srt_to_ass(avctx, &buffer, avpkt->data, x1, y1, x2, y2);
ret = ff_ass_add_rect_bprint(sub, &buffer, ts_start, ts_end-ts_start);
srt_to_ass(avctx, &buffer, avpkt->data, x1, y1, x2, y2);
ret = ff_ass_add_rect_bprint(sub, &buffer, ts_start, ts_end-ts_start);