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You can use this example from librealsense.

This translated code should give you a grayscale depth map for visualization (input depth should be of type CV_16U, output depth will be of type CV_8U):

void make_depth_histogram(const Mat &depth, Mat &normalized_depth) {
  normalized_depth = Mat(depth.size(), CV_8U);
  int width = depth.cols, height = depth.rows;

  static uint32_t histogram[0x10000];
  memset(histogram, 0, sizeof(histogram));

  for(int i = 0; i < height; ++i) {
    for (int j = 0; j < width; ++j) {
      ++histogram[depth.at<ushort>(i,j)];
    }
  }

  for(int i = 2; i < 0x10000; ++i) histogram[i] += histogram[i-1]; // Build a cumulative histogram for the indices in [1,0xFFFF]

  for(int i = 0; i < height; ++i) {
    for (int j = 0; j < width; ++j) {
      if (uint16_t d = depth.at<ushort>(i,j)) {
        int f = histogram[d] * 255 / histogram[0xFFFF]; // 0-255 based on histogram location
        normalized_depth.at<uchar>(i,j) = static_cast<uchar>(f);
      } else {
        normalized_depth.at<uchar>(i,j) = 0;
      }
    }
  }
}