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