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()).

The function reduce reduces the matrix to a vector by treating the matrix rows/columns as a set of 1D vectors and performing the specified operation on the vectors until a single row/column is obtained. For example, the function can be used to compute horizontal and vertical projections of a raster image. In case of CV_REDUCE_SUM and CV_REDUCE_AVG , the output may have a larger element bit-depth to preserve accuracy. And multi-channel arrays are also supported in these two reduction modes.

总结一句话：参数的选择要对应

示例：

double a[5][4] ={{ 4, 0, 2, 5 },{ 1, 1, 0, 7 },{ 0, 5, 2, 0 },{ 0, 3, 4, 0 },{ 8, 0, 1, 2 }};Mat ma(5, 4, CV_64FC1, a);Mat mb(5, 1, CV_64FC1, Scalar(0));Mat mc(1, 4, CV_64FC1, Scalar(0)); cout << "原矩阵：" << endl;cout << ma << endl;reduce(ma, mb, 1, CV_REDUCE_SUM);cout << "列向量" << endl;cout << mb << endl;reduce(ma, mc, 0, CV_REDUCE_SUM);cout << "行向量" << endl;cout << mc << endl;

Tips: 异常信息提示

cv::reduce for SUM and 8U input can return only 32S, 32F or 64F types. 

所以在进行计算的时候一定要注意矩阵的数据类型，不然会出现以下错误提示信息：

cv::reduce gives unsupported format exception

   uchar matrix[5][6] = {  { 1, 2, 3, 4, 5, 6 }, { 7, 8, 9, 10, 11, 12 }, 

                { 13, 14, 15, 16, 17, 18 }, { 19, 20, 21, 22, 23, 24 }, { 25, 26, 27, 28, 29, 30 } };Mat matr(Size(6, 5), CV_8UC1, matrix);Mat mb(5, 1, CV_8UC1, Scalar(0));Mat mc(1, 6, CV_8UC1, Scalar(0));reduce(matr, mb, 1, CV_REDUCE_SUM);cout << "列向量" << endl;cout << mb << endl;

参考链接：

http://answers.opencv.org/question/3698/cvreduce-gives-unsupported-format-exception/