圖像拼接在實際的應用場景很廣，比如無人機航拍，遙感圖像等等，圖像拼接是進一步做圖像理解基礎步驟，拼接效果的好壞直接影響接下來的工作，所以一個好的圖像拼接算法非常重要。
再舉一個身邊的例子吧，你用你的手機對某一場景拍照，但是你沒有辦法一次將所有你要拍的景物全部拍下來，所以你對該場景從左往右依次拍了好幾張圖，來把你要拍的所有景物記錄下來。那么我們能不能把這些圖像拼接成一個大圖呢？我們利用opencv就可以做到圖像拼接的效果！
比如我們有對這兩張圖進行拼接。
從上面兩張圖可以看出，這兩張圖有比較多的重疊部分，這也是拼接的基本要求。
那么要實現圖像拼接需要那幾步呢？簡單來說有以下幾步：
- 對每幅圖進行特征點提取
- 對對特征點進行匹配
- 進行圖像配准
- 把圖像拷貝到另一幅圖像的特定位置
- 對重疊邊界進行特殊處理
好吧，那就開始正式實現圖像配准。
第一步就是特征點提取。現在CV領域有很多特征點的定義，比如sift、surf、harris角點、ORB都是很有名的特征因子，都可以用來做圖像拼接的工作，他們各有優勢。本文將使用ORB和SURF進行圖像拼接，用其他方法進行拼接也是類似的。
基於SURF的圖像拼接
用SIFT算法來實現圖像拼接是很常用的方法，但是因為SIFT計算量很大，所以在速度要求很高的場合下不再適用。所以，它的改進方法SURF因為在速度方面有了明顯的提高（速度是SIFT的3倍），所以在圖像拼接領域還是大有作為。雖說SURF精確度和穩定性不及SIFT，但是其綜合能力還是優越一些。下面將詳細介紹拼接的主要步驟。
1.特征點提取和匹配
1 //提取特征點
2 SurfFeatureDetector Detector(2000); 3 vector<KeyPoint> keyPoint1, keyPoint2; 4 Detector.detect(image1, keyPoint1); 5 Detector.detect(image2, keyPoint2); 6
7 //特征點描述，為下邊的特征點匹配做准備
8 SurfDescriptorExtractor Descriptor; 9 Mat imageDesc1, imageDesc2; 10 Descriptor.compute(image1, keyPoint1, imageDesc1); 11 Descriptor.compute(image2, keyPoint2, imageDesc2); 12
13 FlannBasedMatcher matcher; 14 vector<vector<DMatch> > matchePoints; 15 vector<DMatch> GoodMatchePoints; 16
17 vector<Mat> train_desc(1, imageDesc1); 18 matcher.add(train_desc); 19 matcher.train(); 20
21 matcher.knnMatch(imageDesc2, matchePoints, 2); 22 cout << "total match points: " << matchePoints.size() << endl; 23
24 // Lowe's algorithm,獲取優秀匹配點
25 for (int i = 0; i < matchePoints.size(); i++) 26 { 27 if (matchePoints[i][0].distance < 0.4 * matchePoints[i][1].distance) 28 { 29 GoodMatchePoints.push_back(matchePoints[i][0]); 30 } 31 } 32
33 Mat first_match; 34 drawMatches(image02, keyPoint2, image01, keyPoint1, GoodMatchePoints, first_match); 35 imshow("first_match ", first_match);
2.圖像配准
這樣子我們就可以得到了兩幅待拼接圖的匹配點集，接下來我們進行圖像的配准，即將兩張圖像轉換為同一坐標下，這里我們需要使用findHomography函數來求得變換矩陣。但是需要注意的是，findHomography函數所要用到的點集是Point2f類型的，所有我們需要對我們剛得到的點集GoodMatchePoints再做一次處理，使其轉換為Point2f類型的點集。
1 vector<Point2f> imagePoints1, imagePoints2; 2
3 for (int i = 0; i<GoodMatchePoints.size(); i++) 4 { 5 imagePoints2.push_back(keyPoint2[GoodMatchePoints[i].queryIdx].pt); 6 imagePoints1.push_back(keyPoint1[GoodMatchePoints[i].trainIdx].pt); 7 }
這樣子，我們就可以拿着imagePoints1, imagePoints2去求變換矩陣了，並且實現圖像配准。值得注意的是findHomography函數的參數中我們選澤了CV_RANSAC，這表明我們選擇RANSAC算法繼續篩選可靠地匹配點，這使得匹配點解更為精確。
1 //獲取圖像1到圖像2的投影映射矩陣 尺寸為3*3
2 Mat homo = findHomography(imagePoints1, imagePoints2, CV_RANSAC); 3 ////也可以使用getPerspectiveTransform方法獲得透視變換矩陣，不過要求只能有4個點，效果稍差
4 //Mat homo=getPerspectiveTransform(imagePoints1,imagePoints2);
5 cout << "變換矩陣為：\n" << homo << endl << endl; //輸出映射矩陣 6
7 //圖像配准
8 Mat imageTransform1, imageTransform2; 9 warpPerspective(image01, imageTransform1, homo, Size(MAX(corners.right_top.x, corners.right_bottom.x), image02.rows)); 10 //warpPerspective(image01, imageTransform2, adjustMat*homo, Size(image02.cols*1.3, image02.rows*1.8));
11 imshow("直接經過透視矩陣變換", imageTransform1); 12 imwrite("trans1.jpg", imageTransform1);
3. 圖像拷貝
拷貝的思路很簡單，就是將左圖直接拷貝到配准圖上就可以了。
1 //創建拼接后的圖,需提前計算圖的大小
2 int dst_width = imageTransform1.cols; //取最右點的長度為拼接圖的長度
3 int dst_height = image02.rows; 4
5 Mat dst(dst_height, dst_width, CV_8UC3); 6 dst.setTo(0); 7
8 imageTransform1.copyTo(dst(Rect(0, 0, imageTransform1.cols, imageTransform1.rows))); 9 image02.copyTo(dst(Rect(0, 0, image02.cols, image02.rows))); 10
11 imshow("b_dst", dst);
4.圖像融合（去裂縫處理）
從上圖可以看出，兩圖的拼接並不自然，原因就在於拼接圖的交界處，兩圖因為光照色澤的原因使得兩圖交界處的過渡很糟糕，所以需要特定的處理解決這種不自然。這里的處理思路是加權融合，在重疊部分由前一幅圖像慢慢過渡到第二幅圖像，即將圖像的重疊區域的像素值按一定的權值相加合成新的圖像。
1 //優化兩圖的連接處，使得拼接自然
2 void OptimizeSeam(Mat& img1, Mat& trans, Mat& dst) 3 { 4 int start = MIN(corners.left_top.x, corners.left_bottom.x);//開始位置，即重疊區域的左邊界
5
6 double processWidth = img1.cols - start;//重疊區域的寬度
7 int rows = dst.rows; 8 int cols = img1.cols; //注意，是列數*通道數
9 double alpha = 1;//img1中像素的權重
10 for (int i = 0; i < rows; i++) 11 { 12 uchar* p = img1.ptr<uchar>(i); //獲取第i行的首地址
13 uchar* t = trans.ptr<uchar>(i); 14 uchar* d = dst.ptr<uchar>(i); 15 for (int j = start; j < cols; j++) 16 { 17 //如果遇到圖像trans中無像素的黑點，則完全拷貝img1中的數據
18 if (t[j * 3] == 0 && t[j * 3 + 1] == 0 && t[j * 3 + 2] == 0) 19 { 20 alpha = 1; 21 } 22 else
23 { 24 //img1中像素的權重，與當前處理點距重疊區域左邊界的距離成正比，實驗證明，這種方法確實好
25 alpha = (processWidth - (j - start)) / processWidth; 26 } 27
28 d[j * 3] = p[j * 3] * alpha + t[j * 3] * (1 - alpha); 29 d[j * 3 + 1] = p[j * 3 + 1] * alpha + t[j * 3 + 1] * (1 - alpha); 30 d[j * 3 + 2] = p[j * 3 + 2] * alpha + t[j * 3 + 2] * (1 - alpha); 31
32 } 33 } 34 }
多嘗試幾張，驗證拼接效果
測試一
測試二
測試三
最后給出完整的SURF算法實現的拼接代碼。
1 #include "highgui/highgui.hpp"
2 #include "opencv2/nonfree/nonfree.hpp"
3 #include "opencv2/legacy/legacy.hpp"
4 #include <iostream>
5
6 using namespace cv; 7 using namespace std; 8
9 void OptimizeSeam(Mat& img1, Mat& trans, Mat& dst); 10
11 typedef struct
12 { 13 Point2f left_top; 14 Point2f left_bottom; 15 Point2f right_top; 16 Point2f right_bottom; 17 }four_corners_t; 18
19 four_corners_t corners; 20
21 void CalcCorners(const Mat& H, const Mat& src) 22 { 23 double v2[] = { 0, 0, 1 };//左上角
24 double v1[3];//變換后的坐標值
25 Mat V2 = Mat(3, 1, CV_64FC1, v2); //列向量
26 Mat V1 = Mat(3, 1, CV_64FC1, v1); //列向量
27
28 V1 = H * V2; 29 //左上角(0,0,1)
30 cout << "V2: " << V2 << endl; 31 cout << "V1: " << V1 << endl; 32 corners.left_top.x = v1[0] / v1[2]; 33 corners.left_top.y = v1[1] / v1[2]; 34
35 //左下角(0,src.rows,1)
36 v2[0] = 0; 37 v2[1] = src.rows; 38 v2[2] = 1; 39 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
40 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
41 V1 = H * V2; 42 corners.left_bottom.x = v1[0] / v1[2]; 43 corners.left_bottom.y = v1[1] / v1[2]; 44
45 //右上角(src.cols,0,1)
46 v2[0] = src.cols; 47 v2[1] = 0; 48 v2[2] = 1; 49 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
50 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
51 V1 = H * V2; 52 corners.right_top.x = v1[0] / v1[2]; 53 corners.right_top.y = v1[1] / v1[2]; 54
55 //右下角(src.cols,src.rows,1)
56 v2[0] = src.cols; 57 v2[1] = src.rows; 58 v2[2] = 1; 59 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
60 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
61 V1 = H * V2; 62 corners.right_bottom.x = v1[0] / v1[2]; 63 corners.right_bottom.y = v1[1] / v1[2]; 64
65 } 66
67 int main(int argc, char *argv[]) 68 { 69 Mat image01 = imread("g5.jpg", 1); //右圖
70 Mat image02 = imread("g4.jpg", 1); //左圖
71 imshow("p2", image01); 72 imshow("p1", image02); 73
74 //灰度圖轉換
75 Mat image1, image2; 76 cvtColor(image01, image1, CV_RGB2GRAY); 77 cvtColor(image02, image2, CV_RGB2GRAY); 78
79
80 //提取特征點
81 SurfFeatureDetector Detector(2000); 82 vector<KeyPoint> keyPoint1, keyPoint2; 83 Detector.detect(image1, keyPoint1); 84 Detector.detect(image2, keyPoint2); 85
86 //特征點描述，為下邊的特征點匹配做准備
87 SurfDescriptorExtractor Descriptor; 88 Mat imageDesc1, imageDesc2; 89 Descriptor.compute(image1, keyPoint1, imageDesc1); 90 Descriptor.compute(image2, keyPoint2, imageDesc2); 91
92 FlannBasedMatcher matcher; 93 vector<vector<DMatch> > matchePoints; 94 vector<DMatch> GoodMatchePoints; 95
96 vector<Mat> train_desc(1, imageDesc1); 97 matcher.add(train_desc); 98 matcher.train(); 99
100 matcher.knnMatch(imageDesc2, matchePoints, 2); 101 cout << "total match points: " << matchePoints.size() << endl; 102
103 // Lowe's algorithm,獲取優秀匹配點
104 for (int i = 0; i < matchePoints.size(); i++) 105 { 106 if (matchePoints[i][0].distance < 0.4 * matchePoints[i][1].distance) 107 { 108 GoodMatchePoints.push_back(matchePoints[i][0]); 109 } 110 } 111
112 Mat first_match; 113 drawMatches(image02, keyPoint2, image01, keyPoint1, GoodMatchePoints, first_match); 114 imshow("first_match ", first_match); 115
116 vector<Point2f> imagePoints1, imagePoints2; 117
118 for (int i = 0; i<GoodMatchePoints.size(); i++) 119 { 120 imagePoints2.push_back(keyPoint2[GoodMatchePoints[i].queryIdx].pt); 121 imagePoints1.push_back(keyPoint1[GoodMatchePoints[i].trainIdx].pt); 122 } 123
124
125
126 //獲取圖像1到圖像2的投影映射矩陣 尺寸為3*3
127 Mat homo = findHomography(imagePoints1, imagePoints2, CV_RANSAC); 128 ////也可以使用getPerspectiveTransform方法獲得透視變換矩陣，不過要求只能有4個點，效果稍差
129 //Mat homo=getPerspectiveTransform(imagePoints1,imagePoints2);
130 cout << "變換矩陣為：\n" << homo << endl << endl; //輸出映射矩陣 131
132 //計算配准圖的四個頂點坐標
133 CalcCorners(homo, image01); 134 cout << "left_top:" << corners.left_top << endl; 135 cout << "left_bottom:" << corners.left_bottom << endl; 136 cout << "right_top:" << corners.right_top << endl; 137 cout << "right_bottom:" << corners.right_bottom << endl; 138
139 //圖像配准
140 Mat imageTransform1, imageTransform2; 141 warpPerspective(image01, imageTransform1, homo, Size(MAX(corners.right_top.x, corners.right_bottom.x), image02.rows)); 142 //warpPerspective(image01, imageTransform2, adjustMat*homo, Size(image02.cols*1.3, image02.rows*1.8));
143 imshow("直接經過透視矩陣變換", imageTransform1); 144 imwrite("trans1.jpg", imageTransform1); 145
146
147 //創建拼接后的圖,需提前計算圖的大小
148 int dst_width = imageTransform1.cols; //取最右點的長度為拼接圖的長度
149 int dst_height = image02.rows; 150
151 Mat dst(dst_height, dst_width, CV_8UC3); 152 dst.setTo(0); 153
154 imageTransform1.copyTo(dst(Rect(0, 0, imageTransform1.cols, imageTransform1.rows))); 155 image02.copyTo(dst(Rect(0, 0, image02.cols, image02.rows))); 156
157 imshow("b_dst", dst); 158
159
160 OptimizeSeam(image02, imageTransform1, dst); 161
162
163 imshow("dst", dst); 164 imwrite("dst.jpg", dst); 165
166 waitKey(); 167
168 return 0; 169 } 170
171
172 //優化兩圖的連接處，使得拼接自然
173 void OptimizeSeam(Mat& img1, Mat& trans, Mat& dst) 174 { 175 int start = MIN(corners.left_top.x, corners.left_bottom.x);//開始位置，即重疊區域的左邊界
176
177 double processWidth = img1.cols - start;//重疊區域的寬度
178 int rows = dst.rows; 179 int cols = img1.cols; //注意，是列數*通道數
180 double alpha = 1;//img1中像素的權重
181 for (int i = 0; i < rows; i++) 182 { 183 uchar* p = img1.ptr<uchar>(i); //獲取第i行的首地址
184 uchar* t = trans.ptr<uchar>(i); 185 uchar* d = dst.ptr<uchar>(i); 186 for (int j = start; j < cols; j++) 187 { 188 //如果遇到圖像trans中無像素的黑點，則完全拷貝img1中的數據
189 if (t[j * 3] == 0 && t[j * 3 + 1] == 0 && t[j * 3 + 2] == 0) 190 { 191 alpha = 1; 192 } 193 else
194 { 195 //img1中像素的權重，與當前處理點距重疊區域左邊界的距離成正比，實驗證明，這種方法確實好
196 alpha = (processWidth - (j - start)) / processWidth; 197 } 198
199 d[j * 3] = p[j * 3] * alpha + t[j * 3] * (1 - alpha); 200 d[j * 3 + 1] = p[j * 3 + 1] * alpha + t[j * 3 + 1] * (1 - alpha); 201 d[j * 3 + 2] = p[j * 3 + 2] * alpha + t[j * 3 + 2] * (1 - alpha); 202
203 } 204 } 205 }
基於ORB的圖像拼接
利用ORB進行圖像拼接的思路跟上面的思路基本一樣，只是特征提取和特征點匹配的方式略有差異罷了。這里就不再詳細介紹思路了，直接貼代碼看效果。
1 #include "highgui/highgui.hpp"
2 #include "opencv2/nonfree/nonfree.hpp"
3 #include "opencv2/legacy/legacy.hpp"
4 #include <iostream>
5
6 using namespace cv; 7 using namespace std; 8
9 void OptimizeSeam(Mat& img1, Mat& trans, Mat& dst); 10
11 typedef struct
12 { 13 Point2f left_top; 14 Point2f left_bottom; 15 Point2f right_top; 16 Point2f right_bottom; 17 }four_corners_t; 18
19 four_corners_t corners; 20
21 void CalcCorners(const Mat& H, const Mat& src) 22 { 23 double v2[] = { 0, 0, 1 };//左上角
24 double v1[3];//變換后的坐標值
25 Mat V2 = Mat(3, 1, CV_64FC1, v2); //列向量
26 Mat V1 = Mat(3, 1, CV_64FC1, v1); //列向量
27
28 V1 = H * V2; 29 //左上角(0,0,1)
30 cout << "V2: " << V2 << endl; 31 cout << "V1: " << V1 << endl; 32 corners.left_top.x = v1[0] / v1[2]; 33 corners.left_top.y = v1[1] / v1[2]; 34
35 //左下角(0,src.rows,1)
36 v2[0] = 0; 37 v2[1] = src.rows; 38 v2[2] = 1; 39 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
40 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
41 V1 = H * V2; 42 corners.left_bottom.x = v1[0] / v1[2]; 43 corners.left_bottom.y = v1[1] / v1[2]; 44
45 //右上角(src.cols,0,1)
46 v2[0] = src.cols; 47 v2[1] = 0; 48 v2[2] = 1; 49 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
50 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
51 V1 = H * V2; 52 corners.right_top.x = v1[0] / v1[2]; 53 corners.right_top.y = v1[1] / v1[2]; 54
55 //右下角(src.cols,src.rows,1)
56 v2[0] = src.cols; 57 v2[1] = src.rows; 58 v2[2] = 1; 59 V2 = Mat(3, 1, CV_64FC1, v2); //列向量
60 V1 = Mat(3, 1, CV_64FC1, v1); //列向量
61 V1 = H * V2; 62 corners.right_bottom.x = v1[0] / v1[2]; 63 corners.right_bottom.y = v1[1] / v1[2]; 64
65 } 66
67 int main(int argc, char *argv[]) 68 { 69 Mat image01 = imread("t1.jpg", 1); //右圖
70 Mat image02 = imread("t2.jpg", 1); //左圖
71 imshow("p2", image01); 72 imshow("p1", image02); 73
74 //灰度圖轉換
75 Mat image1, image2; 76 cvtColor(image01, image1, CV_RGB2GRAY); 77 cvtColor(image02, image2, CV_RGB2GRAY); 78
79
80 //提取特征點
81 OrbFeatureDetector surfDetector(3000); 82 vector<KeyPoint> keyPoint1, keyPoint2; 83 surfDetector.detect(image1, keyPoint1); 84 surfDetector.detect(image2, keyPoint2); 85
86 //特征點描述，為下邊的特征點匹配做准備
87 OrbDescriptorExtractor SurfDescriptor; 88 Mat imageDesc1, imageDesc2; 89 SurfDescriptor.compute(image1, keyPoint1, imageDesc1); 90 SurfDescriptor.compute(image2, keyPoint2, imageDesc2); 91
92 flann::Index flannIndex(imageDesc1, flann::LshIndexParams(12, 20, 2), cvflann::FLANN_DIST_HAMMING); 93
94 vector<DMatch> GoodMatchePoints; 95
96 Mat macthIndex(imageDesc2.rows, 2, CV_32SC1), matchDistance(imageDesc2.rows, 2, CV_32FC1); 97 flannIndex.knnSearch(imageDesc2, macthIndex, matchDistance, 2, flann::SearchParams()); 98
99 // Lowe's algorithm,獲取優秀匹配點
100 for (int i = 0; i < matchDistance.rows; i++) 101 { 102 if (matchDistance.at<float>(i, 0) < 0.4 * matchDistance.at<float>(i, 1)) 103 { 104 DMatch dmatches(i, macthIndex.at<int>(i, 0), matchDistance.at<float>(i, 0)); 105 GoodMatchePoints.push_back(dmatches); 106 } 107 } 108
109 Mat first_match; 110 drawMatches(image02, keyPoint2, image01, keyPoint1, GoodMatchePoints, first_match); 111 imshow("first_match ", first_match); 112
113 vector<Point2f> imagePoints1, imagePoints2; 114
115 for (int i = 0; i<GoodMatchePoints.size(); i++) 116 { 117 imagePoints2.push_back(keyPoint2[GoodMatchePoints[i].queryIdx].pt); 118 imagePoints1.push_back(keyPoint1[GoodMatchePoints[i].trainIdx].pt); 119 } 120
121
122
123 //獲取圖像1到圖像2的投影映射矩陣 尺寸為3*3
124 Mat homo = findHomography(imagePoints1, imagePoints2, CV_RANSAC); 125 ////也可以使用getPerspectiveTransform方法獲得透視變換矩陣，不過要求只能有4個點，效果稍差
126 //Mat homo=getPerspectiveTransform(imagePoints1,imagePoints2);
127 cout << "變換矩陣為：\n" << homo << endl << endl; //輸出映射矩陣 128
129 //計算配准圖的四個頂點坐標
130 CalcCorners(homo, image01); 131 cout << "left_top:" << corners.left_top << endl; 132 cout << "left_bottom:" << corners.left_bottom << endl; 133 cout << "right_top:" << corners.right_top << endl; 134 cout << "right_bottom:" << corners.right_bottom << endl; 135
136 //圖像配准
137 Mat imageTransform1, imageTransform2; 138 warpPerspective(image01, imageTransform1, homo, Size(MAX(corners.right_top.x, corners.right_bottom.x), image02.rows)); 139 //warpPerspective(image01, imageTransform2, adjustMat*homo, Size(image02.cols*1.3, image02.rows*1.8));
140 imshow("直接經過透視矩陣變換", imageTransform1); 141 imwrite("trans1.jpg", imageTransform1); 142
143
144 //創建拼接后的圖,需提前計算圖的大小
145 int dst_width = imageTransform1.cols; //取最右點的長度為拼接圖的長度
146 int dst_height = image02.rows; 147
148 Mat dst(dst_height, dst_width, CV_8UC3); 149 dst.setTo(0); 150
151 imageTransform1.copyTo(dst(Rect(0, 0, imageTransform1.cols, imageTransform1.rows))); 152 image02.copyTo(dst(Rect(0, 0, image02.cols, image02.rows))); 153
154 imshow("b_dst", dst); 155
156
157 OptimizeSeam(image02, imageTransform1, dst); 158
159
160 imshow("dst", dst); 161 imwrite("dst.jpg", dst); 162
163 waitKey(); 164
165 return 0; 166 } 167
168
169 //優化兩圖的連接處，使得拼接自然
170 void OptimizeSeam(Mat& img1, Mat& trans, Mat& dst) 171 { 172 int start = MIN(corners.left_top.x, corners.left_bottom.x);//開始位置，即重疊區域的左邊界
173
174 double processWidth = img1.cols - start;//重疊區域的寬度
175 int rows = dst.rows; 176 int cols = img1.cols; //注意，是列數*通道數
177 double alpha = 1;//img1中像素的權重
178 for (int i = 0; i < rows; i++) 179 { 180 uchar* p = img1.ptr<uchar>(i); //獲取第i行的首地址
181 uchar* t = trans.ptr<uchar>(i); 182 uchar* d = dst.ptr<uchar>(i); 183 for (int j = start; j < cols; j++) 184 { 185 //如果遇到圖像trans中無像素的黑點，則完全拷貝img1中的數據
186 if (t[j * 3] == 0 && t[j * 3 + 1] == 0 && t[j * 3 + 2] == 0) 187 { 188 alpha = 1; 189 } 190 else
191 { 192 //img1中像素的權重，與當前處理點距重疊區域左邊界的距離成正比，實驗證明，這種方法確實好
193 alpha = (processWidth - (j - start)) / processWidth; 194 } 195
196 d[j * 3] = p[j * 3] * alpha + t[j * 3] * (1 - alpha); 197 d[j * 3 + 1] = p[j * 3 + 1] * alpha + t[j * 3 + 1] * (1 - alpha); 198 d[j * 3 + 2] = p[j * 3 + 2] * alpha + t[j * 3 + 2] * (1 - alpha); 199
200 } 201 } 202 }
看一看拼接效果，我覺得還是不錯的。
看一下這一組圖片，這組圖片產生了鬼影，為什么？因為兩幅圖中的人物走動了啊！所以要做圖像拼接，盡量保證使用的是靜態圖片，不要加入一些動態因素干擾拼接。
opencv自帶的拼接算法stitch
opencv其實自己就有實現圖像拼接的算法，當然效果也是相當好的，但是因為其實現很復雜，而且代碼量很龐大，其實在一些小應用下的拼接有點殺雞用牛刀的感覺。最近在閱讀sticth源碼時，發現其中有幾個很有意思的地方。
1.opencv stitch選擇的特征檢測方式
一直很好奇opencv stitch算法到底選用了哪個算法作為其特征檢測方式，是ORB，SIFT還是SURF？讀源碼終於看到答案。
1 #ifdef HAVE_OPENCV_NONFREE 2 stitcher.setFeaturesFinder(new detail::SurfFeaturesFinder()); 3 #else
4 stitcher.setFeaturesFinder(new detail::OrbFeaturesFinder()); 5 #endif
在源碼createDefault函數中（默認設置），第一選擇是SURF,第二選擇才是ORB（沒有NONFREE模塊才選），所以既然大牛們這么選擇，必然是經過綜合考慮的，所以應該SURF算法在圖像拼接有着更優秀的效果。
2.opencv stitch獲取匹配點的方式
以下代碼是opencv stitch源碼中的特征點提取部分，作者使用了兩次特征點提取的思路：先對圖一進行特征點提取和篩選匹配（1->2），再對圖二進行特征點的提取和匹配（2->1），這跟我們平時的一次提取的思路不同，這種二次提取的思路可以保證更多的匹配點被選中，匹配點越多，findHomography求出的變換越准確。這個思路值得借鑒。
1 matches_info.matches.clear(); 2
3 Ptr<flann::IndexParams> indexParams = new flann::KDTreeIndexParams(); 4 Ptr<flann::SearchParams> searchParams = new flann::SearchParams(); 5
6 if (features2.descriptors.depth() == CV_8U) 7 { 8 indexParams->setAlgorithm(cvflann::FLANN_INDEX_LSH); 9 searchParams->setAlgorithm(cvflann::FLANN_INDEX_LSH); 10 } 11
12 FlannBasedMatcher matcher(indexParams, searchParams); 13 vector< vector<DMatch> > pair_matches; 14 MatchesSet matches; 15
16 // Find 1->2 matches
17 matcher.knnMatch(features1.descriptors, features2.descriptors, pair_matches, 2); 18 for (size_t i = 0; i < pair_matches.size(); ++i) 19 { 20 if (pair_matches[i].size() < 2) 21 continue; 22 const DMatch& m0 = pair_matches[i][0]; 23 const DMatch& m1 = pair_matches[i][1]; 24 if (m0.distance < (1.f - match_conf_) * m1.distance) 25 { 26 matches_info.matches.push_back(m0); 27 matches.insert(make_pair(m0.queryIdx, m0.trainIdx)); 28 } 29 } 30 LOG("\n1->2 matches: " << matches_info.matches.size() << endl); 31
32 // Find 2->1 matches
33 pair_matches.clear(); 34 matcher.knnMatch(features2.descriptors, features1.descriptors, pair_matches, 2); 35 for (size_t i = 0; i < pair_matches.size(); ++i) 36 { 37 if (pair_matches[i].size() < 2) 38 continue; 39 const DMatch& m0 = pair_matches[i][0]; 40 const DMatch& m1 = pair_matches[i][1]; 41 if (m0.distance < (1.f - match_conf_) * m1.distance) 42 if (matches.find(make_pair(m0.trainIdx, m0.queryIdx)) == matches.end()) 43 matches_info.matches.push_back(DMatch(m0.trainIdx, m0.queryIdx, m0.distance)); 44 } 45 LOG("1->2 & 2->1 matches: " << matches_info.matches.size() << endl);
這里我仿照opencv源碼二次提取特征點的思路對我原有拼接代碼進行改寫，實驗證明獲取的匹配點確實較一次提取要多。
1 //提取特征點
2 SiftFeatureDetector Detector(1000); // 海塞矩陣閾值，在這里調整精度，值越大點越少，越精准
3 vector<KeyPoint> keyPoint1, keyPoint2; 4 Detector.detect(image1, keyPoint1); 5 Detector.detect(image2, keyPoint2); 6
7 //特征點描述，為下邊的特征點匹配做准備
8 SiftDescriptorExtractor Descriptor; 9 Mat imageDesc1, imageDesc2; 10 Descriptor.compute(image1, keyPoint1, imageDesc1); 11 Descriptor.compute(image2, keyPoint2, imageDesc2); 12
13 FlannBasedMatcher matcher; 14 vector<vector<DMatch> > matchePoints; 15 vector<DMatch> GoodMatchePoints; 16
17 MatchesSet matches; 18
19 vector<Mat> train_desc(1, imageDesc1); 20 matcher.add(train_desc); 21 matcher.train(); 22
23 matcher.knnMatch(imageDesc2, matchePoints, 2); 24
25 // Lowe's algorithm,獲取優秀匹配點
26 for (int i = 0; i < matchePoints.size(); i++) 27 { 28 if (matchePoints[i][0].distance < 0.4 * matchePoints[i][1].distance) 29 { 30 GoodMatchePoints.push_back(matchePoints[i][0]); 31 matches.insert(make_pair(matchePoints[i][0].queryIdx, matchePoints[i][0].trainIdx)); 32 } 33 } 34 cout<<"\n1->2 matches: " << GoodMatchePoints.size() << endl; 35
36 #if 1
37
38 FlannBasedMatcher matcher2; 39 matchePoints.clear(); 40 vector<Mat> train_desc2(1, imageDesc2); 41 matcher2.add(train_desc2); 42 matcher2.train(); 43
44 matcher2.knnMatch(imageDesc1, matchePoints, 2); 45 // Lowe's algorithm,獲取優秀匹配點
46 for (int i = 0; i < matchePoints.size(); i++) 47 { 48 if (matchePoints[i][0].distance < 0.4 * matchePoints[i][1].distance) 49 { 50 if (matches.find(make_pair(matchePoints[i][0].trainIdx, matchePoints[i][0].queryIdx)) == matches.end()) 51 { 52 GoodMatchePoints.push_back(DMatch(matchePoints[i][0].trainIdx, matchePoints[i][0].queryIdx, matchePoints[i][0].distance)); 53 } 54
55 } 56 } 57 cout<<"1->2 & 2->1 matches: " << GoodMatchePoints.size() << endl; 58 #endif
最后再看一下opencv stitch的拼接效果吧~速度雖然比較慢，但是效果還是很好的。
1 #include <iostream>
2 #include <opencv2/core/core.hpp>
3 #include <opencv2/highgui/highgui.hpp>
4 #include <opencv2/imgproc/imgproc.hpp>
5 #include <opencv2/stitching/stitcher.hpp>
6 using namespace std; 7 using namespace cv; 8 bool try_use_gpu = false; 9 vector<Mat> imgs; 10 string result_name = "dst1.jpg"; 11 int main(int argc, char * argv[]) 12 { 13 Mat img1 = imread("34.jpg"); 14 Mat img2 = imread("35.jpg"); 15
16 imshow("p1", img1); 17 imshow("p2", img2); 18
19 if (img1.empty() || img2.empty()) 20 { 21 cout << "Can't read image" << endl; 22 return -1; 23 } 24 imgs.push_back(img1); 25 imgs.push_back(img2); 26
27
28 Stitcher stitcher = Stitcher::createDefault(try_use_gpu); 29 // 使用stitch函數進行拼接
30 Mat pano; 31 Stitcher::Status status = stitcher.stitch(imgs, pano); 32 if (status != Stitcher::OK) 33 { 34 cout << "Can't stitch images, error code = " << int(status) << endl; 35 return -1; 36 } 37 imwrite(result_name, pano); 38 Mat pano2 = pano.clone(); 39 // 顯示源圖像，和結果圖像
40 imshow("全景圖像", pano); 41 if (waitKey() == 27) 42 return 0; 43 }
