关于mpeg编码中一段半象素搜索匹配块的问题？

[jesjes]

mpeg编码中有一段半象素搜索匹配块，代码如下：
dmin = 65536;
imin <<= 1;
jmin <<= 1;
ilow = imin - (imin>0);
ihigh = imin + (imin<((xmax-16)<<1));
jlow = jmin - (jmin>0);
jhigh = jmin + (jmin<((ymax-h)<<1));

for (j=jlow; j<=jhigh; j++)
for (i=ilow; i<=ihigh; i++)
{
d = dist1(ref+(i>>1)+lx*(j>>1),blk,lx,i&1,j&1,h,dmin);

if (d<dmin)
{
dmin = d;
imin = i;
jmin = j;
}
}
编的比较高深，和我想象的做插值然后比较似乎不一样，哪位可以解释一下，感谢！