今天在网上看到一个比较健壮的图像特征匹配算法，遂拿出来与大家分享
1.首先看看类定义：
class RobustMatcher
{
private:
// pointer to the feature point detector object
cv::Ptr<cv::FeatureDetector> detector;
// pointer to the feature descriptor extractor object
cv::Ptr<cv::DescriptorExtractor> extractor;
// pointer to the matcher object
cv::Ptr<cv::DescriptorMatcher > matcher;
float ratio; // max ratio between 1st and 2nd NN
bool refineF; // if true will refine the F matrix
double distance; // min distance to epipolar
double confidence; // confidence level (probability)
public:
RobustMatcher() : ratio(0.65f), refineF(true),
confidence(0.99), distance(3.0)
{
// ORB is the default feature
detector= new cv::OrbFeatureDetector();
extractor= new cv::OrbDescriptorExtractor();
matcher= new cv::BruteForceMatcher<cv::HammingLUT>;
}
// Set the feature detector
void setFeatureDetector(cv::Ptr<cv::FeatureDetector>& detect)
{
detector= detect;
}
// Set the descriptor extractor
void setDescriptorExtractor(cv::Ptr<cv::DescriptorExtractor>& desc)
{
extractor= desc;
}
// Set the matcher
void setDescriptorMatcher(cv::Ptr<cv::DescriptorMatcher>& match)
{
matcher= match;
}
// Set confidence level
void setConfidenceLevel(double conf)
{
confidence= conf;
}
//Set MinDistanceToEpipolar
void setMinDistanceToEpipolar(double dist)
{
distance= dist;
}
//Set ratio
void setRatio(float rat)
{
ratio= rat;
}
cv::Mat match(cv::Mat& image1, cv::Mat& image2, // input images
// output matches and keypoints
std::vector<cv::DMatch>& matches,
std::vector<cv::KeyPoint>& keypoints1,
std::vector<cv::KeyPoint>& keypoints2);
cv::Mat ransacTest(
const std::vector<cv::DMatch>& matches,
const std::vector<cv::KeyPoint>& keypoints1,
const std::vector<cv::KeyPoint>& keypoints2,
std::vector<cv::DMatch>& outMatches);
void symmetryTest(
const std::vector<std::vector<cv::DMatch> >& matches1,
const std::vector<std::vector<cv::DMatch> >& matches2,
std::vector<cv::DMatch>& symMatches);
int ratioTest(std::vector<std::vector<cv::DMatch>>& matches);
};
2.cpp中的内容为：
int RobustMatcher::ratioTest(std::vector<std::vector<cv::DMatch> >
&matches)
{
int removed=0;
// for all matches
for (std::vector<std::vector<cv::DMatch> >::iterator
matchIterator= matches.begin();
matchIterator!= matches.end(); ++matchIterator)
{
// if 2 NN has been identified
if (matchIterator->size() > 1)
{
// check distance ratio
if ((*matchIterator)[0].distance/
(*matchIterator)[1].distance > ratio)
{
matchIterator->clear(); // remove match
removed++;
}
} else
{ // does not have 2 neighbours
matchIterator->clear(); // remove match
removed++;
}
}
return removed;//返回被删除的点数量
}
// Insert symmetrical matches in symMatches vector
void RobustMatcher::symmetryTest(
const std::vector<std::vector<cv::DMatch> >& matches1,
const std::vector<std::vector<cv::DMatch> >& matches2,
std::vector<cv::DMatch>& symMatches)
{
// for all matches image 1 -> image 2
for (std::vector<std::vector<cv::DMatch> >::
const_iterator matchIterator1= matches1.begin();
matchIterator1!= matches1.end(); ++matchIterator1)
{
// ignore deleted matches
if (matchIterator1->size() < 2)
continue;
// for all matches image 2 -> image 1
for (std::vector<std::vector<cv::DMatch> >::
const_iterator matchIterator2= matches2.begin();
matchIterator2!= matches2.end();
++matchIterator2)
{
// ignore deleted matches
if (matchIterator2->size() < 2)
continue;
// Match symmetry test
if ((*matchIterator1)[0].queryIdx ==
(*matchIterator2)[0].trainIdx &&
(*matchIterator2)[0].queryIdx ==
(*matchIterator1)[0].trainIdx)
{
// add symmetrical match
symMatches.push_back(
cv::DMatch((*matchIterator1)[0].queryIdx,
(*matchIterator1)[0].trainIdx,
(*matchIterator1)[0].distance));
break; // next match in image 1 -> image 2
}
}
}
}
// Identify good matches using RANSAC
// Return fundemental matrix
cv::Mat RobustMatcher::ransacTest(
const std::vector<cv::DMatch>& matches,
const std::vector<cv::KeyPoint>& keypoints1,
const std::vector<cv::KeyPoint>& keypoints2,
std::vector<cv::DMatch>& outMatches)
{
// Convert keypoints into Point2f
std::vector<cv::Point2f> points1, points2;
cv::Mat fundemental;
for (std::vector<cv::DMatch>::
const_iterator it= matches.begin();
it!= matches.end(); ++it)
{
// Get the position of left keypoints
float x= keypoints1[it->queryIdx].pt.x;
float y= keypoints1[it->queryIdx].pt.y;
points1.push_back(cv::Point2f(x,y));
// Get the position of right keypoints
x= keypoints2[it->trainIdx].pt.x;
y= keypoints2[it->trainIdx].pt.y;
points2.push_back(cv::Point2f(x,y));
}
// Compute F matrix using RANSAC
std::vector<uchar> inliers(points1.size(),0);
if (points1.size()>0&&points2.size()>0)
{
cv::Mat fundemental= cv::findFundamentalMat(
cv::Mat(points1),cv::Mat(points2), // matching points
inliers, // match status (inlier or outlier)
CV_FM_RANSAC, // RANSAC method
distance, // distance to epipolar line
confidence); // confidence probability
// extract the surviving (inliers) matches
std::vector<uchar>::const_iterator
itIn= inliers.begin();
std::vector<cv::DMatch>::const_iterator
itM= matches.begin();
// for all matches
for ( ;itIn!= inliers.end(); ++itIn, ++itM)
{
if (*itIn)
{ // it is a valid match
outMatches.push_back(*itM);
}
}
if (refineF)
{
// The F matrix will be recomputed with
// all accepted matches
// Convert keypoints into Point2f
// for final F computation
points1.clear();
points2.clear();
for (std::vector<cv::DMatch>::
const_iterator it= outMatches.begin();
it!= outMatches.end(); ++it)
{
// Get the position of left keypoints
float x= keypoints1[it->queryIdx].pt.x;
float y= keypoints1[it->queryIdx].pt.y;
points1.push_back(cv::Point2f(x,y));
// Get the position of right keypoints
x= keypoints2[it->trainIdx].pt.x;
y= keypoints2[it->trainIdx].pt.y;
points2.push_back(cv::Point2f(x,y));
}
// Compute 8-point F from all accepted matches
if (points1.size()>0&&points2.size()>0)
{
fundemental= cv::findFundamentalMat(
cv::Mat(points1),cv::Mat(points2), // matches
CV_FM_8POINT); // 8-point method
}
}
}
return fundemental;
}
// Match feature points using symmetry test and RANSAC
// returns fundemental matrix
cv::Mat RobustMatcher:: match(cv::Mat& image1,
cv::Mat& image2, // input images
// output matches and keypoints
std::vector<cv::DMatch>& matches,
std::vector<cv::KeyPoint>& keypoints1,
std::vector<cv::KeyPoint>& keypoints2)
{
detector->detect(image1,keypoints1);
detector->detect(image2,keypoints2);
cv::Mat descriptors1, descriptors2;
extractor->compute(image1,keypoints1,descriptors1);
extractor->compute(image2,keypoints2,descriptors2);
// Construction of the matcher
//cv::BruteForceMatcher<cv::L2<float>> matcher;
// from image 1 to image 2
// based on k nearest neighbours (with k=2)
std::vector<std::vector<cv::DMatch> > matches1;
matcher->knnMatch(descriptors1,descriptors2,
matches1, // vector of matches (up to 2 per entry)
2); // return 2 nearest neighbours
// from image 2 to image 1
// based on k nearest neighbours (with k=2)
std::vector<std::vector<cv::DMatch> > matches2;
matcher->knnMatch(descriptors2,descriptors1,
matches2, // vector of matches (up to 2 per entry)
2); // return 2 nearest neighbours
// > than threshold
// clean image 1 -> image 2 matches
int removed= ratioTest(matches1);
// clean image 2 -> image 1 matches
removed= ratioTest(matches2);
std::vector<cv::DMatch> symMatches;
symmetryTest(matches1,matches2,symMatches);
//=========================================测试代码
cv::Mat img_matches;
cv::drawMatches( image1, keypoints1, image2, keypoints2,
symMatches, img_matches, cv::Scalar::all(-1), cv::Scalar::all(-1),
std::vector<char>(), cv::DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
/*imshow("Test",img_matches);*/
//cvWaitKey(0);
//=========================================测试代码
cv::Mat fundemental= ransacTest(symMatches,
keypoints1, keypoints2, matches);
//=========================================测试代码
std::vector<cv::Point2f> obj;
std::vector<cv::Point2f> scene;
for( int i = 0; i < symMatches.size(); i++ )
{
//-- Get the keypoints from the good matches
obj.push_back( keypoints1[ symMatches[i].queryIdx ].pt );
scene.push_back( keypoints2[ symMatches[i].trainIdx ].pt );
}
cv::Mat H = cv::findHomography( obj, scene, CV_RANSAC ,2);
//-- Get the corners from the image_1 ( the object to be "detected" )
std::vector<cv::Point2f> obj_corners(4);
obj_corners[0]=cvPoint(0,0);
obj_corners[1]=cvPoint(image1.cols, 0 );
obj_corners[2]=cvPoint(image1.cols,image1.rows);
obj_corners[3]=cvPoint(0,image1.rows);
std::vector<cv::Point2f> scene_corners(4);
cv::perspectiveTransform( obj_corners, scene_corners, H);
for( int i = 0; i < 4; i++ )
{
scene_corners[i].x+=image1.cols;
}
line( img_matches, scene_corners[0], scene_corners[1], cv::Scalar(0, 255, 0), 2 );
line( img_matches, scene_corners[1], scene_corners[2], cv::Scalar( 0, 255, 0), 2 );
line( img_matches, scene_corners[2], scene_corners[3], cv::Scalar( 0, 255, 0), 2 );
line( img_matches, scene_corners[3], scene_corners[0], cv::Scalar( 0, 255, 0), 2 );
imshow("Test",img_matches);
cvWaitKey(0);
//=========================================测试代码
// return the found fundemental matrix
return fundemental;
}
剔除低质量匹配点
ratioTest用来剔除距离比例相差过大的配对点，配对点之间的距离相差越大，能匹配上的概率也就越小。这里使用一个参数ratio来控制剔除距离相差在一定范围之外的特征点。
symmetryTest用来判断两个图像间的特征点匹配是否是一一映射，对于不是的点则剔除掉
3.具体调用的例子：
void main()
{
// set parameters
int numKeyPoints = 1500;
//Instantiate robust matcher
RobustMatcher rmatcher;
//instantiate detector, extractor, matcher
cv::Ptr<cv::FeatureDetector> detector = new cv::OrbFeatureDetector(numKeyPoints);
cv::Ptr<cv::DescriptorExtractor> extractor = new cv::OrbDescriptorExtractor;
cv::Ptr<cv::DescriptorMatcher> matcher = new cv::BruteForceMatcher<cv::HammingLUT>;
rmatcher.setFeatureDetector(detector);
rmatcher.setDescriptorExtractor(extractor);
rmatcher.setDescriptorMatcher(matcher);
//Load input image detect keypoints
cv::Mat img1;
std::vector<cv::KeyPoint> img1_keypoints;
cv::Mat img1_descriptors;
cv::Mat img2;
std::vector<cv::KeyPoint> img2_keypoints;
cv::Mat img2_descriptors;
std::vector<cv::DMatch> matches;
img1 = cv::imread("C:\\temp\\PyramidPattern.jpg", CV_LOAD_IMAGE_GRAYSCALE);
/*img2 = cv::imread("C:\\temp\\PyramidPatternTest.bmp", CV_LOAD_IMAGE_GRAYSCALE);*/
//img2 = cv::imread("C:\\temp\\test1.jpg", CV_LOAD_IMAGE_GRAYSCALE);
img2 = cv::imread("C:\\temp\\test2.jpg", CV_LOAD_IMAGE_GRAYSCALE);
rmatcher.match(img1, img2, matches, img1_keypoints, img2_keypoints);
}
最后，对于OpenCV中的所有特征匹配算法都可以用这个办法来做，比如SIFT, SURF等等。只需要简单的替换第一步中的extractor和detector就可以了。
匹配效果还不错，上个图
但是缺点也是存在的，如果完整匹配下来，大约一帧耗时100+ms,不适合实时性要求较高的应用
