static int mov2textsub(AVBitStreamFilterContext *bsfc, AVCodecContext *avctx, const char *args,
                     uint8_t **poutbuf, int *poutbuf_size,
                     const uint8_t *buf, int buf_size, int keyframe){
    if (buf_size < 2) return 0;
    *poutbuf_size = FFMIN(buf_size - 2, AV_RB16(buf));
    *poutbuf = av_malloc(*poutbuf_size + FF_INPUT_BUFFER_PADDING_SIZE);
    memcpy(*poutbuf, buf + 2, *poutbuf_size);
    return 1;
}