计算机视觉OpenCV篇 - 轮廓（寻找/绘制轮廓）

Posted 2021-11-30 carsonzhu

什么是轮廓？

轮廓是一系列相连的点组成的曲线，代表了物体的基本外形。

轮廓与边缘好像挺像的？

是的，确实挺像，那么区别是什么呢？简而言之，轮廓是连续的，而边缘并不全都连续（见下图示例）。其实边缘主要是作为图像的特征使用，比如可以用边缘特征可以区分脸和手，而轮廓主要用来分析物体的形态，比如物体的周长和面积等，可以说边缘包括轮廓。

边缘和轮廓的区别（图片来源：http://pic.ex2tron.top/cv2_understand_contours.jpg）

寻找轮廓的操作一般用于二值化图，所以通常会使用阈值分割或Canny边缘检测先得到二值图。

【注：寻找轮廓是针对白色物体的，一定要保证物体是白色，而背景是黑色，不然很多人在寻找轮廓时会找到图片最外面的一个框】

 

OpenCV4.1.0 C++ Sample Code:

/**
 * @function findContours_Demo.cpp
 * @brief Demo code to find contours in an image
 * @author OpenCV team
 */

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>

using namespace cv;
using namespace std;

Mat src_gray;
int thresh = 100;
RNG rng(12345);

/// Function header
void thresh_callback(int, void* );

/**
 * @function main
 */
int main( int argc, char** argv )
{
    /// Load source image
    CommandLineParser parser( argc, argv, "{@input | ../data/HappyFish.jpg | input image}" );
    Mat src = imread( parser.get<String>( "@input" ) );
    if( src.empty() )
    {
      cout << "Could not open or find the image!\\n" << endl;
      cout << "Usage: " << argv[0] << " <Input image>" << endl;
      return -1;
    }

    /// Convert image to gray and blur it
    cvtColor( src, src_gray, COLOR_BGR2GRAY );
    blur( src_gray, src_gray, Size(3,3) );

    /// Create Window
    const char* source_window = "Source";
    namedWindow( source_window );
    imshow( source_window, src );

    const int max_thresh = 255;
    createTrackbar( "Canny thresh:", source_window, &thresh, max_thresh, thresh_callback );
    thresh_callback( 0, 0 );

    waitKey();
    return 0;
}

/**
 * @function thresh_callback
 */
void thresh_callback(int, void* )
{
    /// Detect edges using Canny
    Mat canny_output;
    Canny( src_gray, canny_output, thresh, thresh*2 );

    /// Find contours
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    findContours( canny_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE );

    /// Draw contours
    Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
    for( size_t i = 0; i< contours.size(); i++ )
    {
        Scalar color = Scalar( rng.uniform(0, 256), rng.uniform(0,256), rng.uniform(0,256) );
        drawContours( drawing, contours, (int)i, color, 2, LINE_8, hierarchy, 0 );
    }

    /// Show in a window
    imshow( "Contours", drawing );
}

Result: