 2179

I am not sure what happens in OpenCV, but as far the paper goes this is what SURF does. The SURF descriptor divides a small image patch into 4x4 sub regions and computes wavelet responses over each sub region in a clever fashion. Basically it is a 4 tuple descripor < sum(dx), sum(dy), sum(|dx|), sum(|dy|)>, where dx, dy are wavelet responses in each sub-region. The descriptor is constructed by concatenating all the responses and normalizing the magnitude, which results in a 64 dimensional descriptor. It is clear from the description that normalized sum(dx) and sum(dy) values would lie between -1 and 1, while sum(|dx|) and sum(|dy|) would lie between 0 to 1. If you see the 128 dimensional descriptor, the summation for dx and |dx| is computed separately for dx >= 0 and dy < 0, which doubles the size of the 64 dimensional descriptor.

Upvotes: 1

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