Opencv函数：reduce

参考资料：

1、http://docs.opencv.org/modules/core/doc/operations_on_arrays.html?highlight=reduce#void reduce(InputArray src, OutputArray dst, int dim, int rtype, int dtype)

Reduces a matrix to a vector.

C++:  void  reduce (InputArray  src, OutputArray  dst, int  dim, int  rtype, int  dtype=-1  )

Python:   cv2. reduce (src, dim, rtype [, dst [, dtype ] ] ) → dst

C:  void  cvReduce (const CvArr*  src, CvArr*  dst, int  dim=-1, int  op=CV_ REDUCE_SUM )

Python:   cv. Reduce (src, dst, dim=-1, op=CV_ REDUCE_SUM ) → None

Parameters:

src – input 2D matrix. dst – output vector. Its size and type is defined by dim and dtype parameters. dim – dimension index along which the matrix is reduced. 0 means that the matrix is reduced to a single row. 1 means that the matrix is reduced to a single column. rtype – reduction operation that could be one of the following: CV_REDUCE_SUM: the output is the sum of all rows/columns of the matrix. CV_REDUCE_AVG: the output is the mean vector of all rows/columns of the matrix. CV_REDUCE_MAX: the output is the maximum (column/row-wise) of all rows/columns of the matrix. CV_REDUCE_MIN: the output is the minimum (column/row-wise) of all rows/columns of the matrix. dtype – when negative, the output vector will have the same type as the input matrix, otherwise, its type will  be CV_MAKE_TYPE(CV_MAT_DEPTH(dtype), src.channels()).

2、用法：

reduce(trainData,dst,0,CV_REDUCE_SUM); 

// 是将训练矩阵按行进行累加，得到一个列向量。训练数据矩阵大小为1133*400

// 执行会出现错误

3、错误：

4、原因分析： 主要是最后一个dtype参数，不能默认，因为按行累加后，可能会超过uchar表达的范围。因此要改写

reduce(trainData,dst,0,CV_REDUCE_SUM,CV_32S); // 可以顺利通过