用opencv检测convexity defects

一概念：

Convexity hull, Convexity defects

如上图所示，黑色的轮廓线为convexity hull, 而convexity hull与手掌之间的部分为convexity defects. 每个convexity defect区域有四个特征量：起始点（startPoint），结束点(endPoint)，距离convexity hull最远点(farPoint)，最远点到convexity hull的距离(depth)。

二．OpenCV中的相关函数

void convexityDefects(InputArray contour, InputArray convexhull, OutputArrayconvexityDefects)

参数：

coutour: 输入参数，检测到的轮廓，可以调用findContours函数得到；

convexhull: 输入参数，检测到的凸包，可以调用convexHull函数得到。注意，convexHull函数可以得到vector<vector<Point>>和vector<vector<int>>两种类型结果，这里的convexhull应该为vector<vector<int>>类型，否则通不过ASSERT检查；

convexityDefects：输出参数，检测到的最终结果，应为vector<vector<Vec4i>>类型，Vec4i存储了起始点（startPoint），结束点(endPoint)，距离convexity hull最远点(farPoint)以及最远点到convexity hull的距离(depth)

三．代码

//http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/hull/hull.html
//http://www.codeproject.com/Articles/782602/Beginners-guide-to-understand-Fingertips-counting#include "opencv2/highgui/highgui.hpp"#include "opencv2/imgproc/imgproc.hpp"#include <iostream>#include <stdio.h>#include <stdlib.h>using namespace cv;using namespace std;Mat src; Mat src_gray;int thresh = 100;int max_thresh = 255;RNG rng(12345);/// Function headervoid thresh_callback(int, void* );/** @function main */
int main( int argc, char** argv ){/// Load source image and convert it to graysrc = imread( argv[1], 1 );/// Convert image to gray and blur itcvtColor( src, src_gray, CV_BGR2GRAY );blur( src_gray, src_gray, Size(3,3) );/// Create Windowchar* source_window = "Source";namedWindow( source_window, CV_WINDOW_AUTOSIZE );imshow( source_window, src );createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );thresh_callback( 0, 0 );waitKey(0);return(0);}/** @function thresh_callback */void thresh_callback(int, void* ){Mat src_copy = src.clone();Mat threshold_output;vector<vector<Point> > contours;vector<Vec4i> hierarchy;/// Detect edges using Thresholdthreshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );/// Find contoursfindContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );/// Find the convex hull object for each contourvector<vector<Point> >hull( contours.size() );// Int type hullvector<vector<int>> hullsI( contours.size() );// Convexity defectsvector<vector<Vec4i>> defects( contours.size() );for( size_t i = 0; i < contours.size(); i++ ){  convexHull( Mat(contours[i]), hull[i], false ); // find int type hullconvexHull( Mat(contours[i]), hullsI[i], false ); // get convexity defectsconvexityDefects(Mat(contours[i]),hullsI[i], defects[i]);}/// Draw contours + hull resultsMat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );for( size_t i = 0; i< contours.size(); i++ ){Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );drawContours( drawing, contours, i, color, 1, 8, vector<Vec4i>(), 0, Point() );drawContours( drawing, hull, i, color, 1, 8, vector<Vec4i>(), 0, Point() );// draw defectssize_t count = contours[i].size();std::cout<<"Count : "<<count<<std::endl;if( count < 300 )continue;vector<Vec4i>::iterator d =defects[i].begin();while( d!=defects[i].end() ) {Vec4i& v=(*d);//if(IndexOfBiggestContour == i){int startidx=v[0]; Point ptStart( contours[i][startidx] ); // point of the contour where the defect beginsint endidx=v[1]; Point ptEnd( contours[i][endidx] ); // point of the contour where the defect endsint faridx=v[2]; Point ptFar( contours[i][faridx] );// the farthest from the convex hull point within the defectint depth = v[3] / 256; // distance between the farthest point and the convex hullif(depth > 20 && depth < 80){line( drawing, ptStart, ptFar, CV_RGB(0,255,0), 2 );line( drawing, ptEnd, ptFar, CV_RGB(0,255,0), 2 );circle( drawing, ptStart,   4, Scalar(255,0,100), 2 );circle( drawing, ptEnd,   4, Scalar(255,0,100), 2 );circle( drawing, ptFar,   4, Scalar(100,0,255), 2 );}/*printf("start(%d,%d) end(%d,%d), far(%d,%d)\n",ptStart.x, ptStart.y, ptEnd.x, ptEnd.y, ptFar.x, ptFar.y);*/}d++;}}/// Show in a windownamedWindow( "Hull demo", CV_WINDOW_AUTOSIZE );imshow( "Hull demo", drawing );//imwrite("convexity_defects.jpg", drawing);}

四．结果

原图

Convexity defects图，蓝色点是convexity defects的起始点和结束点，红色点是最远点。（为什么有的起始点和结束点中间没有最远点呢？因为只画出了depth范围在20到80之间的convexity defects的起始点、结束点和最远点）

五．参考

[1] Gary Bradski, Adrian Kaehler. Learning OpenCV: Computer Vision with the OpenCV Library. Page258~259.

[2] http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/hull/hull.html

[3] http://www.codeproject.com/Articles/782602/Beginners-guide-to-understand-Fingertips-counting

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