// construct the image pyramids（構造影象金字塔）
void
BriskScaleSpace::constructPyramid(const cv::Mat& image)
{

  // set correct size:
  pyramid_.clear();

  // fill the pyramid:
  pyramid_.push_back(BriskLayer(image.clone()));
  if (layers_ > 1)
  {
    pyramid_.push_back(BriskLayer(pyramid_.back(), BriskLayer::CommonParams::TWOTHIRDSAMPLE));//d0層是2/3
  }
  const int octaves2 = layers_;

  for (uchar i = 2; i < octaves2; i += 2)
  {
    pyramid_.push_back(BriskLayer(pyramid_[i - 2], BriskLayer::CommonParams::HALFSAMPLE));//c?層是前兩層的1/2
    pyramid_.push_back(BriskLayer(pyramid_[i - 1], BriskLayer::CommonParams::HALFSAMPLE));//d?層是前兩層的1/2（除d0層外）
  }
}
 

//提取特徵點
void
BriskScaleSpace::getKeypoints(const int threshold_, std::vector<cv::KeyPoint>& keypoints)
{
  // make sure keypoints is empty
  keypoints.resize(0);
  keypoints.reserve(2000);

  // assign thresholds
  int safeThreshold_ = (int)(threshold_ * safetyFactor_);
  std::vector<std::vector<cv::KeyPoint> > agastPoints;
  agastPoints.resize(layers_);

  // go through the octaves and intra layers and calculate fast corner scores:
  for (int i = 0; i < layers_; i++)
  {
    // call OAST16_9 without nms
    BriskLayer& l = pyramid_[i];
    l.getAgastPoints(safeThreshold_, agastPoints[i]);
  }

  if (layers_ == 1)
  {
    // just do a simple 2d subpixel refinement...
    const size_t num = agastPoints[0].size();
    for (size_t n = 0; n < num; n++)
    {
      const cv::Point2f& point = agastPoints.at(0)[n].pt;
      // first check if it is a maximum:
      if (!isMax2D(0, (int)point.x, (int)point.y))
        continue;

      // let's do the subpixel and float scale refinement:
      BriskLayer& l = pyramid_[0];
      int s_0_0 = l.getAgastScore(point.x - 1, point.y - 1, 1);
      int s_1_0 = l.getAgastScore(point.x, point.y - 1, 1);
      int s_2_0 = l.getAgastScore(point.x + 1, point.y - 1, 1);
      int s_2_1 = l.getAgastScore(point.x + 1, point.y, 1);
      int s_1_1 = l.getAgastScore(point.x, point.y, 1);
      int s_0_1 = l.getAgastScore(point.x - 1, point.y, 1);
      int s_0_2 = l.getAgastScore(point.x - 1, point.y + 1, 1);
      int s_1_2 = l.getAgastScore(point.x, point.y + 1, 1);
      int s_2_2 = l.getAgastScore(point.x + 1, point.y + 1, 1);
      float delta_x, delta_y;
      float max = subpixel2D(s_0_0, s_0_1, s_0_2, s_1_0, s_1_1, s_1_2, s_2_0, s_2_1, s_2_2, delta_x, delta_y);

      // store:
      keypoints.push_back(cv::KeyPoint(float(point.x) + delta_x, float(point.y) + delta_y, basicSize_, -1, max, 0));

    }

    return;
  }

  float x, y, scale, score;
  for (int i = 0; i < layers_; i++)
  {
    BriskLayer& l = pyramid_[i];
    const size_t num = agastPoints[i].size();
    if (i == layers_ - 1)
    {
      for (size_t n = 0; n < num; n++)
      {
        const cv::Point2f& point = agastPoints.at(i)[n].pt;
        // consider only 2D maxima...
        if (!isMax2D(i, (int)point.x, (int)point.y))
          continue;

        bool ismax;
        float dx, dy;
        getScoreMaxBelow(i, (int)point.x, (int)point.y, l.getAgastScore(point.x, point.y, safeThreshold_), ismax, dx, dy);
        if (!ismax)
          continue;

        // get the patch on this layer:
        int s_0_0 = l.getAgastScore(point.x - 1, point.y - 1, 1);
        int s_1_0 = l.getAgastScore(point.x, point.y - 1, 1);
        int s_2_0 = l.getAgastScore(point.x + 1, point.y - 1, 1);
        int s_2_1 = l.getAgastScore(point.x + 1, point.y, 1);
        int s_1_1 = l.getAgastScore(point.x, point.y, 1);
        int s_0_1 = l.getAgastScore(point.x - 1, point.y, 1);
        int s_0_2 = l.getAgastScore(point.x - 1, point.y + 1, 1);
        int s_1_2 = l.getAgastScore(point.x, point.y + 1, 1);
        int s_2_2 = l.getAgastScore(point.x + 1, point.y + 1, 1);
        float delta_x, delta_y;
        float max = subpixel2D(s_0_0, s_0_1, s_0_2, s_1_0, s_1_1, s_1_2, s_2_0, s_2_1, s_2_2, delta_x, delta_y);

        // store:
        keypoints.push_back(
            cv::KeyPoint((float(point.x) + delta_x) * l.scale() + l.offset(),
                         (float(point.y) + delta_y) * l.scale() + l.offset(), basicSize_ * l.scale(), -1, max, i));
      }
    }
    else
    {
      // not the last layer:
      for (size_t n = 0; n < num; n++)
      {
        const cv::Point2f& point = agastPoints.at(i)[n].pt;

        // first check if it is a maximum:
        if (!isMax2D(i, (int)point.x, (int)point.y))
          continue;

        // let's do the subpixel and float scale refinement:
        bool ismax=false;
        score = refine3D(i, (int)point.x, (int)point.y, x, y, scale, ismax);
        if (!ismax)
        {
          continue;
        }

        // finally store the detected keypoint:
        if (score > float(threshold_))
        {
          keypoints.push_back(cv::KeyPoint(x, y, basicSize_ * scale, -1, score, i));
        }
      }
    }
  }
}