Lesson 30 convex hull (Convex Hull)

1. What is the convex hull

• In a multi-connection or an internal modification of any two edge points are included in a polygon edge or the inside.
  
• Minimum convex point set S containing all points in the convex hull polygon is called

• Convex hull scanning detection algorithm --Graham

2. Graham scan algorithm

• First, choose the lowest point in the Y direction as a starting point p0
• From the polar coordinates p0 start scanning sequentially adding p1 ... .pn (sort order based on the size of the polar angle, counterclockwise direction)
• For each point pi, pi is added if the convex hull points to cause a left turn (counterclockwise method) is added to the convex hull points, whereas if a right turn leads (clockwise) to remove the point from the convex hull in
  

3. Step

1. First, the image from RGB to gray
2. And then converted to a binary image
3. Then obtained by finding the contour candidate point
4. Convex hull API calls
5. Draw display.

4. Related API

4.1 cv::convexHull（）

convexHull(
InputArray points,// 输入候选点，来自findContours
OutputArray hull,// 凸包
bool clockwise,// default true, 顺时针方向
bool returnPoints）// true 表示返回点个数，如果第二个参数是			vector<Point>则自动忽略

5. Routine

#include<opencv2/opencv.hpp>
#include<iostream>
#include<math.h>

using namespace cv;
using namespace std;
void CallBack(int, void*);
Mat src, dst, binout, src_gray;
int Threshold = 10;

int main() {
	
	//输入图像并转化为灰度
	src = imread("D:/resource/images/手.png");
	if (src.empty()) {
		printf("src  couldn't be loaded...");
		return -1;
	}
	imshow("input", src);
	
	cvtColor(src, src_gray, COLOR_BGR2GRAY);
	GaussianBlur(src_gray, src_gray, Size(3, 3),0, 0, BORDER_DEFAULT);
	imshow("gray", src_gray);

	//设置阈值后执行CallBack（）
	namedWindow("output");
	createTrackbar("Threshold", "output", &Threshold, 255, CallBack);
	CallBack(0, 0);

	waitKey(0);
	return 0;
}

void CallBack(int, void*) {

	//二值化
	threshold(src_gray, binout, Threshold, 255, THRESH_BINARY);

	//寻找边缘
	vector<vector<Point>> contours;
	vector<Vec4i> hierachy;
	findContours(binout, contours, hierachy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0));

	//凸包检测
	vector<vector<Point>> convexs(contours.size());
	for(size_t k=0;k<contours.size();k++)
		convexHull(contours[k], convexs[k], false, true);

	//画出所有边缘（蓝色）和凸包（红色）
	dst = Mat::zeros(src.size(), CV_8UC3);
	for (size_t i = 0; i < convexs.size(); i++)
	{
		drawContours(dst, contours, i, Scalar(255, 0, 0), 1, LINE_8, Mat());
		drawContours(dst, convexs, i, Scalar(0, 0, 255), 1, LINE_8, Mat());
	}
		imshow("output", dst);
	cout << contours.size()<<endl;
		

}