approx_poly.cpp

#include <iostream>
using namespace std;

#include <opencv2/opencv.hpp>
using namespace cv;
#pragma comment(lib, "opencv_world340.lib")

int main(void)
{
	Mat cat_img = imread("cat.png", CV_LOAD_IMAGE_GRAYSCALE);

	threshold(cat_img, cat_img, 127, 255, THRESH_BINARY);

	vector<vector<Point> > contours;
	vector<Vec4i> hierarchy;

	findContours(cat_img, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0));

	vector<vector<Point> > contours_poly(contours.size());

	for (int i = 0; i < contours.size(); i++)
		approxPolyDP(Mat(contours[i]), contours_poly[i], 3, true);

	Mat cat_contours = Mat::zeros(cat_img.size(), CV_8UC3);

	for (int i = 0; i < contours_poly.size(); i++)
	{
		if(1 == contours_poly[i].size())
		{
			line(cat_contours, contours_poly[i][0], contours_poly[i][0], Scalar(0, 255, 0), 1, 8, 0);
		}
		else if (2 == contours_poly[i].size())
		{
			line(cat_contours, contours_poly[i][0], contours_poly[i][1], Scalar(0, 0, 255), 1, 8, 0);
		}
		else
		{
			for (int j = 0; j < contours_poly[i].size() - 1; j++)
				line(cat_contours, contours_poly[i][j], contours_poly[i][j + 1], Scalar(255, 0, 0), 1, 8, 0);

			line(cat_contours, contours_poly[i][contours_poly[i].size() - 1], contours_poly[i][0], Scalar(255, 0, 0), 1, 8, 0);
		}
	}

	imshow("cat img", cat_contours);

	waitKey(0);

	destroyAllWindows();

	return 0;
}