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example_14-02.cpp
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example_14-02.cpp
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// Example 14-2. Finding and drawing contours on an input image
#include <opencv2/opencv.hpp>
#include <algorithm>
#include <iostream>
using namespace std;
struct AreaCmp {
AreaCmp(const vector<float>& _areas) : areas(&_areas) {}
bool operator()(int a, int b) const { return (*areas)[a] > (*areas)[b]; }
const vector<float>* areas;
};
int main(int argc, char* argv[]) {
cv::Mat img, img_edge, img_color;
// load image or show help if no image was provided
//
if( argc != 2 || (img = cv::imread(argv[1],cv::IMREAD_GRAYSCALE)).empty() ) {
cout << "\nERROR: You need 2 parameters, you had " << argc << "\n" << endl;
cout << "\nExample 14_2: Drawing Contours\nCall is:\n" << argv[0] << " <image>\n\n"
<< "Example:\n" << argv[0] << " ../box.png\n" << endl;
return -1;
}
cv::threshold(img, img_edge, 128, 255, cv::THRESH_BINARY);
cv::imshow("Image after threshold", img_edge);
vector< vector< cv::Point > > contours;
vector< cv::Vec4i > hierarchy;
cv::findContours(
img_edge,
contours,
hierarchy,
cv::RETR_LIST,
cv::CHAIN_APPROX_SIMPLE
);
cout << "\n\nHit any key to draw the next contour, ESC to quit\n\n";
cout << "Total Contours Detected: " << contours.size() << endl;
vector<int> sortIdx(contours.size());
vector<float> areas(contours.size());
for( int n = 0; n < (int)contours.size(); n++ ) {
sortIdx[n] = n;
areas[n] = contourArea(contours[n], false);
}
// sort contours so that the largest contours go first
//
std::sort( sortIdx.begin(), sortIdx.end(), AreaCmp(areas ));
for( int n = 0; n < (int)sortIdx.size(); n++ ) {
int idx = sortIdx[n];
cv::cvtColor( img, img_color, cv::COLOR_GRAY2BGR );
cv::drawContours(
img_color, contours, idx,
cv::Scalar(0,0,255), 2, 8, hierarchy,
0 // Try different values of max_level, and see what happens
);
cout << "Contour #" << idx << ": area=" << areas[idx] <<
", nvertices=" << contours[idx].size() << endl;
cv::imshow(argv[0], img_color);
int k;
if( (k = cv::waitKey()&255) == 27 )
break;
}
cout << "Finished all contours\n";
return 0;
}