使用for循环访问随机图像像素

问题描述

我正在尝试使用 OpenCV 学习图像处理。我写了一些代码来编辑图像。编辑工作正常，除了更改某些像素的颜色。

I am trying to learn image processing using OpenCV. I wrote some code to edit an image. The edits are working well except for changing the color of some pixels.

我试图访问一些随机像素，并改变它们的颜色（即<$ c $时） c> status == 3 ）。当我运行程序时，我得到[run] Segmentation fault（core dumped），我认为这意味着存在被拒绝的内存访问。

I am trying to access some random pixels, and change their color (that is when status == 3). When I run the program I get "[run] Segmentation fault (core dumped)", which I think means that there is a denied memory access.

  int main( ) {
      Mat originalImage = imread("image.jpg", CV_LOAD_IMAGE_UNCHANGED);
      if (originalImage.empty()){
          cout << "Error : Image cannot be loaded..!!" << endl;
          return -1;
      }
      int orgRows = originalImage.rows;
      int orgCols = originalImage.cols;
      int status; // output indicator

      cout << "Please select the settings" << endl;
      cin >> status;
      Mat displayedImage;
      while (status != 0 ) {
          if(status == 1){
              // some code
          }
          else if (status == 2 ){
              // some code
          }else if (status == 3 ){
              int j;
              int k;
              for (int i = 0; i < 1000; i++) {
                  j = rand()% orgCols;
                  k = rand() % orgRows;
                  Vec3b intensity = originalImage.at<Vec3b>(j, k);
                  intensity[0] = 255;
                  intensity[1] = 255;
                  intensity[2] = 255;
               }
               displayedImage = originalImage;
           }else if (status == 4){
               // some code
           }else if (status == 5 ){
               // some code
           }else{
               // some code
           }
           namedWindow("MyWindow",CV_WINDOW_AUTOSIZE);
           imshow("MyWindow", displayedImage);
           waitKey(1000);
           destroyWindow("MyWindow");
           cout << "Continue ? ... Please select the settings" << endl;
           cin >> status;
      }
  return 0;
}

问题：

导致错误的原因是什么？如何解决？

Question:

What is causing the error and how could it be solved?

推荐答案

你有两个问题，都在这一行：

You have 2 problems, both on this line:

Vec3b intensity = originalImage.at<Vec3b>(j, k);

1. 矩阵作为（row，col）访问，而不是（x，y）。所以你需要使用： Vec3b intensity = originalImage.at< Vec3b>（j，k）;

您正在复制像素bgr值。您在强度上所做的每项更改都不会反映在 originalImage 中。您可以使用 originalImage 中的数据引用轻松纠正此问题： Vec3b&烈度= ... 。

You are copying the pixel bgr value. Every change you do on intensity won't be reflected in originalImage. You can easily correct this using a reference to the data in originalImage: Vec3b& intensity = ....

因此，您需要更改上述内容line to：

As a result, you need to change the mentioned line to:

Vec3b& intensity = originalImage.at<Vec3b>(k, j);

注意：使用 intensity [0] = 255; 工作正常。您不需要使用 intensity.val [0] = 255;

Note: Using intensity[0] = 255; works ok. You don't need to use intensity.val[0] = 255;

请注意，您可以做一些改进。

Note that there are some improvements that you can do.

1）因为您使用 Vec3b type，你假设你的图像被读作 CV_8UC3 ，即3通道矩阵，每通道8位深度。因此，如果您加载16位图像，或单通道（灰度）图像，或带有Alpha通道（RGBA）的图像，您的程序将崩溃。您可以在加载图片时强制执行 CV_8UC3 图片：

1) Since you work on Vec3b type, you make the assumption that your image is read as CV_8UC3, i.e. a 3 channel matrix, 8 bit depth per channel. So, if you load a 16bit image, or a single channel (grayscale) image, or an image with alpha channel (RGBA) your program will crash. You can enforce that you need a CV_8UC3 image while loading the image like:

Mat3b originalImage = imread("image.jpg", IMREAD_COLOR);

现在，你可以在<> ，因为图像类型是固定的。所以你可以这样做：

Now you can access the pixels value without the at<>, since the image type is fixed. So you can do:

Vec3b& intensity = originalImage(k, j);

2）你可以简单地为 originalImage ，如：

2) You can simply assign a new value to originalImage, like:

originalImage.at<Vec3b>(k,j) = Vec3b(255,255,255);
//or
originalImage(k,j) = Vec3b(255,255,255); // If originalImage is a Mat3b

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