关于OpenCV的那些事——ORB的brief描述子（256bit）的match心得

版权声明：本文为博主原创文章，转载须注明出处。 https://blog.csdn.net/aptx704610875/article/details/51503149

用了ORB那么久，今天第一次将256bit的描述子打印出来，直观看到了match的汉明距离。

上代码：

#include <iostream>
#include <bitset>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>

using namespace std;
using namespace cv;

int main()
{
	Mat image = imread("t.png");
	Mat image1 = imread("t1.png");
	Mat gray,gray1;
	cvtColor(image, gray, CV_BGR2GRAY);
	cvtColor(image1, gray1, CV_BGR2GRAY);
	ORB orb(1000);
	vector<KeyPoint> keypoints,keypoints1;
	orb.detect(gray, keypoints);
	orb.detect(gray1, keypoints1);
	Mat descriptors, descriptors1;
	orb.compute(gray, keypoints, descriptors);
	orb.compute(gray1, keypoints1, descriptors1);
	BFMatcher bf(NORM_HAMMING);
	vector<DMatch> matches,good;
	bf.match(descriptors, descriptors1, matches);
	for (size_t i = 0; i < matches.size(); ++i)
	{
		if (matches[i].distance < 20)
			good.push_back(matches[i]);
	}
	cout << good.size() << endl <<endl;
	Mat imagem;
	drawMatches(image, keypoints, image1, keypoints1, good,imagem);
	imshow("1", imagem);
	for (size_t i = 0; i < good.size(); ++i)
	{
		cout << "第" << i+1 << "对匹配上的描述子，汉明距离为：" << (int)good[i].distance << endl;
		for (int j = 0; j < 32; j++)
		{
			cout << bitset<8>( (int)descriptors.at<uchar>(good[i].queryIdx, j) ) << "  ";
			cout << bitset<8>( (int)descriptors1.at<uchar>(good[i].trainIdx, j) ) <<endl;
		}
		cout << endl;
	}
	waitKey(0);
	return 0;
}

查看了Mat descriptors的信息，行数为keypoints的值，列数为32，说明type为CV_8U（256/32=8）通道的话就只能是1通道。（PS: 强制转换到CV_32U等均报错）

不同于brief描述子的uchar类型，sift和surf均采用float型。所以ORB(brief)描述子8维，32bytes（256bits）,CV_8UC1。SIFT描述子128维，512bytes，CV_32UC4（512*8/32 = 32*4，第一个32表示描述子Mat是32列，第二个32表示float类型）。SURF描述子64维，256bytes，CV_32UC2。（OpenCV里compute的描述子的列数都是32）