第33课 点多边形检测

1. 概念介绍

• 点多边形测试就是测试一个点是否在给定多边形的内部、边缘或外部，可用于做归一化显示。

2. 相关API

2.1 cv::pointPolygonTest（）

double  pointPolygonTest(
InputArray  contour,// 输入的轮廓
Point2f  pt, // 测试点
bool  measureDist // 是否返回距离值，如果是false，1表示在内面，0表示在边界上，-1表示在外部; true返回实际距离
)

3. 例程

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

using namespace cv;
using namespace std;


int Threshold=140;
void CallBack(int, void*);

int r = 100;
Mat	src = Mat::zeros(r * 3, r * 3, CV_8UC3);
int main() {

	//绘制一个五边形

	vector<Point2f> pts(5);
	pts[0] = Point(r, r);
	pts[1] = Point(r * 2, r);
	pts[2] = Point(r * 2, r*2);
	pts[3] = Point(r, r*2);
	pts[4] = Point(r - 50, r + 50);

	for (int i = 0; i < 5; i++) {
		line(src, pts[i], pts[(i + 1) % 5], Scalar(0, 255, 255), 2, 8);
	}
	imshow("input", src);

	namedWindow("output");
	createTrackbar("Threshold:", "output", &Threshold, 255, CallBack);
	CallBack(0, 0);

	waitKey(0);
	return -1;

}
void CallBack(int, void*) {
	//提取边缘
	Mat canny_out;
	Canny(src, canny_out, Threshold, Threshold * 2);
	imshow("canny_out", canny_out);
	vector<vector<Point>> contours;
	findContours(canny_out, contours, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE,Point(0,0));

	//将各点与多边形的距离储存在raw_dist中
	Mat raw_dist(src.size(), CV_32FC1);
	for(int row=0;row<src.rows;row++)
		for (int col = 0; col < src.cols; col++) {
			for (size_t i = 0; i < contours.size(); i++) {
				double dist = pointPolygonTest(contours[i], Point2f(static_cast<float>(col), static_cast<float>(row)),true);
				raw_dist.at<float>(row, col) = static_cast<float>(dist);
			}
		}

	//通过颜色渐变表示像素点距离多边形的远近
	double minDist, maxDist;
	minMaxLoc(raw_dist, &minDist, &maxDist);
	for(int row=0;row<src.rows;row++)
		for (int col = 0; col < src.cols; col++) {
			float dist = raw_dist.at<float>(row, col);
			if (dist > 0)  src.at<Vec3b>(row, col)[0] = (uchar)((dist/maxDist)*255);//dist大于0，该像素点在多边形内，越接近多边形中心越蓝
			if (dist < 0)  src.at<Vec3b>(row, col)[1] = (uchar)(abs(dist / minDist) * 255);//dist小于0，该像素点在多边形外，越远离多边形中心越绿
			else   src.at<Vec3b>(row, col)[2] = (uchar)(abs(255-dist));//dist=0;像素点在多边形边上，变红

		}

	imshow("output", src);
	return;
	
}