//单通道获取
  Scalar intensity = img.at<uchar>(y, x);//行 列
  Scalar intensity = img.at<uchar>(Point(x, y));
   0 =< intensity.val[0] <= 255. 
  // 多通道获取   8u
  Vec3b intensity = img.at<Vec3b>(y, x);
  uchar blue = intensity.val[0];
  uchar green = intensity.val[1];
  uchar red = intensity.val[2];
  // 浮点型 像素值获取  32位
  Vec3f intensity = img.at<Vec3f>(y, x);
  float blue = intensity.val[0];
  float green = intensity.val[1];
  float red = intensity.val[2];

  // 修改
  img.at<uchar>(y, x) = 128;

  // 矩阵类型的点   calib3d module, 例如 投影点 projectPoints
  vector<Point2f> points;   32位
  //... fill the array
  Mat pointsMat = Mat(points);
  使用vector<Point2f> 转化成 mat类型 只有一行
  // 获取
  Point2f point = pointsMat.at<Point2f>(i, 0);