libavcodec/ituh263dec.c

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/*
 * ITU H263 bitstream decoder
 * Copyright (c) 2000,2001 Fabrice Bellard
 * H263+ support.
 * Copyright (c) 2001 Juan J. Sierralta P
 * Copyright (c) 2002-2004 Michael Niedermayer <michaelni@gmx.at>
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#define UNCHECKED_BITSTREAM_READER 1

//#define DEBUG
#include <limits.h>

#include "libavutil/mathematics.h"
#include "dsputil.h"
#include "avcodec.h"
#include "mpegvideo.h"
#include "h263.h"
#include "mathops.h"
#include "unary.h"
#include "flv.h"
#include "mpeg4video.h"

//#undef NDEBUG
//#include <assert.h>

// The defines below define the number of bits that are read at once for
// reading vlc values. Changing these may improve speed and data cache needs
// be aware though that decreasing them may need the number of stages that is
// passed to get_vlc* to be increased.
#define MV_VLC_BITS 9
#define H263_MBTYPE_B_VLC_BITS 6
#define CBPC_B_VLC_BITS 3

static const int h263_mb_type_b_map[15]= {
    MB_TYPE_DIRECT2 | MB_TYPE_L0L1,
    MB_TYPE_DIRECT2 | MB_TYPE_L0L1 | MB_TYPE_CBP,
    MB_TYPE_DIRECT2 | MB_TYPE_L0L1 | MB_TYPE_CBP | MB_TYPE_QUANT,
                      MB_TYPE_L0                                 | MB_TYPE_16x16,
                      MB_TYPE_L0   | MB_TYPE_CBP                 | MB_TYPE_16x16,
                      MB_TYPE_L0   | MB_TYPE_CBP | MB_TYPE_QUANT | MB_TYPE_16x16,
                      MB_TYPE_L1                                 | MB_TYPE_16x16,
                      MB_TYPE_L1   | MB_TYPE_CBP                 | MB_TYPE_16x16,
                      MB_TYPE_L1   | MB_TYPE_CBP | MB_TYPE_QUANT | MB_TYPE_16x16,
                      MB_TYPE_L0L1                               | MB_TYPE_16x16,
                      MB_TYPE_L0L1 | MB_TYPE_CBP                 | MB_TYPE_16x16,
                      MB_TYPE_L0L1 | MB_TYPE_CBP | MB_TYPE_QUANT | MB_TYPE_16x16,
    0, //stuffing
    MB_TYPE_INTRA4x4                | MB_TYPE_CBP,
    MB_TYPE_INTRA4x4                | MB_TYPE_CBP | MB_TYPE_QUANT,
};

void ff_h263_show_pict_info(MpegEncContext *s){
    if(s->avctx->debug&FF_DEBUG_PICT_INFO){
    av_log(s->avctx, AV_LOG_DEBUG, "qp:%d %c size:%d rnd:%d%s%s%s%s%s%s%s%s%s %d/%d\n",
         s->qscale, av_get_picture_type_char(s->pict_type),
         s->gb.size_in_bits, 1-s->no_rounding,
         s->obmc ? " AP" : "",
         s->umvplus ? " UMV" : "",
         s->h263_long_vectors ? " LONG" : "",
         s->h263_plus ? " +" : "",
         s->h263_aic ? " AIC" : "",
         s->alt_inter_vlc ? " AIV" : "",
         s->modified_quant ? " MQ" : "",
         s->loop_filter ? " LOOP" : "",
         s->h263_slice_structured ? " SS" : "",
         s->avctx->time_base.den, s->avctx->time_base.num
    );
    }
}

/***********************************************/
/* decoding */

VLC ff_h263_intra_MCBPC_vlc;
VLC ff_h263_inter_MCBPC_vlc;
VLC ff_h263_cbpy_vlc;
static VLC mv_vlc;
static VLC h263_mbtype_b_vlc;
static VLC cbpc_b_vlc;

/* init vlcs */

/* XXX: find a better solution to handle static init */
void h263_decode_init_vlc(MpegEncContext *s)
{
    static int done = 0;

    if (!done) {
        done = 1;

        INIT_VLC_STATIC(&ff_h263_intra_MCBPC_vlc, INTRA_MCBPC_VLC_BITS, 9,
                 ff_h263_intra_MCBPC_bits, 1, 1,
                 ff_h263_intra_MCBPC_code, 1, 1, 72);
        INIT_VLC_STATIC(&ff_h263_inter_MCBPC_vlc, INTER_MCBPC_VLC_BITS, 28,
                 ff_h263_inter_MCBPC_bits, 1, 1,
                 ff_h263_inter_MCBPC_code, 1, 1, 198);
        INIT_VLC_STATIC(&ff_h263_cbpy_vlc, CBPY_VLC_BITS, 16,
                 &ff_h263_cbpy_tab[0][1], 2, 1,
                 &ff_h263_cbpy_tab[0][0], 2, 1, 64);
        INIT_VLC_STATIC(&mv_vlc, MV_VLC_BITS, 33,
                 &mvtab[0][1], 2, 1,
                 &mvtab[0][0], 2, 1, 538);
        init_rl(&ff_h263_rl_inter, ff_h263_static_rl_table_store[0]);
        init_rl(&rl_intra_aic, ff_h263_static_rl_table_store[1]);
        INIT_VLC_RL(ff_h263_rl_inter, 554);
        INIT_VLC_RL(rl_intra_aic, 554);
        INIT_VLC_STATIC(&h263_mbtype_b_vlc, H263_MBTYPE_B_VLC_BITS, 15,
                 &h263_mbtype_b_tab[0][1], 2, 1,
                 &h263_mbtype_b_tab[0][0], 2, 1, 80);
        INIT_VLC_STATIC(&cbpc_b_vlc, CBPC_B_VLC_BITS, 4,
                 &cbpc_b_tab[0][1], 2, 1,
                 &cbpc_b_tab[0][0], 2, 1, 8);
    }
}

int ff_h263_decode_mba(MpegEncContext *s)
{
    int i, mb_pos;

    for(i=0; i<6; i++){
        if(s->mb_num-1 <= ff_mba_max[i]) break;
    }
    mb_pos= get_bits(&s->gb, ff_mba_length[i]);
    s->mb_x= mb_pos % s->mb_width;
    s->mb_y= mb_pos / s->mb_width;

    return mb_pos;
}

static int h263_decode_gob_header(MpegEncContext *s)
{
    unsigned int val, gob_number;
    int left;

    /* Check for GOB Start Code */
    val = show_bits(&s->gb, 16);
    if(val)
        return -1;

        /* We have a GBSC probably with GSTUFF */
    skip_bits(&s->gb, 16); /* Drop the zeros */
    left= get_bits_left(&s->gb);
    //MN: we must check the bits left or we might end in a infinite loop (or segfault)
    for(;left>13; left--){
        if(get_bits1(&s->gb)) break; /* Seek the '1' bit */
    }
    if(left<=13)
        return -1;

    if(s->h263_slice_structured){
        if(get_bits1(&s->gb)==0)
            return -1;

        ff_h263_decode_mba(s);

        if(s->mb_num > 1583)
            if(get_bits1(&s->gb)==0)
                return -1;

        s->qscale = get_bits(&s->gb, 5); /* SQUANT */
        if(get_bits1(&s->gb)==0)
            return -1;
        skip_bits(&s->gb, 2); /* GFID */
    }else{
        gob_number = get_bits(&s->gb, 5); /* GN */
        s->mb_x= 0;
        s->mb_y= s->gob_index* gob_number;
        skip_bits(&s->gb, 2); /* GFID */
        s->qscale = get_bits(&s->gb, 5); /* GQUANT */
    }

    if(s->mb_y >= s->mb_height)
        return -1;

    if(s->qscale==0)
        return -1;

    return 0;
}

const uint8_t *ff_h263_find_resync_marker(const uint8_t *restrict p, const uint8_t * restrict end)
{
    assert(p < end);

    end-=2;
    p++;
    for(;p<end; p+=2){
        if(!*p){
            if     (!p[-1] && p[1]) return p - 1;
            else if(!p[ 1] && p[2]) return p;
        }
    }
    return end+2;
}

int ff_h263_resync(MpegEncContext *s){
    int left, pos, ret;

    if(s->codec_id==CODEC_ID_MPEG4){
        skip_bits1(&s->gb);
        align_get_bits(&s->gb);
    }

    if(show_bits(&s->gb, 16)==0){
        pos= get_bits_count(&s->gb);
        if(CONFIG_MPEG4_DECODER && s->codec_id==CODEC_ID_MPEG4)
            ret= mpeg4_decode_video_packet_header(s);
        else
            ret= h263_decode_gob_header(s);
        if(ret>=0)
            return pos;
    }
    //OK, it's not where it is supposed to be ...
    s->gb= s->last_resync_gb;
    align_get_bits(&s->gb);
    left= get_bits_left(&s->gb);

    for(;left>16+1+5+5; left-=8){
        if(show_bits(&s->gb, 16)==0){
            GetBitContext bak= s->gb;

            pos= get_bits_count(&s->gb);
            if(CONFIG_MPEG4_DECODER && s->codec_id==CODEC_ID_MPEG4)
                ret= mpeg4_decode_video_packet_header(s);
            else
                ret= h263_decode_gob_header(s);
            if(ret>=0)
                return pos;

            s->gb= bak;
        }
        skip_bits(&s->gb, 8);
    }

    return -1;
}

int h263_decode_motion(MpegEncContext * s, int pred, int f_code)
{
    int code, val, sign, shift;
    code = get_vlc2(&s->gb, mv_vlc.table, MV_VLC_BITS, 2);

    if (code == 0)
        return pred;
    if (code < 0)
        return 0xffff;

    sign = get_bits1(&s->gb);
    shift = f_code - 1;
    val = code;
    if (shift) {
        val = (val - 1) << shift;
        val |= get_bits(&s->gb, shift);
        val++;
    }
    if (sign)
        val = -val;
    val += pred;

    /* modulo decoding */
    if (!s->h263_long_vectors) {
        val = sign_extend(val, 5 + f_code);
    } else {
        /* horrible h263 long vector mode */
        if (pred < -31 && val < -63)
            val += 64;
        if (pred > 32 && val > 63)
            val -= 64;

    }
    return val;
}


/* Decodes RVLC of H.263+ UMV */
static int h263p_decode_umotion(MpegEncContext * s, int pred)
{
   int code = 0, sign;

   if (get_bits1(&s->gb)) /* Motion difference = 0 */
      return pred;

   code = 2 + get_bits1(&s->gb);

   while (get_bits1(&s->gb))
   {
      code <<= 1;
      code += get_bits1(&s->gb);
   }
   sign = code & 1;
   code >>= 1;

   code = (sign) ? (pred - code) : (pred + code);
   av_dlog(s->avctx,"H.263+ UMV Motion = %d\n", code);
   return code;

}

static void preview_obmc(MpegEncContext *s){
    GetBitContext gb= s->gb;

    int cbpc, i, pred_x, pred_y, mx, my;
    int16_t *mot_val;
    const int xy= s->mb_x + 1 + s->mb_y * s->mb_stride;
    const int stride= s->b8_stride*2;

    for(i=0; i<4; i++)
        s->block_index[i]+= 2;
    for(i=4; i<6; i++)
        s->block_index[i]+= 1;
    s->mb_x++;

    assert(s->pict_type == AV_PICTURE_TYPE_P);

    do{
        if (get_bits1(&s->gb)) {
            /* skip mb */
            mot_val = s->current_picture.f.motion_val[0][s->block_index[0]];
            mot_val[0       ]= mot_val[2       ]=
            mot_val[0+stride]= mot_val[2+stride]= 0;
            mot_val[1       ]= mot_val[3       ]=
            mot_val[1+stride]= mot_val[3+stride]= 0;

            s->current_picture.f.mb_type[xy] = MB_TYPE_SKIP | MB_TYPE_16x16 | MB_TYPE_L0;
            goto end;
        }
        cbpc = get_vlc2(&s->gb, ff_h263_inter_MCBPC_vlc.table, INTER_MCBPC_VLC_BITS, 2);
    }while(cbpc == 20);

    if(cbpc & 4){
        s->current_picture.f.mb_type[xy] = MB_TYPE_INTRA;
    }else{
        get_vlc2(&s->gb, ff_h263_cbpy_vlc.table, CBPY_VLC_BITS, 1);
        if (cbpc & 8) {
            if(s->modified_quant){
                if(get_bits1(&s->gb)) skip_bits(&s->gb, 1);
                else                  skip_bits(&s->gb, 5);
            }else
                skip_bits(&s->gb, 2);
        }

        if ((cbpc & 16) == 0) {
                s->current_picture.f.mb_type[xy] = MB_TYPE_16x16 | MB_TYPE_L0;
                /* 16x16 motion prediction */
                mot_val= h263_pred_motion(s, 0, 0, &pred_x, &pred_y);
                if (s->umvplus)
                   mx = h263p_decode_umotion(s, pred_x);
                else
                   mx = h263_decode_motion(s, pred_x, 1);

                if (s->umvplus)
                   my = h263p_decode_umotion(s, pred_y);
                else
                   my = h263_decode_motion(s, pred_y, 1);

                mot_val[0       ]= mot_val[2       ]=
                mot_val[0+stride]= mot_val[2+stride]= mx;
                mot_val[1       ]= mot_val[3       ]=
                mot_val[1+stride]= mot_val[3+stride]= my;
        } else {
            s->current_picture.f.mb_type[xy] = MB_TYPE_8x8 | MB_TYPE_L0;
            for(i=0;i<4;i++) {
                mot_val = h263_pred_motion(s, i, 0, &pred_x, &pred_y);
                if (s->umvplus)
                  mx = h263p_decode_umotion(s, pred_x);
                else
                  mx = h263_decode_motion(s, pred_x, 1);

                if (s->umvplus)
                  my = h263p_decode_umotion(s, pred_y);
                else
                  my = h263_decode_motion(s, pred_y, 1);
                if (s->umvplus && (mx - pred_x) == 1 && (my - pred_y) == 1)
                  skip_bits1(&s->gb); /* Bit stuffing to prevent PSC */
                mot_val[0] = mx;
                mot_val[1] = my;
            }
        }
    }
end:

    for(i=0; i<4; i++)
        s->block_index[i]-= 2;
    for(i=4; i<6; i++)
        s->block_index[i]-= 1;
    s->mb_x--;

    s->gb= gb;
}

static void h263_decode_dquant(MpegEncContext *s){
    static const int8_t quant_tab[4] = { -1, -2, 1, 2 };

    if(s->modified_quant){
        if(get_bits1(&s->gb))
            s->qscale= modified_quant_tab[get_bits1(&s->gb)][ s->qscale ];
        else
            s->qscale= get_bits(&s->gb, 5);
    }else
        s->qscale += quant_tab[get_bits(&s->gb, 2)];
    ff_set_qscale(s, s->qscale);
}

static int h263_decode_block(MpegEncContext * s, DCTELEM * block,
                             int n, int coded)
{
    int code, level, i, j, last, run;
    RLTable *rl = &ff_h263_rl_inter;
    const uint8_t *scan_table;
    GetBitContext gb= s->gb;

    scan_table = s->intra_scantable.permutated;
    if (s->h263_aic && s->mb_intra) {
        rl = &rl_intra_aic;
        i = 0;
        if (s->ac_pred) {
            if (s->h263_aic_dir)
                scan_table = s->intra_v_scantable.permutated; /* left */
            else
                scan_table = s->intra_h_scantable.permutated; /* top */
        }
    } else if (s->mb_intra) {
        /* DC coef */
        if(s->codec_id == CODEC_ID_RV10){
#if CONFIG_RV10_DECODER
          if (s->rv10_version == 3 && s->pict_type == AV_PICTURE_TYPE_I) {
            int component, diff;
            component = (n <= 3 ? 0 : n - 4 + 1);
            level = s->last_dc[component];
            if (s->rv10_first_dc_coded[component]) {
                diff = rv_decode_dc(s, n);
                if (diff == 0xffff)
                    return -1;
                level += diff;
                level = level & 0xff; /* handle wrap round */
                s->last_dc[component] = level;
            } else {
                s->rv10_first_dc_coded[component] = 1;
            }
          } else {
                level = get_bits(&s->gb, 8);
                if (level == 255)
                    level = 128;
          }
#endif
        }else{
            level = get_bits(&s->gb, 8);
            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);
                if(s->error_recognition >= FF_ER_COMPLIANT)
                    return -1;
            }
            if (level == 255)
                level = 128;
        }
        block[0] = level;
        i = 1;
    } else {
        i = 0;
    }
    if (!coded) {
        if (s->mb_intra && s->h263_aic)
            goto not_coded;
        s->block_last_index[n] = i - 1;
        return 0;
    }
retry:
    for(;;) {
        code = get_vlc2(&s->gb, rl->vlc.table, TEX_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 */
            if (CONFIG_FLV_DECODER && s->h263_flv > 1) {
                ff_flv2_decode_ac_esc(&s->gb, &level, &run, &last);
            } else {
                last = get_bits1(&s->gb);
                run = get_bits(&s->gb, 6);
                level = (int8_t)get_bits(&s->gb, 8);
                if(level == -128){
                    if (s->codec_id == CODEC_ID_RV10) {
                        /* XXX: should patch encoder too */
                        level = get_sbits(&s->gb, 12);
                    }else{
                        level = get_bits(&s->gb, 5);
                        level |= get_sbits(&s->gb, 6)<<5;
                    }
                }
            }
        } else {
            run = rl->table_run[code];
            level = rl->table_level[code];
            last = code >= rl->last;
            if (get_bits1(&s->gb))
                level = -level;
        }
        i += run;
        if (i >= 64){
            if(s->alt_inter_vlc && rl == &ff_h263_rl_inter && !s->mb_intra){
                //Looks like a hack but no, it's the way it is supposed to work ...
                rl = &rl_intra_aic;
                i = 0;
                s->gb= gb;
                s->dsp.clear_block(block);
                goto retry;
            }
            av_log(s->avctx, AV_LOG_ERROR, "run overflow at %dx%d i:%d\n", s->mb_x, s->mb_y, s->mb_intra);
            return -1;
        }
        j = scan_table[i];
        block[j] = level;
        if (last)
            break;
        i++;
    }
not_coded:
    if (s->mb_intra && s->h263_aic) {
        h263_pred_acdc(s, block, n);
        i = 63;
    }
    s->block_last_index[n] = i;
    return 0;
}

static int h263_skip_b_part(MpegEncContext *s, int cbp)
{
    LOCAL_ALIGNED_16(DCTELEM, dblock, [64]);
    int i, mbi;

    /* we have to set s->mb_intra to zero to decode B-part of PB-frame correctly
     * but real value should be restored in order to be used later (in OBMC condition)
     */
    mbi = s->mb_intra;
    s->mb_intra = 0;
    for (i = 0; i < 6; i++) {
        if (h263_decode_block(s, dblock, i, cbp&32) < 0)
            return -1;
        cbp+=cbp;
    }
    s->mb_intra = mbi;
    return 0;
}

static int h263_get_modb(GetBitContext *gb, int pb_frame, int *cbpb)
{
    int c, mv = 1;

    if (pb_frame < 3) { // h.263 Annex G and i263 PB-frame
        c = get_bits1(gb);
        if (pb_frame == 2 && c)
            mv = !get_bits1(gb);
    } else { // h.263 Annex M improved PB-frame
        mv = get_unary(gb, 0, 4) + 1;
        c = mv & 1;
        mv = !!(mv & 2);
    }
    if(c)
        *cbpb = get_bits(gb, 6);
    return mv;
}

int ff_h263_decode_mb(MpegEncContext *s,
                      DCTELEM block[6][64])
{
    int cbpc, cbpy, i, cbp, pred_x, pred_y, mx, my, dquant;
    int16_t *mot_val;
    const int xy= s->mb_x + s->mb_y * s->mb_stride;
    int cbpb = 0, pb_mv_count = 0;

    assert(!s->h263_pred);

    if (s->pict_type == AV_PICTURE_TYPE_P) {
        do{
            if (get_bits1(&s->gb)) {
                /* skip mb */
                s->mb_intra = 0;
                for(i=0;i<6;i++)
                    s->block_last_index[i] = -1;
                s->mv_dir = MV_DIR_FORWARD;
                s->mv_type = MV_TYPE_16X16;
                s->current_picture.f.mb_type[xy] = MB_TYPE_SKIP | MB_TYPE_16x16 | MB_TYPE_L0;
                s->mv[0][0][0] = 0;
                s->mv[0][0][1] = 0;
                s->mb_skipped = !(s->obmc | s->loop_filter);
                goto end;
            }
            cbpc = get_vlc2(&s->gb, ff_h263_inter_MCBPC_vlc.table, INTER_MCBPC_VLC_BITS, 2);
            if (cbpc < 0){
                av_log(s->avctx, AV_LOG_ERROR, "cbpc damaged at %d %d\n", s->mb_x, s->mb_y);
                return -1;
            }
        }while(cbpc == 20);

        s->dsp.clear_blocks(s->block[0]);

        dquant = cbpc & 8;
        s->mb_intra = ((cbpc & 4) != 0);
        if (s->mb_intra) goto intra;

        if(s->pb_frame && get_bits1(&s->gb))
            pb_mv_count = h263_get_modb(&s->gb, s->pb_frame, &cbpb);
        cbpy = get_vlc2(&s->gb, ff_h263_cbpy_vlc.table, CBPY_VLC_BITS, 1);

        if(s->alt_inter_vlc==0 || (cbpc & 3)!=3)
            cbpy ^= 0xF;

        cbp = (cbpc & 3) | (cbpy << 2);
        if (dquant) {
            h263_decode_dquant(s);
        }

        s->mv_dir = MV_DIR_FORWARD;
        if ((cbpc & 16) == 0) {
            s->current_picture.f.mb_type[xy] = MB_TYPE_16x16 | MB_TYPE_L0;
            /* 16x16 motion prediction */
            s->mv_type = MV_TYPE_16X16;
            h263_pred_motion(s, 0, 0, &pred_x, &pred_y);
            if (s->umvplus)
               mx = h263p_decode_umotion(s, pred_x);
            else
               mx = h263_decode_motion(s, pred_x, 1);

            if (mx >= 0xffff)
                return -1;

            if (s->umvplus)
               my = h263p_decode_umotion(s, pred_y);
            else
               my = h263_decode_motion(s, pred_y, 1);

            if (my >= 0xffff)
                return -1;
            s->mv[0][0][0] = mx;
            s->mv[0][0][1] = my;

            if (s->umvplus && (mx - pred_x) == 1 && (my - pred_y) == 1)
               skip_bits1(&s->gb); /* Bit stuffing to prevent PSC */
        } else {
            s->current_picture.f.mb_type[xy] = MB_TYPE_8x8 | MB_TYPE_L0;
            s->mv_type = MV_TYPE_8X8;
            for(i=0;i<4;i++) {
                mot_val = h263_pred_motion(s, i, 0, &pred_x, &pred_y);
                if (s->umvplus)
                  mx = h263p_decode_umotion(s, pred_x);
                else
                  mx = h263_decode_motion(s, pred_x, 1);
                if (mx >= 0xffff)
                    return -1;

                if (s->umvplus)
                  my = h263p_decode_umotion(s, pred_y);
                else
                  my = h263_decode_motion(s, pred_y, 1);
                if (my >= 0xffff)
                    return -1;
                s->mv[0][i][0] = mx;
                s->mv[0][i][1] = my;
                if (s->umvplus && (mx - pred_x) == 1 && (my - pred_y) == 1)
                  skip_bits1(&s->gb); /* Bit stuffing to prevent PSC */
                mot_val[0] = mx;
                mot_val[1] = my;
            }
        }
    } else if(s->pict_type==AV_PICTURE_TYPE_B) {
        int mb_type;
        const int stride= s->b8_stride;
        int16_t *mot_val0 = s->current_picture.f.motion_val[0][2 * (s->mb_x + s->mb_y * stride)];
        int16_t *mot_val1 = s->current_picture.f.motion_val[1][2 * (s->mb_x + s->mb_y * stride)];
//        const int mv_xy= s->mb_x + 1 + s->mb_y * s->mb_stride;

        //FIXME ugly
        mot_val0[0       ]= mot_val0[2       ]= mot_val0[0+2*stride]= mot_val0[2+2*stride]=
        mot_val0[1       ]= mot_val0[3       ]= mot_val0[1+2*stride]= mot_val0[3+2*stride]=
        mot_val1[0       ]= mot_val1[2       ]= mot_val1[0+2*stride]= mot_val1[2+2*stride]=
        mot_val1[1       ]= mot_val1[3       ]= mot_val1[1+2*stride]= mot_val1[3+2*stride]= 0;

        do{
            mb_type= get_vlc2(&s->gb, h263_mbtype_b_vlc.table, H263_MBTYPE_B_VLC_BITS, 2);
            if (mb_type < 0){
                av_log(s->avctx, AV_LOG_ERROR, "b mb_type damaged at %d %d\n", s->mb_x, s->mb_y);
                return -1;
            }

            mb_type= h263_mb_type_b_map[ mb_type ];
        }while(!mb_type);

        s->mb_intra = IS_INTRA(mb_type);
        if(HAS_CBP(mb_type)){
            s->dsp.clear_blocks(s->block[0]);
            cbpc = get_vlc2(&s->gb, cbpc_b_vlc.table, CBPC_B_VLC_BITS, 1);
            if(s->mb_intra){
                dquant = IS_QUANT(mb_type);
                goto intra;
            }

            cbpy = get_vlc2(&s->gb, ff_h263_cbpy_vlc.table, CBPY_VLC_BITS, 1);

            if (cbpy < 0){
                av_log(s->avctx, AV_LOG_ERROR, "b cbpy damaged at %d %d\n", s->mb_x, s->mb_y);
                return -1;
            }

            if(s->alt_inter_vlc==0 || (cbpc & 3)!=3)
                cbpy ^= 0xF;

            cbp = (cbpc & 3) | (cbpy << 2);
        }else
            cbp=0;

        assert(!s->mb_intra);

        if(IS_QUANT(mb_type)){
            h263_decode_dquant(s);
        }

        if(IS_DIRECT(mb_type)){
            s->mv_dir = MV_DIR_FORWARD | MV_DIR_BACKWARD | MV_DIRECT;
            mb_type |= ff_mpeg4_set_direct_mv(s, 0, 0);
        }else{
            s->mv_dir = 0;
            s->mv_type= MV_TYPE_16X16;
//FIXME UMV

            if(USES_LIST(mb_type, 0)){
                int16_t *mot_val= h263_pred_motion(s, 0, 0, &mx, &my);
                s->mv_dir = MV_DIR_FORWARD;

                mx = h263_decode_motion(s, mx, 1);
                my = h263_decode_motion(s, my, 1);

                s->mv[0][0][0] = mx;
                s->mv[0][0][1] = my;
                mot_val[0       ]= mot_val[2       ]= mot_val[0+2*stride]= mot_val[2+2*stride]= mx;
                mot_val[1       ]= mot_val[3       ]= mot_val[1+2*stride]= mot_val[3+2*stride]= my;
            }

            if(USES_LIST(mb_type, 1)){
                int16_t *mot_val= h263_pred_motion(s, 0, 1, &mx, &my);
                s->mv_dir |= MV_DIR_BACKWARD;

                mx = h263_decode_motion(s, mx, 1);
                my = h263_decode_motion(s, my, 1);

                s->mv[1][0][0] = mx;
                s->mv[1][0][1] = my;
                mot_val[0       ]= mot_val[2       ]= mot_val[0+2*stride]= mot_val[2+2*stride]= mx;
                mot_val[1       ]= mot_val[3       ]= mot_val[1+2*stride]= mot_val[3+2*stride]= my;
            }
        }

        s->current_picture.f.mb_type[xy] = mb_type;
    } else { /* I-Frame */
        do{
            cbpc = get_vlc2(&s->gb, ff_h263_intra_MCBPC_vlc.table, INTRA_MCBPC_VLC_BITS, 2);
            if (cbpc < 0){
                av_log(s->avctx, AV_LOG_ERROR, "I cbpc damaged at %d %d\n", s->mb_x, s->mb_y);
                return -1;
            }
        }while(cbpc == 8);

        s->dsp.clear_blocks(s->block[0]);

        dquant = cbpc & 4;
        s->mb_intra = 1;
intra:
        s->current_picture.f.mb_type[xy] = MB_TYPE_INTRA;
        if (s->h263_aic) {
            s->ac_pred = get_bits1(&s->gb);
            if(s->ac_pred){
                s->current_picture.f.mb_type[xy] = MB_TYPE_INTRA | MB_TYPE_ACPRED;

                s->h263_aic_dir = get_bits1(&s->gb);
            }
        }else
            s->ac_pred = 0;

        if(s->pb_frame && get_bits1(&s->gb))
            pb_mv_count = h263_get_modb(&s->gb, s->pb_frame, &cbpb);
        cbpy = get_vlc2(&s->gb, ff_h263_cbpy_vlc.table, CBPY_VLC_BITS, 1);
        if(cbpy<0){
            av_log(s->avctx, AV_LOG_ERROR, "I cbpy damaged at %d %d\n", s->mb_x, s->mb_y);
            return -1;
        }
        cbp = (cbpc & 3) | (cbpy << 2);
        if (dquant) {
            h263_decode_dquant(s);
        }

        pb_mv_count += !!s->pb_frame;
    }

    while(pb_mv_count--){
        h263_decode_motion(s, 0, 1);
        h263_decode_motion(s, 0, 1);
    }

    /* decode each block */
    for (i = 0; i < 6; i++) {
        if (h263_decode_block(s, block[i], i, cbp&32) < 0)
            return -1;
        cbp+=cbp;
    }

    if(s->pb_frame && h263_skip_b_part(s, cbpb) < 0)
        return -1;
    if(s->obmc && !s->mb_intra){
        if(s->pict_type == AV_PICTURE_TYPE_P && s->mb_x+1<s->mb_width && s->mb_num_left != 1)
            preview_obmc(s);
    }
end:

        /* per-MB end of slice check */
    {
        int v= show_bits(&s->gb, 16);

        if(get_bits_count(&s->gb) + 16 > s->gb.size_in_bits){
            v>>= get_bits_count(&s->gb) + 16 - s->gb.size_in_bits;
        }

        if(v==0)
            return SLICE_END;
    }

    return SLICE_OK;
}

/* most is hardcoded. should extend to handle all h263 streams */
int h263_decode_picture_header(MpegEncContext *s)
{
    int format, width, height, i;
    uint32_t startcode;

    align_get_bits(&s->gb);

    startcode= get_bits(&s->gb, 22-8);

    for(i= get_bits_left(&s->gb); i>24; i-=8) {
        startcode = ((startcode << 8) | get_bits(&s->gb, 8)) & 0x003FFFFF;

        if(startcode == 0x20)
            break;
    }

    if (startcode != 0x20) {
        av_log(s->avctx, AV_LOG_ERROR, "Bad picture start code\n");
        return -1;
    }
    /* temporal reference */
    i = get_bits(&s->gb, 8); /* picture timestamp */
    if( (s->picture_number&~0xFF)+i < s->picture_number)
        i+= 256;
    s->current_picture_ptr->f.pts =
    s->picture_number= (s->picture_number&~0xFF) + i;

    /* PTYPE starts here */
    if (get_bits1(&s->gb) != 1) {
        /* marker */
        av_log(s->avctx, AV_LOG_ERROR, "Bad marker\n");
        return -1;
    }
    if (get_bits1(&s->gb) != 0) {
        av_log(s->avctx, AV_LOG_ERROR, "Bad H263 id\n");
        return -1;      /* h263 id */
    }
    skip_bits1(&s->gb);         /* split screen off */
    skip_bits1(&s->gb);         /* camera  off */
    skip_bits1(&s->gb);         /* freeze picture release off */

    format = get_bits(&s->gb, 3);
    /*
        0    forbidden
        1    sub-QCIF
        10   QCIF
        7       extended PTYPE (PLUSPTYPE)
    */

    if (format != 7 && format != 6) {
        s->h263_plus = 0;
        /* H.263v1 */
        width = h263_format[format][0];
        height = h263_format[format][1];
        if (!width)
            return -1;

        s->pict_type = AV_PICTURE_TYPE_I + get_bits1(&s->gb);

        s->h263_long_vectors = get_bits1(&s->gb);

        if (get_bits1(&s->gb) != 0) {
            av_log(s->avctx, AV_LOG_ERROR, "H263 SAC not supported\n");
            return -1; /* SAC: off */
        }
        s->obmc= get_bits1(&s->gb); /* Advanced prediction mode */
        s->unrestricted_mv = s->h263_long_vectors || s->obmc;

        s->pb_frame = get_bits1(&s->gb);
        s->chroma_qscale= s->qscale = get_bits(&s->gb, 5);
        skip_bits1(&s->gb); /* Continuous Presence Multipoint mode: off */

        s->width = width;
        s->height = height;
        s->avctx->sample_aspect_ratio= (AVRational){12,11};
        s->avctx->time_base= (AVRational){1001, 30000};
    } else {
        int ufep;

        /* H.263v2 */
        s->h263_plus = 1;
        ufep = get_bits(&s->gb, 3); /* Update Full Extended PTYPE */

        /* ufep other than 0 and 1 are reserved */
        if (ufep == 1) {
            /* OPPTYPE */
            format = get_bits(&s->gb, 3);
            av_dlog(s->avctx, "ufep=1, format: %d\n", format);
            s->custom_pcf= get_bits1(&s->gb);
            s->umvplus = get_bits1(&s->gb); /* Unrestricted Motion Vector */
            if (get_bits1(&s->gb) != 0) {
                av_log(s->avctx, AV_LOG_ERROR, "Syntax-based Arithmetic Coding (SAC) not supported\n");
            }
            s->obmc= get_bits1(&s->gb); /* Advanced prediction mode */
            s->h263_aic = get_bits1(&s->gb); /* Advanced Intra Coding (AIC) */
            s->loop_filter= get_bits1(&s->gb);
            s->unrestricted_mv = s->umvplus || s->obmc || s->loop_filter;

            s->h263_slice_structured= get_bits1(&s->gb);
            if (get_bits1(&s->gb) != 0) {
                av_log(s->avctx, AV_LOG_ERROR, "Reference Picture Selection not supported\n");
            }
            if (get_bits1(&s->gb) != 0) {
                av_log(s->avctx, AV_LOG_ERROR, "Independent Segment Decoding not supported\n");
            }
            s->alt_inter_vlc= get_bits1(&s->gb);
            s->modified_quant= get_bits1(&s->gb);
            if(s->modified_quant)
                s->chroma_qscale_table= ff_h263_chroma_qscale_table;

            skip_bits(&s->gb, 1); /* Prevent start code emulation */

            skip_bits(&s->gb, 3); /* Reserved */
        } else if (ufep != 0) {
            av_log(s->avctx, AV_LOG_ERROR, "Bad UFEP type (%d)\n", ufep);
            return -1;
        }

        /* MPPTYPE */
        s->pict_type = get_bits(&s->gb, 3);
        switch(s->pict_type){
        case 0: s->pict_type= AV_PICTURE_TYPE_I;break;
        case 1: s->pict_type= AV_PICTURE_TYPE_P;break;
        case 2: s->pict_type= AV_PICTURE_TYPE_P;s->pb_frame = 3;break;
        case 3: s->pict_type= AV_PICTURE_TYPE_B;break;
        case 7: s->pict_type= AV_PICTURE_TYPE_I;break; //ZYGO
        default:
            return -1;
        }
        skip_bits(&s->gb, 2);
        s->no_rounding = get_bits1(&s->gb);
        skip_bits(&s->gb, 4);

        /* Get the picture dimensions */
        if (ufep) {
            if (format == 6) {
                /* Custom Picture Format (CPFMT) */
                s->aspect_ratio_info = get_bits(&s->gb, 4);
                av_dlog(s->avctx, "aspect: %d\n", s->aspect_ratio_info);
                /* aspect ratios:
                0 - forbidden
                1 - 1:1
                2 - 12:11 (CIF 4:3)
                3 - 10:11 (525-type 4:3)
                4 - 16:11 (CIF 16:9)
                5 - 40:33 (525-type 16:9)
                6-14 - reserved
                */
                width = (get_bits(&s->gb, 9) + 1) * 4;
                skip_bits1(&s->gb);
                height = get_bits(&s->gb, 9) * 4;
                av_dlog(s->avctx, "\nH.263+ Custom picture: %dx%d\n",width,height);
                if (s->aspect_ratio_info == FF_ASPECT_EXTENDED) {
                    /* aspected dimensions */
                    s->avctx->sample_aspect_ratio.num= get_bits(&s->gb, 8);
                    s->avctx->sample_aspect_ratio.den= get_bits(&s->gb, 8);
                }else{
                    s->avctx->sample_aspect_ratio= ff_h263_pixel_aspect[s->aspect_ratio_info];
                }
            } else {
                width = h263_format[format][0];
                height = h263_format[format][1];
                s->avctx->sample_aspect_ratio= (AVRational){12,11};
            }
            if ((width == 0) || (height == 0))
                return -1;
            s->width = width;
            s->height = height;

            if(s->custom_pcf){
                int gcd;
                s->avctx->time_base.den= 1800000;
                s->avctx->time_base.num= 1000 + get_bits1(&s->gb);
                s->avctx->time_base.num*= get_bits(&s->gb, 7);
                if(s->avctx->time_base.num == 0){
                    av_log(s, AV_LOG_ERROR, "zero framerate\n");
                    return -1;
                }
                gcd= av_gcd(s->avctx->time_base.den, s->avctx->time_base.num);
                s->avctx->time_base.den /= gcd;
                s->avctx->time_base.num /= gcd;
            }else{
                s->avctx->time_base= (AVRational){1001, 30000};
            }
        }

        if(s->custom_pcf){
            skip_bits(&s->gb, 2); //extended Temporal reference
        }

        if (ufep) {
            if (s->umvplus) {
                if(get_bits1(&s->gb)==0) /* Unlimited Unrestricted Motion Vectors Indicator (UUI) */
                    skip_bits1(&s->gb);
            }
            if(s->h263_slice_structured){
                if (get_bits1(&s->gb) != 0) {
                    av_log(s->avctx, AV_LOG_ERROR, "rectangular slices not supported\n");
                }
                if (get_bits1(&s->gb) != 0) {
                    av_log(s->avctx, AV_LOG_ERROR, "unordered slices not supported\n");
                }
            }
        }

        s->qscale = get_bits(&s->gb, 5);
    }

    s->mb_width = (s->width  + 15) / 16;
    s->mb_height = (s->height  + 15) / 16;
    s->mb_num = s->mb_width * s->mb_height;

    if (s->pb_frame) {
        skip_bits(&s->gb, 3); /* Temporal reference for B-pictures */
        if (s->custom_pcf)
            skip_bits(&s->gb, 2); //extended Temporal reference
        skip_bits(&s->gb, 2); /* Quantization information for B-pictures */
    }

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

    if(s->h263_slice_structured){
        if (get_bits1(&s->gb) != 1) {
            av_log(s->avctx, AV_LOG_ERROR, "SEPB1 marker missing\n");
            return -1;
        }

        ff_h263_decode_mba(s);

        if (get_bits1(&s->gb) != 1) {
            av_log(s->avctx, AV_LOG_ERROR, "SEPB2 marker missing\n");
            return -1;
        }
    }
    s->f_code = 1;

    if(s->h263_aic){
         s->y_dc_scale_table=
         s->c_dc_scale_table= ff_aic_dc_scale_table;
    }else{
        s->y_dc_scale_table=
        s->c_dc_scale_table= ff_mpeg1_dc_scale_table;
    }

        ff_h263_show_pict_info(s);
    if (s->pict_type == AV_PICTURE_TYPE_I && s->codec_tag == AV_RL32("ZYGO")){
        int i,j;
        for(i=0; i<85; i++) av_log(s->avctx, AV_LOG_DEBUG, "%d", get_bits1(&s->gb));
        av_log(s->avctx, AV_LOG_DEBUG, "\n");
        for(i=0; i<13; i++){
            for(j=0; j<3; j++){
                int v= get_bits(&s->gb, 8);
                v |= get_sbits(&s->gb, 8)<<8;
                av_log(s->avctx, AV_LOG_DEBUG, " %5d", v);
            }
            av_log(s->avctx, AV_LOG_DEBUG, "\n");
        }
        for(i=0; i<50; i++) av_log(s->avctx, AV_LOG_DEBUG, "%d", get_bits1(&s->gb));
    }

    return 0;
}

---