scale_slice       534 libavfilter/vf_scale.c         scale_slice(link, out, in, scale->isws[0], 0, (link->h+1)/2, 2, 0);
scale_slice       535 libavfilter/vf_scale.c         scale_slice(link, out, in, scale->isws[1], 0,  link->h   /2, 2, 1);
scale_slice       537 libavfilter/vf_scale.c         scale_slice(link, out, in, scale->sws, 0, link->h, 1, 0);