有時候想知道多邊形的面積，我們可以怎麼做？現在給定幾個座標點，只要呼叫內建的函式contourArea，就會回傳結果數值喔！這篇就叫做計算輪廓面積 (Calculate Contour Area)。
讀取一張600×600的影像，四個座標點分別為(70,70)、(70,170)、(170,170)、(170,70)，面積為10000，因為四邊長都是100，所以用心算就能得出結果。
簡單地把它寫成程式：
/**
Theme: Calculate Contour Area
IDE: Xcode 7
Language: C++
Date: 105/03/27
Author: HappyMan
Blog: https://cg2010studio.wordpress.com/
*/
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
using namespace cv;
using namespace std;
int main(int argc, char** argv)
{
// Read the file
image = imread("cake.jpg", CV_LOAD_IMAGE_COLOR);
if(!image.data) {
// Check for invalid input
cout << "Could not open or find the image" << endl;
return -1;
}
line(image, Point(70,70), Point(70,170), Scalar(255,0,0), 2, 8);
line(image, Point(70,170), Point(170,170), Scalar(0,255,0), 2, 8);
line(image, Point(170,170), Point(170,70), Scalar(0,0,255), 2, 8);
line(image, Point(170,70), Point(70,70), Scalar(255,255,0), 2, 8);
vector<Point> contour;
contour.push_back(Point(70,70));
contour.push_back(Point(70,170));
contour.push_back(Point(170,170));
contour.push_back(Point(170,70));
double area = contourArea(contour,false); // Find the area of contour
cout << "Happy Area: " << area << endl;
// Create a window to display
namedWindow("Happy window", WINDOW_AUTOSIZE);
imshow("Happy window", image);
waitKey(0);
// Wait for a keystroke in the window
return 0;
}
輸出：
Happy Area: 10000
那如果座標點變更為：(70,70)、(470,170)、(370,370)、(270,370)，面積為何呢？這就令人傷透腦筋⋯⋯於是就交給厲害的程式來幫我們計算囉～
輸出：
Happy Area: 60000
倘若這些點無法構成一個凸多邊形的話，結果會如何呢？
參考：OpenCV – contourArea。
隨意留個言吧：）～