how to specify SURF parameters at FeatureDetector object ?

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i try to get SURF descriptor with OpenCV 2.4.6.0, but FeatureDetector object cannot be specified SURF parameters. using FeatureDetector object, I can get descriptors Mat, then it contains unkown data.

i.e. -431602080.

about 35000's above data is contained in descriptors Mat. code is below,

void getDescriptors(char* image) {

///// load image
cv::Mat img = cv::imread(image, CV_LOAD_IMAGE_GRAYSCALE);
if(img.empty()) return;

///// detect keypoints /////
cv::vector<cv::KeyPoint> keyPoints; // keypoint vector

// create feature detector
cv::Ptr<cv::FeatureDetector> detector = cv::FeatureDetector::create("SURF");
if(detector.empty()) return;

// detect key points (feature point)
detector->detect(image, keyPoints);

///// compute descriptors /////
cv::Mat descriptor; // descriptors mat

// create descriptor extractor
cv::Ptr<cv::DescriptorExtractor> extractor = cv::DescriptorExtractor::create("SURF");
if(extractor.empty()) return;

// compute descriptor
extractor->compute(image, keyPoints, descriptor);

return descriptor;
}