opencv reduce函数

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二，reduce() 的使用陷阱。

函数原型：void cv::reduce(InputArray _src, OutputArray _dst, int dim, int op, int dtype=-1)

对dtype采用默认值方式使用函数reduce()，出现了点异常问题，然后追加try-catch，发现输入输出的数据类型不匹配，于是就结合着reduce() 原代码做进一步分析，先是尝试着将输出创建为各种指定的格式，不行！
原因在于对下面这条语句没有理解好：

if( dtype < 0 )
dtype = _dst.fixedType() ? _dst.type() : stype;

上网、上Q，折腾许久，终于想到了要把
dtype指定一个初值！由于输入的数据类型是8U，对于求和操作CV_REDUCE_SUM，那么输出是32S就可以，对应的
dtype=CV_32S 就行，此时输出的矩阵也就只需要定义一下就行，不必再进行其它操作。例如：

Mat matIn=imread(''lena.jpg",0);

Mat matOut;

reduce(matIn, matOut, 1, CV_REDUCE_SUM, CV_32S);

以下为函数reduce()
源码，方便对照学习。

void cv::reduce(InputArray _src, OutputArray _dst, int dim, int op, int dtype){    Mat src = _src.getMat();    CV_Assert( src.dims <= 2 );    int op0 = op;    int stype = src.type(), sdepth = src.depth(), cn = src.channels();    if( dtype < 0 )        dtype = _dst.fixedType() ? _dst.type() : stype;    int ddepth = CV_MAT_DEPTH(dtype);    _dst.create(dim == 0 ? 1 : src.rows, dim == 0 ? src.cols : 1,                CV_MAKETYPE(dtype >= 0 ? dtype : stype, cn));    Mat dst = _dst.getMat(), temp = dst;        CV_Assert( op == CV_REDUCE_SUM || op == CV_REDUCE_MAX ||        op == CV_REDUCE_MIN || op == CV_REDUCE_AVG );    CV_Assert( src.channels() == dst.channels() );    if( op == CV_REDUCE_AVG )    {        op = CV_REDUCE_SUM;        if( sdepth < CV_32S && ddepth < CV_32S )        {            temp.create(dst.rows, dst.cols, CV_32SC(cn));            ddepth = CV_32S;        }    }    ReduceFunc func = 0;    if( dim == 0 )    {        if( op == CV_REDUCE_SUM )        {            if(sdepth == CV_8U && ddepth == CV_32S)                func = reduceR_<uchar,int,OpAdd<int> >;            else if(sdepth == CV_8U && ddepth == CV_32F)                func = reduceR_<uchar,float,OpAdd<int> >;            else if(sdepth == CV_8U && ddepth == CV_64F)                func = reduceR_<uchar,double,OpAdd<int> >;            else if(sdepth == CV_16U && ddepth == CV_32F)                func = reduceR_<ushort,float,OpAdd<float> >;            else if(sdepth == CV_16U && ddepth == CV_64F)                func = reduceR_<ushort,double,OpAdd<double> >;            else if(sdepth == CV_16S && ddepth == CV_32F)                func = reduceR_<short,float,OpAdd<float> >;            else if(sdepth == CV_16S && ddepth == CV_64F)                func = reduceR_<short,double,OpAdd<double> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceR_<float,float,OpAdd<float> >;            else if(sdepth == CV_32F && ddepth == CV_64F)                func = reduceR_<float,double,OpAdd<double> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceR_<double,double,OpAdd<double> >;        }        else if(op == CV_REDUCE_MAX)        {            if(sdepth == CV_8U && ddepth == CV_8U)                func = reduceR_<uchar, uchar, OpMax<uchar> >;            else if(sdepth == CV_16U && ddepth == CV_16U)                func = reduceR_<ushort, ushort, OpMax<ushort> >;            else if(sdepth == CV_16S && ddepth == CV_16S)                func = reduceR_<short, short, OpMax<short> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceR_<float, float, OpMax<float> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceR_<double, double, OpMax<double> >;        }        else if(op == CV_REDUCE_MIN)        {            if(sdepth == CV_8U && ddepth == CV_8U)                func = reduceR_<uchar, uchar, OpMin<uchar> >;            else if(sdepth == CV_16U && ddepth == CV_16U)                func = reduceR_<ushort, ushort, OpMin<ushort> >;            else if(sdepth == CV_16S && ddepth == CV_16S)                func = reduceR_<short, short, OpMin<short> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceR_<float, float, OpMin<float> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceR_<double, double, OpMin<double> >;        }    }    else    {        if(op == CV_REDUCE_SUM)        {            if(sdepth == CV_8U && ddepth == CV_32S)                func = reduceC_<uchar,int,OpAdd<int> >;            else if(sdepth == CV_8U && ddepth == CV_32F)                func = reduceC_<uchar,float,OpAdd<int> >;            else if(sdepth == CV_8U && ddepth == CV_64F)                func = reduceC_<uchar,double,OpAdd<int> >;            else if(sdepth == CV_16U && ddepth == CV_32F)                func = reduceC_<ushort,float,OpAdd<float> >;            else if(sdepth == CV_16U && ddepth == CV_64F)                func = reduceC_<ushort,double,OpAdd<double> >;            else if(sdepth == CV_16S && ddepth == CV_32F)                func = reduceC_<short,float,OpAdd<float> >;            else if(sdepth == CV_16S && ddepth == CV_64F)                func = reduceC_<short,double,OpAdd<double> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceC_<float,float,OpAdd<float> >;            else if(sdepth == CV_32F && ddepth == CV_64F)                func = reduceC_<float,double,OpAdd<double> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceC_<double,double,OpAdd<double> >;        }        else if(op == CV_REDUCE_MAX)        {            if(sdepth == CV_8U && ddepth == CV_8U)                func = reduceC_<uchar, uchar, OpMax<uchar> >;            else if(sdepth == CV_16U && ddepth == CV_16U)                func = reduceC_<ushort, ushort, OpMax<ushort> >;            else if(sdepth == CV_16S && ddepth == CV_16S)                func = reduceC_<short, short, OpMax<short> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceC_<float, float, OpMax<float> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceC_<double, double, OpMax<double> >;        }        else if(op == CV_REDUCE_MIN)        {            if(sdepth == CV_8U && ddepth == CV_8U)                func = reduceC_<uchar, uchar, OpMin<uchar> >;            else if(sdepth == CV_16U && ddepth == CV_16U)                func = reduceC_<ushort, ushort, OpMin<ushort> >;            else if(sdepth == CV_16S && ddepth == CV_16S)                func = reduceC_<short, short, OpMin<short> >;            else if(sdepth == CV_32F && ddepth == CV_32F)                func = reduceC_<float, float, OpMin<float> >;            else if(sdepth == CV_64F && ddepth == CV_64F)                func = reduceC_<double, double, OpMin<double> >;        }    }    if( !func )        CV_Error( CV_StsUnsupportedFormat,        "Unsupported combination of input and output array formats" );    func( src, temp );    if( op0 == CV_REDUCE_AVG )        temp.convertTo(dst, dst.type(), 1./(dim == 0 ? src.rows : src.cols));}

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