请解释此代码如何从图像中去除噪声

Question

下面的代码实际上是如何从图像中去除噪点的？我试图了解发生了什么，但似乎无法掌握整体思路。我已经尝试过了，它有效（但效果不佳）。请粗略解释一下。谢谢。

void averageFilter(PIXEL_ARRAY* img, PIXEL_ARRAY* orig_img, int N) 
{

  int i, j, n, m;
  int red_avg, blue_avg, green_avg;
  int radius, out_of_bounds, idx, curr_idx;
  int32_t pixel;

  if (N % 2 == 0) {
    printf("ERROR: Please use an odd sized window\n");
    exit(1);
  }

  radius = N / 2;

  for (i = 0; i < img->sizeY; i++) {
    for (j = 0; j < img->sizeX; j++) {
      out_of_bounds = 0;

      red_avg = 0;
      blue_avg = 0;
      green_avg = 0;

      for (n = i - radius; n <= i + radius; n++) {

    for (m = j - radius; m <= j + radius; m++) {
      if (n < 0 || m < 0 || n >= img->sizeY || m >= img->sizeX) {
        out_of_bounds++;
        continue;
      }
      idx = m + n * img->sizeX;
      /* Shift, mask and add */


      red_avg += ((orig_img->data[idx] >> 16) & 0xFF);
      green_avg += ((orig_img->data[idx] >> 8) & 0xFF);
      blue_avg += (orig_img->data[idx] & 0xFF);

    }
      }

      /* Divide the total sum by the amount of pixels in the window */
      red_avg /= (N * N - out_of_bounds);
      green_avg /= (N * N - out_of_bounds);
      blue_avg /= (N * N - out_of_bounds);

      /* Set the average to the current pixel */
      curr_idx = j + i * img->sizeX;
      pixel = (red_avg << 16) + (green_avg << 8) + blue_avg;
      img->data[curr_idx] = pixel;
    }
  }
}

Answer 1

for (i = 0; i < img->sizeY; i++) {
    for (j = 0; j < img->sizeX; j++) {`

对于网格中的任何像素...

for (n = i - radius; n <= i + radius; n++) {
 for (m = j - radius; m <= j + radius; m++) {

访问我们像素的radius 内的位置...

 if (n < 0 || m < 0 || n >= img->sizeY || m >= img->sizeX) {
        out_of_bounds++;
        continue;

（记住我们找到了多少）

  idx = m + n * img->sizeX;

当我们找到一个位置时，我们是

• n 像素向上（主像素 Y +/- 半径），

• m 跨像素（主像素-X +/- 半径）， 所以……

• n 行大小 X 像素，

• 加上m 这一行，是...

idx: 我们所在位置的像素索引

 red_avg += ((orig_img->data[idx] >> 16) & 0xFF);
 green_avg += ((orig_img->data[idx] >> 8) & 0xFF);
 blue_avg += (orig_img->data[idx] & 0xFF);

统计我们访问的每个位置的原始图像的 RGB 数据

   /* Divide the total sum by the amount of pixels in the window */
   red_avg /= (N * N - out_of_bounds);
   green_avg /= (N * N - out_of_bounds);
   blue_avg /= (N * N - out_of_bounds);
   /* Set the average to the current pixel */

...平均每个主像素的radius 内的所有位置...

      curr_idx = j + i * img->sizeX;
      pixel = (red_avg << 16) + (green_avg << 8) + blue_avg;
      img->data[curr_idx] = pixel;

...并将输出文件中的 main-pixel-index 设置为平均值。

Answer 2

代码探索每个像素的邻域，找到每个 R、G、B 分量的平均值，并将它们写入输出图像。所以它是一个平滑滤波器。我已经注释了代码：

void averageFilter(PIXEL_ARRAY* img, PIXEL_ARRAY* orig_img, int N) 
{

  int i, j, n, m;
  int red_avg, blue_avg, green_avg;
  int radius, out_of_bounds, idx, curr_idx;
  int32_t pixel;

  if (N % 2 == 0) {
    printf("ERROR: Please use an odd sized window\n");
    exit(1);
  }

  radius = N / 2;                                   // distance from pixel to explore

  for (i = 0; i < img->sizeY; i++) {                // parse each image pixel
    for (j = 0; j < img->sizeX; j++) {
      out_of_bounds = 0;

      red_avg = 0;                                  // init the averages
      blue_avg = 0;
      green_avg = 0;

      for (n = i - radius; n <= i + radius; n++) {  // within the area to explore

    for (m = j - radius; m <= j + radius; m++) {
      if (n < 0 || m < 0 || n >= img->sizeY || m >= img->sizeX) {   // off the map?
        out_of_bounds++;                            // count pixels off the map
        continue;                                   // and skip the summing
      }
      idx = m + n * img->sizeX;        // locate index of the pixel in source 1D array
      /* Shift, mask and add */


      red_avg += ((orig_img->data[idx] >> 16) & 0xFF);   // extract each R,G,B in the region
      green_avg += ((orig_img->data[idx] >> 8) & 0xFF);  // and sum them
      blue_avg += (orig_img->data[idx] & 0xFF);

    }
      }

      /* Divide the total sum by the amount of pixels in the window */
      red_avg /= (N * N - out_of_bounds);           // produce an average R,G,B within the region
      green_avg /= (N * N - out_of_bounds);
      blue_avg /= (N * N - out_of_bounds);

      /* Set the average to the current pixel */
      curr_idx = j + i * img->sizeX;                // locate index in destination array
      pixel = (red_avg << 16) + (green_avg << 8) + blue_avg;  // merge the components
      img->data[curr_idx] = pixel;                  // set its value to the average of the region
    }
  }
}