h261dec.c

mediastreamer2是开源的网络传输媒体流的库

    s->mv_dir = MV_DIR_FORWARD;
    s->mv_type = MV_TYPE_16X16;
    s->current_picture.mb_type[xy]= MB_TYPE_16x16 | MB_TYPE_L0;
    s->mv[0][0][0] = h->current_mv_x * 2;//gets divided by 2 in motion compensation
    s->mv[0][0][1] = h->current_mv_y * 2;

intra:
    /* decode each block */
    if(s->mb_intra || HAS_CBP(h->mtype)){
        s->dsp.clear_blocks(s->block[0]);
        for (i = 0; i < 6; i++) {
            if (h261_decode_block(h, s->block[i], i, cbp&32) < 0){
                return SLICE_ERROR;
            }
            cbp+=cbp;
        }
    }else{
        for (i = 0; i < 6; i++)
            s->block_last_index[i]= -1;
    }

    MPV_decode_mb(s, s->block);

    return SLICE_OK;
}

/**
 * decodes a macroblock
 * @return <0 if an error occured
 */
static int h261_decode_block(H261Context * h, DCTELEM * block,
                             int n, int coded)
{
    MpegEncContext * const s = &h->s;
    int code, level, i, j, run;
    RLTable *rl = &h261_rl_tcoeff;
    const uint8_t *scan_table;

    // For the variable length encoding there are two code tables, one being used for
    // the first transmitted LEVEL in INTER, INTER+MC and INTER+MC+FIL blocks, the second
    // for all other LEVELs except the first one in INTRA blocks which is fixed length
    // coded with 8 bits.
    // NOTE: the two code tables only differ in one VLC so we handle that manually.
    scan_table = s->intra_scantable.permutated;
    if (s->mb_intra){
        /* DC coef */
        level = get_bits(&s->gb, 8);
        // 0 (00000000b) and -128 (10000000b) are FORBIDDEN
        if((level&0x7F) == 0){
            av_log(s->avctx, AV_LOG_ERROR, "illegal dc %d at %d %d\n", level, s->mb_x, s->mb_y);
            return -1;
        }
        // The code 1000 0000 is not used, the reconstruction level of 1024 being coded as 1111 1111.
        if (level == 255)
            level = 128;
        block[0] = level;
        i = 1;
    }else if(coded){
        // Run  Level   Code
        // EOB                  Not possible for first level when cbp is available (that's why the table is different)
        // 0    1               1s
        // *    *               0*
        int check = show_bits(&s->gb, 2);
        i = 0;
        if ( check & 0x2 ){
            skip_bits(&s->gb, 2);
            block[0] = ( check & 0x1 ) ? -1 : 1;
            i = 1;
        }
    }else{
        i = 0;
    }
    if(!coded){
        s->block_last_index[n] = i - 1;
        return 0;
    }
    for(;;){
        code = get_vlc2(&s->gb, rl->vlc.table, TCOEFF_VLC_BITS, 2);
        if (code < 0){
            av_log(s->avctx, AV_LOG_ERROR, "illegal ac vlc code at %dx%d\n", s->mb_x, s->mb_y);
            return -1;
        }
        if (code == rl->n) {
            /* escape */
            // The remaining combinations of (run, level) are encoded with a 20-bit word consisting of 6 bits escape, 6 bits run and 8 bits level.
            run = get_bits(&s->gb, 6);
            level = get_sbits(&s->gb, 8);
        }else if(code == 0){
            break;
        }else{
            run = rl->table_run[code];
            level = rl->table_level[code];
            if (get_bits1(&s->gb))
                level = -level;
        }
        i += run;
        if (i >= 64){
            av_log(s->avctx, AV_LOG_ERROR, "run overflow at %dx%d\n", s->mb_x, s->mb_y);
            return -1;
        }
        j = scan_table[i];
        block[j] = level;
        i++;
    }
    s->block_last_index[n] = i-1;
    return 0;
}

/**
 * decodes the H261 picture header.
 * @return <0 if no startcode found
 */
static int h261_decode_picture_header(H261Context *h){
    MpegEncContext * const s = &h->s;
    int format, i;
    uint32_t startcode= 0;

    for(i= s->gb.size_in_bits - get_bits_count(&s->gb); i>24; i-=1){
        startcode = ((startcode << 1) | get_bits(&s->gb, 1)) & 0x000FFFFF;

        if(startcode == 0x10)
            break;
    }

    if (startcode != 0x10){
        av_log(s->avctx, AV_LOG_ERROR, "Bad picture start code\n");
        return -1;
    }

    /* temporal reference */
    i= get_bits(&s->gb, 5); /* picture timestamp */
    if(i < (s->picture_number&31))
        i += 32;
    s->picture_number = (s->picture_number&~31) + i;

    s->avctx->time_base= (AVRational){1001, 30000};
    s->current_picture.pts= s->picture_number;


    /* PTYPE starts here */
    skip_bits1(&s->gb); /* split screen off */
    skip_bits1(&s->gb); /* camera  off */
    skip_bits1(&s->gb); /* freeze picture release off */

    format = get_bits1(&s->gb);

    //only 2 formats possible
    if (format == 0){//QCIF
        s->width = 176;
        s->height = 144;
        s->mb_width = 11;
        s->mb_height = 9;
    }else{//CIF
        s->width = 352;
        s->height = 288;
        s->mb_width = 22;
        s->mb_height = 18;
    }

    s->mb_num = s->mb_width * s->mb_height;

    skip_bits1(&s->gb); /* still image mode off */
    skip_bits1(&s->gb); /* Reserved */

    /* PEI */
    while (get_bits1(&s->gb) != 0){
        skip_bits(&s->gb, 8);
    }

    // h261 has no I-FRAMES, but if we pass I_TYPE for the first frame, the codec crashes if it does
    // not contain all I-blocks (e.g. when a packet is lost)
    s->pict_type = P_TYPE;

    h->gob_number = 0;
    return 0;
}

static int h261_decode_gob(H261Context *h){
    MpegEncContext * const s = &h->s;

    ff_set_qscale(s, s->qscale);

    /* decode mb's */
    while(h->current_mba <= MBA_STUFFING)
    {
        int ret;
        /* DCT & quantize */
        ret= h261_decode_mb(h);
        if(ret<0){
            if(ret==SLICE_END){
                h261_decode_mb_skipped(h, h->current_mba, 33);
                return 0;
            }
            av_log(s->avctx, AV_LOG_ERROR, "Error at MB: %d\n", s->mb_x + s->mb_y*s->mb_stride);
            return -1;
        }

        h261_decode_mb_skipped(h, h->current_mba-h->mba_diff, h->current_mba-1);
    }

    return -1;
}

/**
 * returns the number of bytes consumed for building the current frame
 */
static int get_consumed_bytes(MpegEncContext *s, int buf_size){
    int pos= get_bits_count(&s->gb)>>3;
    if(pos==0) pos=1; //avoid infinite loops (i doubt that is needed but ...)
    if(pos+10>buf_size) pos=buf_size; // oops ;)

    return pos;
}

static int h261_decode_frame(AVCodecContext *avctx,
                             void *data, int *data_size,
                             const uint8_t *buf, int buf_size)
{
    H261Context *h= avctx->priv_data;
    MpegEncContext *s = &h->s;
    int ret;
    AVFrame *pict = data;

#ifdef DEBUG
    av_log(avctx, AV_LOG_DEBUG, "*****frame %d size=%d\n", avctx->frame_number, buf_size);
    av_log(avctx, AV_LOG_DEBUG, "bytes=%x %x %x %x\n", buf[0], buf[1], buf[2], buf[3]);
#endif
    s->flags= avctx->flags;
    s->flags2= avctx->flags2;

    h->gob_start_code_skipped=0;

retry:

    init_get_bits(&s->gb, buf, buf_size*8);

    if(!s->context_initialized){
        if (MPV_common_init(s) < 0) //we need the idct permutaton for reading a custom matrix
            return -1;
    }

    //we need to set current_picture_ptr before reading the header, otherwise we cannot store anyting im there
    if(s->current_picture_ptr==NULL || s->current_picture_ptr->data[0]){
        int i= ff_find_unused_picture(s, 0);
        s->current_picture_ptr= &s->picture[i];
    }

    ret = h261_decode_picture_header(h);

    /* skip if the header was thrashed */
    if (ret < 0){
        av_log(s->avctx, AV_LOG_ERROR, "header damaged\n");
        return -1;
    }

    if (s->width != avctx->coded_width || s->height != avctx->coded_height){
        ParseContext pc= s->parse_context; //FIXME move this demuxing hack to libavformat
        s->parse_context.buffer=0;
        MPV_common_end(s);
        s->parse_context= pc;
    }
    if (!s->context_initialized) {
        avcodec_set_dimensions(avctx, s->width, s->height);

        goto retry;
    }

    // for hurry_up==5
    s->current_picture.pict_type= s->pict_type;
    s->current_picture.key_frame= s->pict_type == I_TYPE;

    /* skip everything if we are in a hurry>=5 */
    if(avctx->hurry_up>=5) return get_consumed_bytes(s, buf_size);
    if(  (avctx->skip_frame >= AVDISCARD_NONREF && s->pict_type==B_TYPE)
       ||(avctx->skip_frame >= AVDISCARD_NONKEY && s->pict_type!=I_TYPE)
       || avctx->skip_frame >= AVDISCARD_ALL)
        return get_consumed_bytes(s, buf_size);

    if(MPV_frame_start(s, avctx) < 0)
        return -1;

    ff_er_frame_start(s);

    /* decode each macroblock */
    s->mb_x=0;
    s->mb_y=0;

    while(h->gob_number < (s->mb_height==18 ? 12 : 5)){
        if(ff_h261_resync(h)<0)
            break;
        h261_decode_gob(h);
    }
    MPV_frame_end(s);

assert(s->current_picture.pict_type == s->current_picture_ptr->pict_type);
assert(s->current_picture.pict_type == s->pict_type);
    *pict= *(AVFrame*)s->current_picture_ptr;
    ff_print_debug_info(s, pict);

    *data_size = sizeof(AVFrame);

    return get_consumed_bytes(s, buf_size);
}

static int h261_decode_end(AVCodecContext *avctx)
{
    H261Context *h= avctx->priv_data;
    MpegEncContext *s = &h->s;

    MPV_common_end(s);
    return 0;
}

AVCodec h261_decoder = {
    "h261",
    CODEC_TYPE_VIDEO,
    CODEC_ID_H261,
    sizeof(H261Context),
    h261_decode_init,
    NULL,
    h261_decode_end,
    h261_decode_frame,
    CODEC_CAP_DR1,
};