static int find_image_range(int *pfirst_index, int *plast_index,
                            const char *path)
{
    char buf[1024];
    int range, last_index, range1, first_index;

    /* find the first image */
    for(first_index = 0; first_index < 5; first_index++) {
        if (av_get_frame_filename(buf, sizeof(buf), path, first_index) < 0){
            *pfirst_index =
            *plast_index = 1;
            if (avio_check(buf, AVIO_FLAG_READ) > 0)
                return 0;
            return -1;
        }
        if (avio_check(buf, AVIO_FLAG_READ) > 0)
            break;
    }
    if (first_index == 5)
        goto fail;

    /* find the last image */
    last_index = first_index;
    for(;;) {
        range = 0;
        for(;;) {
            if (!range)
                range1 = 1;
            else
                range1 = 2 * range;
            if (av_get_frame_filename(buf, sizeof(buf), path,
                                      last_index + range1) < 0)
                goto fail;
            if (avio_check(buf, AVIO_FLAG_READ) <= 0)
                break;
            range = range1;
            /* just in case... */
            if (range >= (1 << 30))
                goto fail;
        }
        /* we are sure than image last_index + range exists */
        if (!range)
            break;
        last_index += range;
    }
    *pfirst_index = first_index;
    *plast_index = last_index;
    return 0;
 fail:
    return -1;
}