thanks, scharfi... the idea was frighteningly obvious, no?
i initially tried using dumb bob, but this would hit unmoving areas. leak's optimised kernelbob is almost as fast and doesn't suffer from that.
now what's needed is some sanity-checking in it. this will involve masking, and will probably slow it down a fair amount. right now it isn't so good with scenechanges, and as you say suffers from lack of subpel accuracy (though if you're downscaling the output it isn't really a big deal).
manao: thanks again for MVtools, masktools, etc. it's scary how useful these tools are
[edit]
scharfi: if you're upscaling, you should probably make the blocksize 8 to avoid too much jumping around of the vectors.