OpenCV每日函数图像过滤模块 (12) getStructuringElement函数

一、概述

返回指定大小和形状的结构元素，用于形态学操作。该函数构造并返回可以进一步传递给 erode、dilate 或morphologyEx 的结构元素。但是您也可以自己构建任意二值mask并将其用作结构元素。

结构元素是一个小的二进制图像，即一个小的像素矩阵，每个像素的值为零或一：

矩阵维度指定结构元素的大小。
1 和 0 的模式指定了结构元素的形状。
结构元素的原点通常是其像素之一，尽管通常原点可以在结构元素之外。

某些情况下，您可能需要椭圆形/圆形的内核。所以为了这个目的。您只需传递内核的形状和大小，即可获得所需的内核。

cv2.getStructuringElement(cv2.MORPH_RECT,(5,5))
cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(5,5))
cv2.getStructuringElement(cv2.MORPH_CROSS,(5,5))

比如使用上面的代码可以得到，如下的结构元素。矩形，椭圆、十字形状。

二、getStructuringElement函数

1、函数原型

cv::getStructuringElement (int shape, Size ksize, Point anchor=Point(-1,-1))

2、参数详解

shape	可能是 MorphShapes 之一的元素形状，矩形、十字、椭圆
ksize	结构元素的大小。
anchor	元素内的锚点位置。默认值 (-1,-1) 表示锚点位于中心。请注意，只有十字形元素的形状取决于锚点位置。在其他情况下，anchor 只是调节形态操作的结果移动了多少。

三、OpenCV源码

1、源码路径

opencv\modules\imgproc\src\morph.dispatch.cpp

2、源码代码

Mat getStructuringElement(int shape, Size ksize, Point anchor)
{
    int i, j;
    int r = 0, c = 0;
    double inv_r2 = 0;

    CV_Assert( shape == MORPH_RECT || shape == MORPH_CROSS || shape == MORPH_ELLIPSE );

    anchor = normalizeAnchor(anchor, ksize);

    if( ksize == Size(1,1) )
        shape = MORPH_RECT;

    if( shape == MORPH_ELLIPSE )
    {
        r = ksize.height/2;
        c = ksize.width/2;
        inv_r2 = r ? 1./((double)r*r) : 0;
    }

    Mat elem(ksize, CV_8U);

    for( i = 0; i < ksize.height; i++ )
    {
        uchar* ptr = elem.ptr(i);
        int j1 = 0, j2 = 0;

        if( shape == MORPH_RECT || (shape == MORPH_CROSS && i == anchor.y) )
            j2 = ksize.width;
        else if( shape == MORPH_CROSS )
            j1 = anchor.x, j2 = j1 + 1;
        else
        {
            int dy = i - r;
            if( std::abs(dy) <= r )
            {
                int dx = saturate_cast<int>(c*std::sqrt((r*r - dy*dy)*inv_r2));
                j1 = std::max( c - dx, 0 );
                j2 = std::min( c + dx + 1, ksize.width );
            }
        }

        for( j = 0; j < j1; j++ )
            ptr[j] = 0;
        for( ; j < j2; j++ )
            ptr[j] = 1;
        for( ; j < ksize.width; j++ )
            ptr[j] = 0;
    }

    return elem;
}

四、效果图像示例

以下代码摘自官方文档，介绍了结构元素在腐蚀、膨胀中的使用。

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>
using namespace cv;
using namespace std;
Mat src, erosion_dst, dilation_dst;
int erosion_elem = 0;
int erosion_size = 0;
int dilation_elem = 0;
int dilation_size = 0;
int const max_elem = 2;
int const max_kernel_size = 21;
void Erosion( int, void* );
void Dilation( int, void* );
int main( int argc, char** argv )
{
  CommandLineParser parser( argc, argv, "{@input | LinuxLogo.jpg | input image}" );
  src = imread( samples::findFile( parser.get<String>( "@input" ) ), IMREAD_COLOR );
  if( src.empty() )
  {
    cout << "Could not open or find the image!\n" << endl;
    cout << "Usage: " << argv[0] << " <Input image>" << endl;
    return -1;
  }
  namedWindow( "Erosion Demo", WINDOW_AUTOSIZE );
  namedWindow( "Dilation Demo", WINDOW_AUTOSIZE );
  moveWindow( "Dilation Demo", src.cols, 0 );
  createTrackbar( "Element:\n 0: Rect \n 1: Cross \n 2: Ellipse", "Erosion Demo",
          &erosion_elem, max_elem,
          Erosion );
  createTrackbar( "Kernel size:\n 2n +1", "Erosion Demo",
          &erosion_size, max_kernel_size,
          Erosion );
  createTrackbar( "Element:\n 0: Rect \n 1: Cross \n 2: Ellipse", "Dilation Demo",
          &dilation_elem, max_elem,
          Dilation );
  createTrackbar( "Kernel size:\n 2n +1", "Dilation Demo",
          &dilation_size, max_kernel_size,
          Dilation );
  Erosion( 0, 0 );
  Dilation( 0, 0 );
  waitKey(0);
  return 0;
}
void Erosion( int, void* )
{
  int erosion_type = 0;
  if( erosion_elem == 0 ){ erosion_type = MORPH_RECT; }
  else if( erosion_elem == 1 ){ erosion_type = MORPH_CROSS; }
  else if( erosion_elem == 2) { erosion_type = MORPH_ELLIPSE; }
  Mat element = getStructuringElement( erosion_type,
                       Size( 2*erosion_size + 1, 2*erosion_size+1 ),
                       Point( erosion_size, erosion_size ) );
  erode( src, erosion_dst, element );
  imshow( "Erosion Demo", erosion_dst );
}
void Dilation( int, void* )
{
  int dilation_type = 0;
  if( dilation_elem == 0 ){ dilation_type = MORPH_RECT; }
  else if( dilation_elem == 1 ){ dilation_type = MORPH_CROSS; }
  else if( dilation_elem == 2) { dilation_type = MORPH_ELLIPSE; }
  Mat element = getStructuringElement( dilation_type,
                       Size( 2*dilation_size + 1, 2*dilation_size+1 ),
                       Point( dilation_size, dilation_size ) );
  dilate( src, dilation_dst, element );
  imshow( "Dilation Demo", dilation_dst );
}

OpenCV每日函数 图像过滤模块 (12) getStructuringElement函数