static void do_subtitle_out(AVFormatContext *s,
                            AVOutputStream *ost,
                            AVInputStream *ist,
                            AVSubtitle *sub,
                            int64_t pts)
{
    static uint8_t *subtitle_out = NULL;
    int subtitle_out_max_size = 65536;
    int subtitle_out_size, nb, i;
    AVCodecContext *enc;
    AVPacket pkt;

    if (pts == AV_NOPTS_VALUE) {
        fprintf(stderr, "Subtitle packets must have a pts\n");
        if (exit_on_error)
            av_exit(1);
        return;
    }

    enc = ost->st->codec;

    if (!subtitle_out) {
        subtitle_out = av_malloc(subtitle_out_max_size);
    }

    /* Note: DVB subtitle need one packet to draw them and one other
       packet to clear them */
    /* XXX: signal it in the codec context ? */
    if (enc->codec_id == CODEC_ID_DVB_SUBTITLE)
        nb = 2;
    else
        nb = 1;

    for(i = 0; i < nb; i++) {
        subtitle_out_size = avcodec_encode_subtitle(enc, subtitle_out,
                                                    subtitle_out_max_size, sub);

        av_init_packet(&pkt);
        pkt.stream_index = ost->index;
        pkt.data = subtitle_out;
        pkt.size = subtitle_out_size;
        pkt.pts = av_rescale_q(pts, ist->st->time_base, ost->st->time_base);
        if (enc->codec_id == CODEC_ID_DVB_SUBTITLE) {
            /* XXX: the pts correction is handled here. Maybe handling
               it in the codec would be better */
            if (i == 0)
                pkt.pts += 90 * sub->start_display_time;
            else
                pkt.pts += 90 * sub->end_display_time;
        }
        write_frame(s, &pkt, ost->st->codec, bitstream_filters[ost->file_index][pkt.stream_index]);
    }
}