Three methods of accessing image pixels in opencv

Pointer to access pixels

这种方法最快,但是有点抽象
例1：Simple pixel manipulation

 for (int i = 0; i < _img.rows; i++)  //行循环
 {
    
   uchar *data = _img.ptr<uchar>(i);  //获取第i行的首地址
   for (int j = 0; j < _img.cols; j++)  //列循环
   {
    
        data[j] =  data[j]/2;   //处理每个像素 
   }    
 }

Dynamic address manipulation of pixels

This method is simple and clear in line with everyone's understanding of pixels;
例2：Simple pixel manipulation

  for (size_t i = 0; i < _img.rows; i++)  //行循环
  {
    
     for (size_t j = 0; j < _img.cols; j++)  //列循环
     {
    
          _img.at<cv::Vec3b>(i,j) = cv::Vec3b(0,0,0);   //处理每个像素
     }
     
  }

例3：Determine the pixel value under certain conditions,Operations on pixel values

 for (size_t i = 0; i < _img.rows; i++)   //行循环
  {
    
     for (size_t j = 0; j < _img.cols; j++)   //列循环
     {
    
        //The following pixel operations
        if (_img.at<cv::Vec3b>(i,j)[0] >= 100 && _img.at<cv::Vec3b>(i,j)[0] <= 124 &&
            _img.at<cv::Vec3b>(i,j)[1] >= 43 && _img.at<cv::Vec3b>(i,j)[1] <= 255 &&
            _img.at<cv::Vec3b>(i,j)[2] >= 46 && _img.at<cv::Vec3b>(i,j)[2] <=255)
        {
    
           _img.at<cv::Vec3b>(i,j) = cv::Vec3b(0,0,0);;
        }else{
    
            _img.at<cv::Vec3b>(i,j) = cv::Vec3b(255,255,255);
        }
     }     
  }

迭代器操作像素

This method is to get the image matrixbegin和end,然后增加迭代从begin到end,将*操作符添加在迭代指针前,即可访问当前指向的内容.
例3

cv::Mat_<cv::Vec3b>::iterator it = _img.begin<cv::Vec3b>();  //初始位置的迭代器
cv::Mat_<cv::Vec3b>::iterator itend = _img.end<cv::Vec3b>();  //终止位置的迭代器
//存取彩色图像像素
for(;it != itend; ++it)
{
    
   (*it)[0] =  (*it)[0]/2;
   (*it)[1] =  (*it)[1]/2;
   (*it)[2] =  (*it)[2]/2;
}

So far, several methods have been introduced！

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