学步园

        OpenCV中对Mat类型的操作，具有很多技巧，下面列几个函数：

1）在逐步访问Mat类型数据时，一次可以访问多个元素（减少循环次数）

template<typename T, typename DT, typename WT> static void
cvtScaleAbs_( const T* src, size_t sstep,
              DT* dst, size_t dstep, Size size,
              WT scale, WT shift )
{
    sstep /= sizeof(src[0]);
    dstep /= sizeof(dst[0]);

    for( ; size.height--; src += sstep, dst += dstep )
    {
        int x = 0;
        #if CV_ENABLE_UNROLLED
        for( ; x <= size.width - 4; x += 4 )
        {
            DT t0, t1;
            t0 = saturate_cast<DT>(std::abs(src[x]*scale + shift));
            t1 = saturate_cast<DT>(std::abs(src[x+1]*scale + shift));
            dst[x] = t0; dst[x+1] = t1;
            t0 = saturate_cast<DT>(std::abs(src[x+2]*scale + shift));
            t1 = saturate_cast<DT>(std::abs(src[x+3]*scale + shift));
            dst[x+2] = t0; dst[x+3] = t1;
        }
        #endif
        for( ; x < size.width; x++ )
            dst[x] = saturate_cast<DT>(std::abs(src[x]*scale + shift));
    }
}

template<typename T> static void
copyMask_(const uchar* _src, size_t sstep, const uchar* mask, size_t mstep, uchar* _dst, size_t dstep, Size size)
{
    for( ; size.height--; mask += mstep, _src += sstep, _dst += dstep )
    {
        const T* src = (const T*)_src;
        T* dst = (T*)_dst;
        int x = 0;
         #if CV_ENABLE_UNROLLED
        for( ; x <= size.width - 4; x += 4 )
        {
            if( mask[x] )
                dst[x] = src[x];
            if( mask[x+1] )
                dst[x+1] = src[x+1];
            if( mask[x+2] )
                dst[x+2] = src[x+2];
            if( mask[x+3] )
                dst[x+3] = src[x+3];
        }
        #endif
        for( ; x < size.width; x++ )
            if( mask[x] )
                dst[x] = src[x];
    }
}

2）在确定目标类型和源类型后，可以逐行复制数据

void Mat::copyTo( OutputArray _dst ) const
{
    int dtype = _dst.type();
    if( _dst.fixedType() && dtype != type() )
    {
        CV_Assert( channels() == CV_MAT_CN(dtype) );
        convertTo( _dst, dtype );
        return;
    }

    if( empty() )
    {
        _dst.release();
        return;
    }

    if( dims <= 2 )
    {
        _dst.create( rows, cols, type() );
        Mat dst = _dst.getMat();
        if( data == dst.data )
            return;

        if( rows > 0 && cols > 0 )
        {
            const uchar* sptr = data;
            uchar* dptr = dst.data;

            Size sz = getContinuousSize(*this, dst);
            size_t len = sz.width*elemSize();

            for( ; sz.height--; sptr += step, dptr += dst.step )
                memcpy( dptr, sptr, len );
        }
        return;
    }

    _dst.create( dims, size, type() );
    Mat dst = _dst.getMat();
    if( data == dst.data )
        return;

    if( total() != 0 )
    {
        const Mat* arrays[] = { this, &dst };
        uchar* ptrs[2];
        NAryMatIterator it(arrays, ptrs, 2);
        size_t sz = it.size*elemSize();

        for( size_t i = 0; i < it.nplanes; i++, ++it )
            memcpy(ptrs[1], ptrs[0], sz);
    }
}

void repeat(InputArray _src, int ny, int nx, OutputArray _dst)
{
    Mat src = _src.getMat();
    CV_Assert( src.dims <= 2 );
    CV_Assert( ny > 0 && nx > 0 );

    _dst.create(src.rows*ny, src.cols*nx, src.type());
    Mat dst = _dst.getMat();
    Size ssize = src.size(), dsize = dst.size();
    int esz = (int)src.elemSize();
    int x, y;
    ssize.width *= esz; dsize.width *= esz;

	//源数据复制到目标数据区域
    for( y = 0; y < ssize.height; y++ )
    {
        for( x = 0; x < dsize.width; x += ssize.width )
            memcpy( dst.data + y*dst.step + x, src.data + y*src.step, ssize.width );
    }

	// 填充 ssize.height-dsize.height区域
    for( ; y < dsize.height; y++ )
        memcpy( dst.data + y*dst.step, dst.data + (y - ssize.height)*dst.step, dsize.width );
}