OpenCV matrix element type.type() and its access method.at<>()

1. The member function type() of cv::Mat can obtain the data type of the Mat matrix elements, but the type() function returns an int value, and further table lookup is required to obtain the corresponding data type.

   cv symbol	c++ type
   8U	unsigned char
   8S	char
   16U	unsigned short
   16S	short
   32s	int
   32F	float
   64F	double

   C(n) indicates the number of channels

   type	C1	C2	C3	C4
   CV_8U	0	8	16	24
   CV_8S	1	9	17	25
   CV_16U	2	10	18	26
   CV_16S	3	11	19	27
   CV_32S	4	12	20	28
   CV_32F	5	13	21	29
   CV_64F	6	14	22	30

2. .at<>() to access elements
   . at<>() needs to know the element type to access.
   Taking the single channel as an example, the data type accepted by the at method is uchar, not CV_8U, that is, img.at<uchar>(2,3)
   taking three-channel CV8SC3 as an example, img.at<cv::Vec<uchar,3>>(行，列）, orimg.at<cv::Vec3i>(行，列）

   type of data	C1	C2	C3	C4	C6
   CV_8U	fly	cv::Vec2b	cv::Vec3b	cv::Vec<uchar,4>	cv::Vec<uchar,6>
   CV_8S	char	cv::Vec<char,2>	cv::Vec<char,3>	cv::Vec<char,4>	cv::Vec<char,6>
   CV_16U	ushort	cv::Vec<ushort,2>	cv::Vec<ushort,3>	cv::Vec<ushort,4>	cv::Vec<ushort,6>
   CV_16S	short	cv::Vec2s	cv::Vec3s	cv::Vec4s	cv::Vec<short,6>
   CV_32S	int	cv::Vec2i	cv::Vec3i	cv::Vec4i	cv::Vec<int,6>
   CV_32F	float	cv::Vec2f	cv::Vec3f	cv::Vec4f	cv::Vec6f
   CV_64F	double	cv::Vec2d	cv::Vec3d	cv::Vec4d	cv::Vec6d