h= FFMIN(h, s->height - y);
h= FFMIN(h, (s->height>>s->avctx->lowres) - y);
void ff_draw_horiz_band(MpegEncContext *s, int y, int h){    if (s->avctx->draw_horiz_band) {        AVFrame *src;        int offset[4];                if(s->picture_structure != PICT_FRAME){            h <<= 1;            y <<= 1;            if(s->first_field  && !(s->avctx->slice_flags&SLICE_FLAG_ALLOW_FIELD)) return;        }        h= FFMIN(h, s->height - y);        if(s->pict_type==B_TYPE || s->low_delay || (s->avctx->slice_flags&SLICE_FLAG_CODED_ORDER))             src= (AVFrame*)s->current_picture_ptr;        else if(s->last_picture_ptr)            src= (AVFrame*)s->last_picture_ptr;        else            return;                    if(s->pict_type==B_TYPE && s->picture_structure == PICT_FRAME && s->out_format != FMT_H264){            offset[0]=            offset[1]=            offset[2]=            offset[3]= 0;        }else{            offset[0]= y * s->linesize;;            offset[1]=             offset[2]= (y >> s->chroma_y_shift) * s->uvlinesize;            offset[3]= 0;        }        emms_c();        s->avctx->draw_horiz_band(s->avctx, src, offset,                                  y, s->picture_structure, h);    }}