Problems with opencv feature descriptors converted from gpu feature descriptors

Question

I've been having some problems with the conversion of a GPU feature descriptor matrix to a CPU feature descriptor matrix with openvc, using:

void downloadDescriptors(const GpuMat& descriptorsGPU, vector<float>& descriptors);

As you can see, this method converts the GpuMat holding the descriptors to a vector of floats holding them. The problem is, when I access some element of this vector, the values returned are quite different from the expected interval of 0 to 255. I've made the following test program to compare the time of extraction and description obtained through SURF_GPU and SURF:

clock_t start;
clock_t end;

SURF_GPU surfGPU;
SURF surf;

Mat img1 = imread("Ipo_SP_01.jpg", CV_LOAD_IMAGE_GRAYSCALE);
Mat outimageGPU;
Mat outimageCPU;
GpuMat imgGPU;
imgGPU.upload(img1);

vector<KeyPoint> keyp_A;
vector<KeyPoint> keyp_B;
GpuMat keypGPU;

vector<float> descriptorsConverted;
Mat descriptorsCPU;
GpuMat descriptorsGPU;

start = (clock() * 1000)/CLOCKS_PER_SEC;
surfGPU(imgGPU, GpuMat(), keypGPU, descriptorsGPU);
end = (clock() * 1000)/CLOCKS_PER_SEC;
cout << "GPU time: " << end - start << endl;
surfGPU.downloadKeypoints(keypGPU, keyp_A);
surfGPU.downloadDescriptors(descriptorsGPU, descriptorsConverted);
cout << "GPU Keypoints = " << keyp_A.size() << endl;

start = (clock() * 1000)/CLOCKS_PER_SEC;
surf(img1, Mat(), keyp_B, descriptorsCPU);
end = (clock() * 1000)/CLOCKS_PER_SEC;
cout << "CPU time: " << end - start << endl;
cout << "CPU Keypoints = " << keyp_B.size() << endl;

drawKeypoints(img1, keyp_A, outimageGPU, Scalar(255, 255, 255), DrawMatchesFlags::DEFAULT);
imwrite("GPU.jpg", outimageGPU);
drawKeypoints(img1, keyp_B, outimageCPU, Scalar(255, 255, 255), DrawMatchesFlags::DEFAULT);
imwrite("CPU.jpg", outimageCPU);

return 0;

Checking the elements of descriptorsConverted, I expected to get values between 0 and 255, like I get when I access the elements of descriptorsCPU. Instead, I got values like:

-0.000621859
0.000627841
-0.000503146
0.000543773
-8.69408e-05
0.000110254
0.000265697
0.000941789
0.0595061
0.0619723

I suspect this problem is related to the type returned by downloadDescriptors, even though it's clear it returns a float vector.

Answer 1

I get similar results for the descriptors and had some initial trouble with matching. I've found that when the descriptors are downloaded to std::vector<float> f_descriptors that the length of the vector is divisible by the descriptor size (i.e., either 64 or 128) so I was able to use the following:

std::vector<float> f_descriptors;
std::vector<cv::KeyPoint> keypoints;
surfGPU( imgGPU, cv::gpu::GpuMat(), keypoints, f_descriptors  );
descriptorsCPU = cv::Mat( (int)f_descriptors.size()/surfGPU.descriptorSize(), surfGPU.descriptorSize(), CV_32F, f_descriptors[0] );

Also, I found that uploading from the descriptorsGPU to descriptorsCPU also does the same thing and is probably safer:

std::vector<cv::KeyPoint> keypoints;
cv::gpu::GpuMat descriptorsGPU
surfGPU( imgGPU, cv::gpu::GpuMat(), keypoints, descriptorsGPU  );
descriptorsGPU.download( descriptorsCPU );