.. _moments:


轮廓矩
**************

目标
=====

在这节教程中您将学到:

.. container:: enumeratevisibleitemswithsquare

   * 使用OpenCV函数 :moments:`moments <>` 计算图像所有的矩(最高到3阶)
   * 使用OpenCV函数 :contour_area:`contourArea <>` 来计算轮廓面积
   * 使用OpenCV函数 :arc_length:`arcLength <>` 来计算轮廓或曲线长度          

原理
======

代码
====

下面是本节教程源码. 你也可以从 `这里 <https://code.ros.org/svn/opencv/trunk/opencv/samples/cpp/tutorial_code/ShapeDescriptors/moments_demo.cpp>`_ 下载.

.. code-block:: cpp 

   #include "opencv2/highgui/highgui.hpp"
   #include "opencv2/imgproc/imgproc.hpp"
   #include <iostream>
   #include <stdio.h>
   #include <stdlib.h>

   using namespace cv;
   using namespace std;

   Mat src; Mat src_gray;
   int thresh = 100;
   int max_thresh = 255;
   RNG rng(12345);

   /// 函数声明
   void thresh_callback(int, void* );

   /** @主函数 */
   int main( int argc, char** argv )
   {
     /// 读入原图像, 返回3通道图像数据
     src = imread( argv[1], 1 );

     /// 把原图像转化成灰度图像并进行平滑
     cvtColor( src, src_gray, CV_BGR2GRAY );
     blur( src_gray, src_gray, Size(3,3) );

     /// 创建新窗口
     char* source_window = "Source";
     namedWindow( source_window, CV_WINDOW_AUTOSIZE );
     imshow( source_window, src );

     createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, thresh_callback );
     thresh_callback( 0, 0 );

     waitKey(0);
     return(0);
   }

   /** @thresh_callback 函数 */
   void thresh_callback(int, void* )
   {
     Mat canny_output;
     vector<vector<Point> > contours;
     vector<Vec4i> hierarchy;

     /// 使用Canndy检测边缘
     Canny( src_gray, canny_output, thresh, thresh*2, 3 );
     /// 找到轮廓  
     findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

     /// 计算矩
     vector<Moments> mu(contours.size() );
     for( int i = 0; i < contours.size(); i++ )
        { mu[i] = moments( contours[i], false ); }

     ///  计算中心矩: 
     vector<Point2f> mc( contours.size() );
     for( int i = 0; i < contours.size(); i++ )
        { mc[i] = Point2f( mu[i].m10/mu[i].m00 , mu[i].m01/mu[i].m00 ); }

     /// 绘制轮廓
     Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
     for( int i = 0; i< contours.size(); i++ )
        { 
          Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
          drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, Point() ); 
          circle( drawing, mc[i], 4, color, -1, 8, 0 );
        }

     /// 显示到窗口中
     namedWindow( "Contours", CV_WINDOW_AUTOSIZE );
     imshow( "Contours", drawing );

     /// 通过m00计算轮廓面积并且和OpenCV函数比较
     printf("\t Info: Area and Contour Length \n");
     for( int i = 0; i< contours.size(); i++ )
        {
          printf(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f \n", i, mu[i].m00, contourArea(contours[i]), arcLength( contours[i], true ) );  
          Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
          drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, Point() ); 
          circle( drawing, mc[i], 4, color, -1, 8, 0 );
        }
   }

代码说明
============

结果
======

#. 如下所示:

   ========== ==========  ==========  
    |MU_0|     |MU_1|      |MU_2|   
   ========== ==========  ========== 

   .. |MU_0|  image:: images/Moments_Source_Image.jpg
                    :width: 250pt
                    :align: middle

   .. |MU_1|  image:: images/Moments_Result1.jpg
                    :width: 250pt
                    :align: middle   

   .. |MU_2|  image:: images/Moments_Result2.jpg
                    :width: 250pt
                    :align: middle   

翻译者
=================
guoming0000@ `OpenCV中文网站 <http://www.opencv.org.cn>`_ <guoming0000@sina.com>