原文網址:http://blog.csdn.net/hujingshuang/article/details/46984411
簡介
ORB的全稱是ORiented Brief,是文章ORB: an efficient alternative to SIFT or SURF中提出的一種新的角點檢測與特徵描述算法。實際上,ORB算法是將FAST角點檢測與BRIEF特徵描述結合並進行了改進。
ORB算法
在上一篇文章《BRIEF特徵點描述算法》中,指出了BRIEF的優缺點,ORB算法就是針對BRIEF算法的缺點1、2提出來的。ORB算法分爲兩個部分:FAST特徵點檢測、BRIEF特徵描述。
FAST特徵檢測
在文章《FAST特徵點檢測算法》中,詳細闡述了FAST算法。但該算法僅僅確定了特徵點的位置,沒有得到其他任何信息。在ORB算法中,依然採用FAST來檢測特徵點的位置,但算法進行了如下改動:(以FAST-9爲例)
1、假設在圖像中要提取N個特徵點,則降低FAST的閾值,使FAST算法檢測到的特徵點大於N;
2、在特徵點位置處,計算特徵點的Harris響應值R,取前N個響應值大的點作爲FAST特徵點(Harris角點響應計算:Harris角點檢測中的數學推導);
3、由於要解決BRIEF算法的旋轉不變性,則需要計算特徵點的主方向。
ORB中利用重心來計算,如下(其中(x,y)是特徵鄰域內的點):
atan2表示反正切,得到的θ值就是FAST特徵點的主方向。
BRIEF特徵描述
在文章《BRIEF特徵點描述算法》種,闡述了BRIEF算法。該算法速度優勢相當明顯,但存在三個致命的缺點。針對尺度不變性,可以像SIFT算法一樣,子尺度空間構造圖像金字塔解決,此處不再說明。ORB算法主要解決前兩天缺點:噪聲敏感、旋轉不變性。
1、解決噪聲敏感問題
BRIEF中,採用了9x9的高斯算子進行濾波,可以一定程度上解決噪聲敏感問題,但一個濾波顯然是不夠的。ORB中提出,利用積分圖像來解決:在31x31的窗口中,產生一對隨機點後,以隨機點爲中心,取5x5的子窗口,比較兩個子窗口內的像素和的大小進行二進制編碼,而非僅僅由兩個隨機點決定二進制編碼。(這一步可有積分圖像完成)
2、解決旋轉不變性
利用FAST中求出的特徵點的主方向θ,對特徵點鄰域進行旋轉,Calonder建議先將每個塊旋轉後,再進行BRIEF描述子的提取,但這種方法代價較大。ORB算法採用的是:每一個特徵點處,對產生的256對隨機點(以256爲例),將其進行旋轉,後進行判別,再二進制編碼。如下:S表示隨機點位置(2xn的矩陣),Sθ表示旋轉後的隨機點的位置(2xn的矩陣),x1=(u1,v1)是一個座標向量,其餘雷同。n=256。
得到新的隨機點位置後,利用積分圖像進行二進制編碼,即可。
實驗
opencv代碼
- #include <iostream>
- #include <opencv2/core/core.hpp>
- #include <opencv2/highgui/highgui.hpp>
- #include <opencv2/legacy/legacy.hpp>
- #include <iostream>
- #include <vector>
- using namespace cv;
- using namespace std;
- int main()
- {
- Mat img_1 = imread("beaver1.png");
- Mat img_2 = imread("beaver2.png");
- if (!img_1.data || !img_2.data)
- {
- cout << "error reading images " << endl;
- return -1;
- }
- ORB orb;
- vector<KeyPoint> keyPoints_1, keyPoints_2;
- Mat descriptors_1, descriptors_2;
- orb(img_1, Mat(), keyPoints_1, descriptors_1);
- orb(img_2, Mat(), keyPoints_2, descriptors_2);
- BruteForceMatcher<HammingLUT> matcher;
- vector<DMatch> matches;
- matcher.match(descriptors_1, descriptors_2, matches);
- double max_dist = 0; double min_dist = 100;
- //-- Quick calculation of max and min distances between keypoints
- for( int i = 0; i < descriptors_1.rows; i++ )
- {
- double dist = matches[i].distance;
- if( dist < min_dist ) min_dist = dist;
- if( dist > max_dist ) max_dist = dist;
- }
- printf("-- Max dist : %f \n", max_dist );
- printf("-- Min dist : %f \n", min_dist );
- //-- Draw only "good" matches (i.e. whose distance is less than 0.6*max_dist )
- //-- PS.- radiusMatch can also be used here.
- std::vector< DMatch > good_matches;
- for( int i = 0; i < descriptors_1.rows; i++ )
- {
- if( matches[i].distance < 0.6*max_dist )
- {
- good_matches.push_back( matches[i]);
- }
- }
- Mat img_matches;
- drawMatches(img_1, keyPoints_1, img_2, keyPoints_2,
- good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
- vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS);
- imshow( "Match", img_matches);
- cvWaitKey();
- return 0;
- }
實驗結果
ORB源碼
- static void//計算Harris角點響應
- HarrisResponses(const Mat& img, vector<KeyPoint>& pts, int blockSize, float harris_k)
- {
- CV_Assert( img.type() == CV_8UC1 && blockSize*blockSize <= 2048 );
- size_t ptidx, ptsize = pts.size();
- const uchar* ptr00 = img.ptr<uchar>();
- int step = (int)(img.step/img.elemSize1());
- int r = blockSize/2;
- float scale = (1 << 2) * blockSize * 255.0f;
- scale = 1.0f / scale;
- float scale_sq_sq = scale * scale * scale * scale;
- AutoBuffer<int> ofsbuf(blockSize*blockSize);
- int* ofs = ofsbuf;
- for( int i = 0; i < blockSize; i++ )
- for( int j = 0; j < blockSize; j++ )
- ofs[i*blockSize + j] = (int)(i*step + j);
- for( ptidx = 0; ptidx < ptsize; ptidx++ )
- {
- int x0 = cvRound(pts[ptidx].pt.x - r);
- int y0 = cvRound(pts[ptidx].pt.y - r);
- const uchar* ptr0 = ptr00 + y0*step + x0;
- int a = 0, b = 0, c = 0;
- for( int k = 0; k < blockSize*blockSize; k++ )
- {
- const uchar* ptr = ptr0 + ofs[k];
- int Ix = (ptr[1] - ptr[-1])*2 + (ptr[-step+1] - ptr[-step-1]) + (ptr[step+1] - ptr[step-1]);
- int Iy = (ptr[step] - ptr[-step])*2 + (ptr[step-1] - ptr[-step-1]) + (ptr[step+1] - ptr[-step+1]);
- a += Ix*Ix;
- b += Iy*Iy;
- c += Ix*Iy;
- }
- pts[ptidx].response = ((float)a * b - (float)c * c -
- harris_k * ((float)a + b) * ((float)a + b))*scale_sq_sq;
- }
- }
- //計算FAST角點的主方向
- static float IC_Angle(const Mat& image, const int half_k, Point2f pt,
- const vector<int> & u_max)
- {
- int m_01 = 0, m_10 = 0;
- const uchar* center = &image.at<uchar> (cvRound(pt.y), cvRound(pt.x));
- // Treat the center line differently, v=0
- for (int u = -half_k; u <= half_k; ++u)
- m_10 += u * center[u];
- // Go line by line in the circular patch
- int step = (int)image.step1();
- for (int v = 1; v <= half_k; ++v)
- {
- // Proceed over the two lines
- int v_sum = 0;
- int d = u_max[v];
- for (int u = -d; u <= d; ++u)
- {
- int val_plus = center[u + v*step], val_minus = center[u - v*step];
- v_sum += (val_plus - val_minus);
- m_10 += u * (val_plus + val_minus);
- }
- m_01 += v * v_sum;
- }
- return fastAtan2((float)m_01, (float)m_10);
- }
- #define GET_VALUE(idx) \
- (x = pattern[idx].x*a - pattern[idx].y*b, \ //計算旋轉後的位置
- y = pattern[idx].x*b + pattern[idx].y*a, \
- ix = cvRound(x), \
- iy = cvRound(y), \
- *(center + iy*step + ix) )
- //判決,並二進制編碼
- for (int i = 0; i < dsize; ++i, pattern += 16)
- {
- int t0, t1, val;
- t0 = GET_VALUE(0); t1 = GET_VALUE(1);
- val = t0 < t1;
- t0 = GET_VALUE(2); t1 = GET_VALUE(3);
- val |= (t0 < t1) << 1;
- t0 = GET_VALUE(4); t1 = GET_VALUE(5);
- val |= (t0 < t1) << 2;
- t0 = GET_VALUE(6); t1 = GET_VALUE(7);
- val |= (t0 < t1) << 3;
- t0 = GET_VALUE(8); t1 = GET_VALUE(9);
- val |= (t0 < t1) << 4;
- t0 = GET_VALUE(10); t1 = GET_VALUE(11);
- val |= (t0 < t1) << 5;
- t0 = GET_VALUE(12); t1 = GET_VALUE(13);
- val |= (t0 < t1) << 6;
- t0 = GET_VALUE(14); t1 = GET_VALUE(15);
- val |= (t0 < t1) << 7;
- desc[i] = (uchar)val;
- }
- //產生512個隨機點的座標位置
- static void makeRandomPattern(int patchSize, Point* pattern, int npoints)
- {
- RNG rng(0x34985739); // we always start with a fixed seed,
- // to make patterns the same on each run
- for( int i = 0; i < npoints; i++ )
- {
- pattern[i].x = rng.uniform(-patchSize/2, patchSize/2+1);
- pattern[i].y = rng.uniform(-patchSize/2, patchSize/2+1);
- }
- }