avctx->has_b_frames= buf[5] >> 7;
static void mpegvideo_extract_headers(AVCodecParserContext *s,                                       AVCodecContext *avctx,                                      const uint8_t *buf, int buf_size){    ParseContext1 *pc = s->priv_data;    const uint8_t *buf_end;    int32_t start_code;    int frame_rate_index, ext_type, bytes_left;    int frame_rate_ext_n, frame_rate_ext_d;    int picture_structure, top_field_first, repeat_first_field, progressive_frame;    int horiz_size_ext, vert_size_ext;    s->repeat_pict = 0;    buf_end = buf + buf_size;    while (buf < buf_end) {        start_code = find_start_code(&buf, buf_end);        bytes_left = buf_end - buf;        switch(start_code) {        case PICTURE_START_CODE:            if (bytes_left >= 2) {                s->pict_type = (buf[1] >> 3) & 7;            }            break;        case SEQ_START_CODE:            if (bytes_left >= 4) {                pc->width  = (buf[0] << 4) | (buf[1] >> 4);                pc->height = ((buf[1] & 0x0f) << 8) | buf[2];                avcodec_set_dimensions(avctx, pc->width, pc->height);                frame_rate_index = buf[3] & 0xf;                pc->frame_rate = avctx->frame_rate = frame_rate_tab[frame_rate_index];                avctx->frame_rate_base = MPEG1_FRAME_RATE_BASE;                avctx->codec_id = CODEC_ID_MPEG1VIDEO;                avctx->sub_id = 1;            }            break;        case EXT_START_CODE:            if (bytes_left >= 1) {                ext_type = (buf[0] >> 4);                switch(ext_type) {                case 0x1: /* sequence extension */                    if (bytes_left >= 6) {                        horiz_size_ext = ((buf[1] & 1) << 1) | (buf[2] >> 7);                        vert_size_ext = (buf[2] >> 5) & 3;                        frame_rate_ext_n = (buf[5] >> 5) & 3;                        frame_rate_ext_d = (buf[5] & 0x1f);                        pc->progressive_sequence = buf[1] & (1 << 3);                        pc->width  |=(horiz_size_ext << 12);                        pc->height |=( vert_size_ext << 12);                        avcodec_set_dimensions(avctx, pc->width, pc->height);                        avctx->frame_rate = pc->frame_rate * (frame_rate_ext_n + 1);                        avctx->frame_rate_base = MPEG1_FRAME_RATE_BASE * (frame_rate_ext_d + 1);                        avctx->codec_id = CODEC_ID_MPEG2VIDEO;                        avctx->sub_id = 2; /* forces MPEG2 */                    }                    break;                case 0x8: /* picture coding extension */                    if (bytes_left >= 5) {                        picture_structure = buf[2]&3;                        top_field_first = buf[3] & (1 << 7);                        repeat_first_field = buf[3] & (1 << 1);                        progressive_frame = buf[4] & (1 << 7);                                            /* check if we must repeat the frame */                        if (repeat_first_field) {                            if (pc->progressive_sequence) {                                if (top_field_first)                                    s->repeat_pict = 4;                                else                                    s->repeat_pict = 2;                            } else if (progressive_frame) {                                s->repeat_pict = 1;                            }                        }                                                /* the packet only represents half a frame                            XXX,FIXME maybe find a different solution */                        if(picture_structure != 3)                            s->repeat_pict = -1;                    }                    break;                }            }            break;        case -1:            goto the_end;        default:            /* we stop parsing when we encounter a slice. It ensures               that this function takes a negligible amount of time */            if (start_code >= SLICE_MIN_START_CODE &&                 start_code <= SLICE_MAX_START_CODE)                goto the_end;            break;        }    } the_end: ;}