gistfile1.txt

static void enable_tracks(AVFormatContext *s)
{
    MOVMuxContext *mov = s->priv_data;
    int i;
    int enabled[AVMEDIA_TYPE_NB];
    int first[AVMEDIA_TYPE_NB];

    for (i = 0; i < AVMEDIA_TYPE_NB; i++) {
        enabled[i] = 0;
        first[i] = -1;
    }

    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];

        if (st->codecpar->codec_type <= AVMEDIA_TYPE_UNKNOWN ||
            st->codecpar->codec_type >= AVMEDIA_TYPE_NB)
            continue;

        // note the first stream number for each type
       if (first[st->codecpar->codec_type] < 0)
           first[st->codecpar->codec_type] = i;

        // note whether each media type has any enabled streams
        if (st->disposition & AV_DISPOSITION_DEFAULT)
            enabled[st->codecpar->codec_type] = 1;

        // clear the enables for each stream and track
        st->disposition &= ~AV_DISPOSITION_DEFAULT;
        mov->tracks[i].flags &= ~MOV_TRACK_ENABLED;
    }

     // for each media type, if no stream was explicitly enabled
    // then enable the first one found for that type
    for (i = 0; i < AVMEDIA_TYPE_NB; i++) {
        switch (i) {
        case AVMEDIA_TYPE_VIDEO:
        case AVMEDIA_TYPE_AUDIO:
        case AVMEDIA_TYPE_SUBTITLE:
            if (!enabled[i] && first[i] >= 0)
                enabled[i] = 1;
            break;
        }
    }

    // translate the first enabled stream of each media type
    // into its stream being default and its output track being enabled
    for (i = 0; i < AVMEDIA_TYPE_NB; i++) {
        switch (i) {
        case AVMEDIA_TYPE_VIDEO:
        case AVMEDIA_TYPE_AUDIO:
        case AVMEDIA_TYPE_SUBTITLE:
            if (enabled[i]) {
                s->streams[first[i]]->disposition |= AV_DISPOSITION_DEFAULT;
                mov->tracks[first[i]].flags |= MOV_TRACK_ENABLED;
            }
            break;
        }
    }
}