实现opencv如下的函数，在pyramid lk算法中用到，其他地方也有用得到，单独拿出来。
代码共享在github: https://github.com/tishi43/pyr_down
template<class CastOp, class VecOp> void
pyrDown_( const Mat& _src, Mat& _dst, int borderType )
{
const int PD_SZ = 5;
typedef typename CastOp::type1 WT;
typedef typename CastOp::rtype T;
CV_Assert( !_src.empty() );
Size ssize = _src.size(), dsize = _dst.size();
int cn = _src.channels();
int bufstep = (int)alignSize(dsize.width*cn, 16);
AutoBuffer<WT> _buf(bufstep*PD_SZ + 16);
WT* buf = alignPtr((WT*)_buf, 16);
int tabL[CV_CN_MAX*(PD_SZ+2)], tabR[CV_CN_MAX*(PD_SZ+2)];
AutoBuffer<int> _tabM(dsize.width*cn);
int* tabM = _tabM;
WT* rows[PD_SZ];
CastOp castOp;
VecOp vecOp;
char path[64] = { 0 };
int pyramid_level = 0;
sprintf(path, "hor%d.txt", pyramid_level);
fopen_s(&f_hor, path, "wb");
sprintf(path, "ver%d.txt", pyramid_level);
fopen_s(&f_ver, "ver.txt", "wb");
CV_Assert( ssize.width > 0 && ssize.height > 0 &&
std::abs(dsize.width*2 - ssize.width) <= 2 &&
std::abs(dsize.height*2 - ssize.height) <= 2 );
int k, x, sy0 = -PD_SZ/2, sy = sy0, width0 = std::min((ssize.width-PD_SZ/2-1)/2 + 1, dsize.width);
for( x = 0; x <= PD_SZ+1; x++ )
{
int sx0 = borderInterpolate(x - PD_SZ/2, ssize.width, borderType)*cn;
int sx1 = borderInterpolate(x + width0*2 - PD_SZ/2, ssize.width, borderType)*cn;
for( k = 0; k < cn; k++ )
{
tabL[x*cn + k] = sx0 + k;
tabR[x*cn + k] = sx1 + k;
}
}
ssize.width *= cn;
dsize.width *= cn;
width0 *= cn;
for( x = 0; x < dsize.width; x++ )
tabM[x] = (x/cn)*2*cn + x % cn;
for( int y = 0; y < dsize.height; y++ )
{
T* dst = _dst.ptr<T>(y);
WT *row0, *row1, *row2, *row3, *row4;
// fill the ring buffer (horizontal convolution and decimation)
for( ; sy <= y*2 + 2; sy++ )
{
WT* row = buf + ((sy - sy0) % PD_SZ)*bufstep;
int _sy = borderInterpolate(sy, ssize.height, borderType);
const T* src = _src.ptr<T>(_sy);
int limit = cn;
const int* tab = tabL;
for( x = 0;;)
{
for( ; x < limit; x++ )
{
row[x] = src[tab[x+cn*2]]*6 + (src[tab[x+cn]] + src[tab[x+cn*3]])*4 +
src[tab[x]] + src[tab[x+cn*4]];
if (sy >= 0)
Log_hor("y=%d x=%d conv=%d\n",sy,x,row[x]);
}
if( x == dsize.width )
break;
if( cn == 1 )
{
for (; x < width0; x++){
row[x] = src[x * 2] * 6 + (src[x * 2 - 1] + src[x * 2 + 1]) * 4 +
src[x * 2 - 2] + src[x * 2 + 2];
if (sy >= 0)
Log_hor("y=%d x=%d conv=%d\n",sy, x, row[x]);
}
}
else if( cn == 3 )
{
for( ; x < width0; x += 3 )
{
const T* s = src + x*2;
WT t0 = s[0]*6 + (s[-3] + s[3])*4 + s[-6] + s[6];
WT t1 = s[1]*6 + (s[-2] + s[4])*4 + s[-5] + s[7];
WT t2 = s[2]*6 + (s[-1] + s[5])*4 + s[-4] + s[8];
row[x] = t0; row[x+1] = t1; row[x+2] = t2;
}
}
else if( cn == 4 )
{
for( ; x < width0; x += 4 )
{
const T* s = src + x*2;
WT t0 = s[0]*6 + (s[-4] + s[4])*4 + s[-8] + s[8];
WT t1 = s[1]*6 + (s[-3] + s[5])*4 + s[-7] + s[9];
row[x] = t0; row[x+1] = t1;
t0 = s[2]*6 + (s[-2] + s[6])*4 + s[-6] + s[10];
t1 = s[3]*6 + (s[-1] + s[7])*4 + s[-5] + s[11];
row[x+2] = t0; row[x+3] = t1;
}
}
else
{
for( ; x < width0; x++ )
{
int sx = tabM[x];
row[x] = src[sx]*6 + (src[sx - cn] + src[sx + cn])*4 +
src[sx - cn*2] + src[sx + cn*2];
}
}
limit = dsize.width;
tab = tabR - x;
}
}
// do vertical convolution and decimation and write the result to the destination image
for( k = 0; k < PD_SZ; k++ )
rows[k] = buf + ((y*2 - PD_SZ/2 + k - sy0) % PD_SZ)*bufstep;
row0 = rows[0]; row1 = rows[1]; row2 = rows[2]; row3 = rows[3]; row4 = rows[4];
//x = vecOp(rows, dst, (int)_dst.step, dsize.width);
x = 0;
for (; x < dsize.width; x++){
dst[x] = castOp(row2[x] * 6 + (row1[x] + row3[x]) * 4 + row0[x] + row4[x]);
Log_ver("y=%d x=%d conv=%d cast=%d(0x%x)\n", y, x, row2[x] * 6 + (row1[x] + row3[x]) * 4 + row0[x] + row4[x]+128, dst[x], dst[x]);
}
}
fclose(f_hor);
fclose(f_ver);
}