学习OpenCV——BOW特征提取函数（特征点篇）

from：  http://www.xuebuyuan.com/582331.html

简单的通过特征点分类的方法：                                                                      

一、train

1.提取+/- sample的feature，每幅图提取出的sift特征个数不定（假设每个feature有128维）

2.利用聚类方法（e.g K-means）将不定数量的feature聚类为固定数量的(比如10个)words即BOW（bag of word）

（本篇文章主要完成以上的工作！）

3.normalize，并作这10个类的直方图e.g [0.1，0.2，0.7，0...0];

4.将each image的这10个word作为feature_instance 和 (手工标记的) label（+/-）进入SVM训练

二、predict

1. 提取test_img的feature（如137个）

2. 分别求each feature与10个类的距离(e.g. 128维欧氏距离)，确定该feature属于哪个类

3. normalize，并作这10个类的直方图e.g [0，0.2，0.2，0.6，0...0];

4. 应用SVM_predict进行结果预测

通过OpenCV实现feature聚类 BOW                                                             

首先在此介绍一下OpenCV的特征描述符与BOW的通用函数。

主要的通用接口有：

 

1.特征点提取

Ptr<FeatureDetector> FeatureDetector::create(const string& detectorType)

	Ptr<FeatureDetector> FeatureDetector::create(const string& detectorType)
// 	"FAST" – FastFeatureDetector 
// 	"STAR" – StarFeatureDetector 
// 	"SIFT" – SIFT (nonfree module)//必须使用 initModule_nonfree()初始化
// 	"SURF" – SURF (nonfree module)//同上; 
// 	"ORB" – ORB 
// 	"MSER" – MSER 
// 	"GFTT" – GoodFeaturesToTrackDetector 
// 	"HARRIS" – GoodFeaturesToTrackDetector with Harris detector enabled 
// 	"Dense" – DenseFeatureDetector 
// 	"SimpleBlob" – SimpleBlobDetector 

根据以上接口，测试不同的特征点：

对同一幅图像进行水平翻转前后的两幅图像检测特征点检测结果，

检测到的特征点的坐标类型为：pt: int / float（与keyPoint的性质有关）

数量分别为num1, num2,

 "FAST" – FastFeatureDetector           pt:int (num1:615  num2:618)
 "STAR" – StarFeatureDetector           pt:int (num1:43   num2:42 )
 "SIFT" – SIFT (nonfree module)          pt:float(num1:155  num2:135)            //必须使用 initModule_nonfree()初始化
 "SURF" – SURF (nonfree module)     pt:float(num1:344  num2:342)           
//同上; 
 "ORB" – ORB                                        pt:float(num1:496  num2:497)
 "MSER" – MSER                                 pt:float(num1:51   num2:45 )
 "GFTT" – GoodFeaturesToTrackDetector        pt:int (num1:744  num2:771)
 "HARRIS" – GoodFeaturesToTrackDetector with Harris detector enabled         pt:float(num1:162  num2:160)
 "Dense" – DenseFeatureDetector          pt:int (num1:3350 num2:3350)

2.特征描述符提取

Ptr<DescriptorExtractor> DescriptorExtractor::create(const string& descriptorExtractorType)

//  Ptr<DescriptorExtractor> DescriptorExtractor::create(const string& descriptorExtractorType)	
// 	"SIFT" – SIFT 
// 	"SURF" – SURF 
// 	"ORB" – ORB 
// 	"BRIEF" – BriefDescriptorExtractor 

3.描述符匹配

Ptr<DescriptorMatcher> descriptorMatcher = DescriptorMatcher::create(const string& descriptorMatcherType)

// 	descriptorMatcherType – Descriptor matcher type. 
//	Now the following matcher types are supported: 
// 		BruteForce (it uses L2 ) 
// 		BruteForce-L1 
// 		BruteForce-Hamming 
// 		BruteForce-Hamming(2) 
// 		FlannBased 
	Ptr<DescriptorMatcher> descriptorMatcher = DescriptorMatcher::create( "BruteForce" );

4.class BOWTrainer

class BOWKmeansTrainer::public BOWTrainer:Kmeans算法训练

BOWKMeansTrainer ::BOWKmeansTrainer(int clusterCount, const TermCriteria& termcrit=TermCriteria(), int attempts=3, int flags=KMEANS_PP_CENTERS)¶

parameter same as 
Kmeans

代码实现：                                                                                                                    

1.画特征点。

2.特征点Kmeans聚类，每一种颜色代表一个类别。

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/nonfree.hpp"

#include <iostream>

using namespace cv;
using namespace std;

#define ClusterNum 10

void DrawAndMatchKeypoints(const Mat& Img1,const Mat& Img2,const vector<KeyPoint>& Keypoints1,
	const vector<KeyPoint>& Keypoints2,const Mat& Descriptors1,const Mat& Descriptors2)
{
	Mat keyP1,keyP2;
	drawKeypoints(Img1,Keypoints1,keyP1,Scalar::all(-1),0);
	drawKeypoints(Img2,Keypoints2,keyP2,Scalar::all(-1),0);
	putText(keyP1, "drawKeyPoints", cvPoint(10,30), FONT_HERSHEY_SIMPLEX, 1 ,Scalar :: all(-1));
	putText(keyP2, "drawKeyPoints", cvPoint(10,30), FONT_HERSHEY_SIMPLEX, 1 ,Scalar :: all(-1));
	imshow("img1 keyPoints",keyP1);
	imshow("img2 keyPoints",keyP2);

	Ptr<DescriptorMatcher> descriptorMatcher = DescriptorMatcher::create( "BruteForce" );
	vector<DMatch> matches;
	descriptorMatcher->match( Descriptors1, Descriptors2, matches );
	Mat show;
	drawMatches(Img1,Keypoints1,Img2,Keypoints2,matches,show,Scalar::all(-1),CV_RGB(255,255,255),Mat(),4);
	putText(show, "drawMatchKeyPoints", cvPoint(10,30), FONT_HERSHEY_SIMPLEX, 1 ,Scalar :: all(-1));  
	imshow("match",show);
}

//测试OpenCV：class BOWTrainer
void BOWKeams(const Mat& img, const vector<KeyPoint>& Keypoints, 
	const Mat& Descriptors, Mat& centers)
{
	//BOW的kmeans算法聚类;
	BOWKMeansTrainer bowK(ClusterNum, 
		cvTermCriteria (CV_TERMCRIT_EPS + CV_TERMCRIT_ITER, 10, 0.1),3,2);
	centers = bowK.cluster(Descriptors);
	cout<<endl<<"< cluster num: "<<centers.rows<<" >"<<endl;
	
	Ptr<DescriptorMatcher> descriptorMatcher = DescriptorMatcher::create( "BruteForce" );
	vector<DMatch> matches;
	descriptorMatcher->match(Descriptors,centers,matches);//const Mat& queryDescriptors, const Mat& trainDescriptors第一个参数是待分类节点，第二个参数是聚类中心;
	Mat demoCluster;
	img.copyTo(demoCluster);
	
	//为每一类keyPoint定义一种颜色
	Scalar color[]={CV_RGB(255,255,255),
     CV_RGB(255,0,0),CV_RGB(0,255,0),CV_RGB(0,0,255),
     CV_RGB(255,255,0),CV_RGB(255,0,255),CV_RGB(0,255,255),
     CV_RGB(123,123,0),CV_RGB(0,123,123),CV_RGB(123,0,123)};


	for (vector<DMatch>::iterator iter=matches.begin();iter!=matches.end();iter++)
	{
		cout<<"< descriptorsIdx:"<<iter->queryIdx<<"  centersIdx:"<<iter->trainIdx
			<<" distincs:"<<iter->distance<<" >"<<endl;
		Point center= Keypoints[iter->queryIdx].pt;
		circle(demoCluster,center,2,color[iter->trainIdx],-1);
	}
	putText(demoCluster, "KeyPoints Clustering: 一种颜色代表一种类型",
		cvPoint(10,30), FONT_HERSHEY_SIMPLEX, 1 ,Scalar :: all(-1));
	imshow("KeyPoints Clusrtering",demoCluster);
	
}




int main()
{
	cv::initModule_nonfree();//使用SIFT/SURF create之前，必须先initModule_<modulename>(); 

	cout << "< Creating detector, descriptor extractor and descriptor matcher ...";
	Ptr<FeatureDetector> detector = FeatureDetector::create( "SIFT" );

	Ptr<DescriptorExtractor> descriptorExtractor = DescriptorExtractor::create( "SIFT" );

	Ptr<DescriptorMatcher> descriptorMatcher = DescriptorMatcher::create( "BruteForce" );



	cout << ">" << endl;

	if( detector.empty() || descriptorExtractor.empty() )
	{
		cout << "Can not create detector or descriptor exstractor or descriptor matcher of given types" << endl;
		return -1;
	}
	cout << endl << "< Reading images..." << endl;
	Mat img1 = imread("D:/demo0.jpg");
	Mat img2 = imread("D:/demo1.jpg");
	cout<<endl<<">"<<endl;


	//detect keypoints;
	cout << endl << "< Extracting keypoints from images..." << endl;
	vector<KeyPoint> keypoints1,keypoints2;
	detector->detect( img1, keypoints1 );
	detector->detect( img2, keypoints2 );
	cout <<"img1:"<< keypoints1.size() << " points  img2:" <<keypoints2.size() 
		<< " points" << endl << ">" << endl;
	
	//compute descriptors for keypoints;
	cout << "< Computing descriptors for keypoints from images..." << endl;
	Mat descriptors1,descriptors2;
	descriptorExtractor->compute( img1, keypoints1, descriptors1 );
	descriptorExtractor->compute( img2, keypoints2, descriptors2 );

	cout<<endl<<"< Descriptoers Size: "<<descriptors2.size()<<" >"<<endl;
	cout<<endl<<"descriptor's col: "<<descriptors2.cols<<endl
		<<"descriptor's row: "<<descriptors2.rows<<endl;
	cout << ">" << endl;

	//Draw And Match img1,img2 keypoints
	//匹配的过程是对特征点的descriptors进行match;
	DrawAndMatchKeypoints(img1,img2,keypoints1,keypoints2,descriptors1,descriptors2);

	Mat center;
	//对img1提取特征点，并聚类
	//测试OpenCV：class BOWTrainer
	BOWKeams(img1,keypoints1,descriptors1,center);


	waitKey();

}

通过Qt实现DrawKeypoints：

void Qt_test1::on_DrawKeypoints_clicked()
{
	//initModule_nonfree();
	Ptr<FeatureDetector> detector = FeatureDetector::create( "FAST" );
	vector<KeyPoint> keypoints;
	detector->detect( src, keypoints );

	Mat DrawKeyP;
	drawKeypoints(src,keypoints,DrawKeyP,Scalar::all(-1),0);
	putText(DrawKeyP, "drawKeyPoints", cvPoint(10,30), 
		FONT_HERSHEY_SIMPLEX, 0.5 ,Scalar :: all(255));
	cvtColor(DrawKeyP, image, CV_RGB2RGBA);
	QImage img = QImage((const unsigned char*)(image.data), 
		image.cols, image.rows, QImage::Format_RGB32);
	QLabel *label = new QLabel(this);
	label->move(50, 50);//图像在窗口中所处的位置;
	label->setPixmap(QPixmap::fromImage(img));
	label->resize(label->pixmap()->size());	
	label->show();
}

由于initModule_nonfree()总是出错，无法对SIFT与SURF特征点提取，

而且无法实现聚类因为运行/BOW的kmeans算法聚类：BOWKMeansTrainer bowK(ClusterNum, cvTermCriteria (CV_TERMCRIT_EPS + CV_TERMCRIT_ITER, 10, 0.1),3,2);总是出错，不知道咋解决~~~~~(>_<)~~~~ 需要继续学习