libavcodec/h261.c

   1 /*
   2  * H261 decoder
   3  * Copyright (c) 2002-2004 Michael Niedermayer <michaelni@gmx.at>
   4  * Copyright (c) 2004 Maarten Daniels
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; either
   9  * version 2 of the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, write to the Free Software
  18  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  19  */
  20
  21 /**
  22  * @file h261.c
  23  * h261codec.
  24  */
  25
  26 #include "common.h"
  27 #include "dsputil.h"
  28 #include "avcodec.h"
  29 #include "mpegvideo.h"
  30 #include "h261data.h"
  31
  32
  33 #define H261_MBA_VLC_BITS 9
  34 #define H261_MTYPE_VLC_BITS 6
  35 #define H261_MV_VLC_BITS 7
  36 #define H261_CBP_VLC_BITS 9
  37 #define TCOEFF_VLC_BITS 9
  38
  39 #define MBA_STUFFING 33
  40 #define MBA_STARTCODE 34
  41 #define IS_FIL(a)    ((a)&MB_TYPE_H261_FIL)
  42
  43 /**
  44  * H261Context
  45  */
  46 typedef struct H261Context{
  47     MpegEncContext s;
  48
  49     int current_mba;
  50     int mba_diff;
  51     int mtype;
  52     int current_mv_x;
  53     int current_mv_y;
  54     int gob_number;
  55     int bits_left; //8 - nr of bits left of the following frame in the last byte in this frame
  56     int last_bits; //bits left of the following frame in the last byte in this frame
  57     int gob_start_code_skipped; // 1 if gob start code is already read before gob header is read
  58 }H261Context;
  59
  60 void ff_h261_loop_filter(H261Context * h){
  61     MpegEncContext * const s = &h->s;
  62     const int linesize  = s->linesize;
  63     const int uvlinesize= s->uvlinesize;
  64     uint8_t *dest_y = s->dest[0];
  65     uint8_t *dest_cb= s->dest[1];
  66     uint8_t *dest_cr= s->dest[2];
  67
  68     s->dsp.h261_loop_filter(dest_y                   , linesize);
  69     s->dsp.h261_loop_filter(dest_y                + 8, linesize);
  70     s->dsp.h261_loop_filter(dest_y + 8 * linesize    , linesize);
  71     s->dsp.h261_loop_filter(dest_y + 8 * linesize + 8, linesize);
  72     s->dsp.h261_loop_filter(dest_cb, uvlinesize);
  73     s->dsp.h261_loop_filter(dest_cr, uvlinesize);
  74 }
  75
  76 static int h261_decode_block(H261Context *h, DCTELEM *block,
  77                              int n, int coded);
  78 static int h261_decode_mb(H261Context *h);
  79 void ff_set_qscale(MpegEncContext * s, int qscale);
  80
  81 /***********************************************/
  82 /* decoding */
  83
  84 static VLC h261_mba_vlc;
  85 static VLC h261_mtype_vlc;
  86 static VLC h261_mv_vlc;
  87 static VLC h261_cbp_vlc;
  88
  89 void init_vlc_rl(RLTable *rl);
  90
  91 static void h261_decode_init_vlc(H261Context *h){
  92     static int done = 0;
  93
  94     if(!done){
  95         done = 1;
  96         init_vlc(&h261_mba_vlc, H261_MBA_VLC_BITS, 35,
  97                  h261_mba_bits, 1, 1,
  98                  h261_mba_code, 1, 1);
  99         init_vlc(&h261_mtype_vlc, H261_MTYPE_VLC_BITS, 10,
 100                  h261_mtype_bits, 1, 1,
 101                  h261_mtype_code, 1, 1);
 102         init_vlc(&h261_mv_vlc, H261_MV_VLC_BITS, 17,
 103                  &h261_mv_tab[0][1], 2, 1,
 104                  &h261_mv_tab[0][0], 2, 1);
 105         init_vlc(&h261_cbp_vlc, H261_CBP_VLC_BITS, 63,
 106                  &h261_cbp_tab[0][1], 2, 1,
 107                  &h261_cbp_tab[0][0], 2, 1);
 108         init_rl(&h261_rl_tcoeff);
 109         init_vlc_rl(&h261_rl_tcoeff);
 110     }
 111 }
 112
 113 static int h261_decode_init(AVCodecContext *avctx){
 114     H261Context *h= avctx->priv_data;
 115     MpegEncContext * const s = &h->s;
 116
 117     // set defaults
 118     MPV_decode_defaults(s);
 119     s->avctx = avctx;
 120
 121     s->width  = s->avctx->coded_width;
 122     s->height = s->avctx->coded_height;
 123     s->codec_id = s->avctx->codec->id;
 124
 125     s->out_format = FMT_H261;
 126     s->low_delay= 1;
 127     avctx->pix_fmt= PIX_FMT_YUV420P;
 128
 129     s->codec_id= avctx->codec->id;
 130
 131     h261_decode_init_vlc(h);
 132
 133     h->bits_left = 0;
 134     h->last_bits = 0;
 135     h->gob_start_code_skipped = 0;
 136
 137     return 0;
 138 }
 139
 140 /**
 141  * decodes the group of blocks header or slice header.
 142  * @return <0 if an error occured
 143  */
 144 static int h261_decode_gob_header(H261Context *h){
 145     unsigned int val;
 146     MpegEncContext * const s = &h->s;
 147
 148     if ( !h->gob_start_code_skipped ){
 149         /* Check for GOB Start Code */
 150         val = show_bits(&s->gb, 15);
 151         if(val)
 152             return -1;
 153
 154         /* We have a GBSC */
 155         skip_bits(&s->gb, 16);
 156     }
 157
 158     h->gob_start_code_skipped = 0;
 159
 160     h->gob_number = get_bits(&s->gb, 4); /* GN */
 161     s->qscale = get_bits(&s->gb, 5); /* GQUANT */
 162
 163     /* Check if gob_number is valid */
 164     if (s->mb_height==18){ //cif
 165         if ((h->gob_number<=0) || (h->gob_number>12))
 166             return -1;
 167     }
 168     else{ //qcif
 169         if ((h->gob_number!=1) && (h->gob_number!=3) && (h->gob_number!=5))
 170             return -1;
 171     }
 172
 173     /* GEI */
 174     while (get_bits1(&s->gb) != 0) {
 175         skip_bits(&s->gb, 8);
 176     }
 177
 178     if(s->qscale==0)
 179         return -1;
 180
 181     // For the first transmitted macroblock in a GOB, MBA is the absolute address. For
 182     // subsequent macroblocks, MBA is the difference between the absolute addresses of
 183     // the macroblock and the last transmitted macroblock.
 184     h->current_mba = 0;
 185     h->mba_diff = 0;
 186
 187     return 0;
 188 }
 189
 190 /**
 191  * decodes the group of blocks / video packet header.
 192  * @return <0 if no resync found
 193  */
 194 static int ff_h261_resync(H261Context *h){
 195     MpegEncContext * const s = &h->s;
 196     int left, ret;
 197
 198     if ( h->gob_start_code_skipped ){
 199         ret= h261_decode_gob_header(h);
 200         if(ret>=0)
 201             return 0;
 202     }
 203     else{
 204         if(show_bits(&s->gb, 15)==0){
 205             ret= h261_decode_gob_header(h);
 206             if(ret>=0)
 207                 return 0;
 208         }
 209         //ok, its not where its supposed to be ...
 210         s->gb= s->last_resync_gb;
 211         align_get_bits(&s->gb);
 212         left= s->gb.size_in_bits - get_bits_count(&s->gb);
 213
 214         for(;left>15+1+4+5; left-=8){
 215             if(show_bits(&s->gb, 15)==0){
 216                 GetBitContext bak= s->gb;
 217
 218                 ret= h261_decode_gob_header(h);
 219                 if(ret>=0)
 220                     return 0;
 221
 222                 s->gb= bak;
 223             }
 224             skip_bits(&s->gb, 8);
 225         }
 226     }
 227
 228     return -1;
 229 }
 230
 231 /**
 232  * decodes skipped macroblocks
 233  * @return 0
 234  */
 235 static int h261_decode_mb_skipped(H261Context *h, int mba1, int mba2 )
 236 {
 237     MpegEncContext * const s = &h->s;
 238     int i;
 239
 240     s->mb_intra = 0;
 241
 242     for(i=mba1; i<mba2; i++){
 243         int j, xy;
 244
 245         s->mb_x= ((h->gob_number-1) % 2) * 11 + i % 11;
 246         s->mb_y= ((h->gob_number-1) / 2) * 3 + i / 11;
 247         xy = s->mb_x + s->mb_y * s->mb_stride;
 248         ff_init_block_index(s);
 249         ff_update_block_index(s);
 250         s->dsp.clear_blocks(s->block[0]);
 251
 252         for(j=0;j<6;j++)
 253             s->block_last_index[j] = -1;
 254
 255         s->mv_dir = MV_DIR_FORWARD;
 256         s->mv_type = MV_TYPE_16X16;
 257         s->current_picture.mb_type[xy]= MB_TYPE_SKIP | MB_TYPE_16x16 | MB_TYPE_L0;
 258         s->mv[0][0][0] = 0;
 259         s->mv[0][0][1] = 0;
 260         s->mb_skiped = 1;
 261
 262         MPV_decode_mb(s, s->block);
 263     }
 264
 265     return 0;
 266 }
 267
 268 static int decode_mv_component(GetBitContext *gb, int v){
 269     int mv_diff = get_vlc2(gb, h261_mv_vlc.table, H261_MV_VLC_BITS, 2);
 270
 271     /* check if mv_diff is valid */
 272     if ( mv_diff < 0 )
 273         return v;
 274
 275     mv_diff = mvmap[mv_diff];
 276
 277     if(mv_diff && !get_bits1(gb))
 278         mv_diff= -mv_diff;
 279
 280     v += mv_diff;
 281     if     (v <=-16) v+= 32;
 282     else if(v >= 16) v-= 32;
 283
 284     return v;
 285 }
 286
 287 static int h261_decode_mb(H261Context *h){
 288     MpegEncContext * const s = &h->s;
 289     int i, cbp, xy, old_mtype;
 290
 291     cbp = 63;
 292     // Read mba
 293     do{
 294         h->mba_diff = get_vlc2(&s->gb, h261_mba_vlc.table, H261_MBA_VLC_BITS, 2);
 295
 296         /* Check for slice end */
 297         /* NOTE: GOB can be empty (no MB data) or exist only of MBA_stuffing */
 298         if (h->mba_diff == MBA_STARTCODE){ // start code
 299             h->gob_start_code_skipped = 1;
 300             return SLICE_END;
 301         }
 302     }
 303     while( h->mba_diff == MBA_STUFFING ); // stuffing
 304
 305     if ( h->mba_diff < 0 ){
 306         if ( get_bits_count(&s->gb) + 7 >= s->gb.size_in_bits )
 307             return SLICE_END;
 308
 309         av_log(s->avctx, AV_LOG_ERROR, "illegal mba at %d %d\n", s->mb_x, s->mb_y);
 310         return SLICE_ERROR;
 311     }
 312
 313     h->mba_diff += 1;
 314     h->current_mba += h->mba_diff;
 315
 316     if ( h->current_mba > MBA_STUFFING )
 317         return SLICE_ERROR;
 318
 319     s->mb_x= ((h->gob_number-1) % 2) * 11 + ((h->current_mba-1) % 11);
 320     s->mb_y= ((h->gob_number-1) / 2) * 3 + ((h->current_mba-1) / 11);
 321     xy = s->mb_x + s->mb_y * s->mb_stride;
 322     ff_init_block_index(s);
 323     ff_update_block_index(s);
 324     s->dsp.clear_blocks(s->block[0]);
 325
 326     // Read mtype
 327     old_mtype = h->mtype;
 328     h->mtype = get_vlc2(&s->gb, h261_mtype_vlc.table, H261_MTYPE_VLC_BITS, 2);
 329     h->mtype = h261_mtype_map[h->mtype];
 330
 331     // Read mquant
 332     if ( IS_QUANT ( h->mtype ) ){
 333         ff_set_qscale(s, get_bits(&s->gb, 5));
 334     }
 335
 336     s->mb_intra = IS_INTRA4x4(h->mtype);
 337
 338     // Read mv
 339     if ( IS_16X16 ( h->mtype ) ){
 340         // Motion vector data is included for all MC macroblocks. MVD is obtained from the macroblock vector by subtracting the
 341         // vector of the preceding macroblock. For this calculation the vector of the preceding macroblock is regarded as zero in the
 342         // following three situations:
 343         // 1) evaluating MVD for macroblocks 1, 12 and 23;
 344         // 2) evaluating MVD for macroblocks in which MBA does not represent a difference of 1;
 345         // 3) MTYPE of the previous macroblock was not MC.
 346         if ( ( h->current_mba == 1 ) || ( h->current_mba == 12 ) || ( h->current_mba == 23 ) ||
 347              ( h->mba_diff != 1) || ( !IS_16X16 ( old_mtype ) ))
 348         {
 349             h->current_mv_x = 0;
 350             h->current_mv_y = 0;
 351         }
 352
 353         h->current_mv_x= decode_mv_component(&s->gb, h->current_mv_x);
 354         h->current_mv_y= decode_mv_component(&s->gb, h->current_mv_y);
 355     }
 356
 357     // Read cbp
 358     if ( HAS_CBP( h->mtype ) ){
 359         cbp = get_vlc2(&s->gb, h261_cbp_vlc.table, H261_CBP_VLC_BITS, 2) + 1;
 360     }
 361
 362     if(s->mb_intra){
 363         s->current_picture.mb_type[xy]= MB_TYPE_INTRA;
 364         goto intra;
 365     }
 366
 367     //set motion vectors
 368     s->mv_dir = MV_DIR_FORWARD;
 369     s->mv_type = MV_TYPE_16X16;
 370     s->current_picture.mb_type[xy]= MB_TYPE_16x16 | MB_TYPE_L0;
 371     if(IS_16X16 ( h->mtype )){
 372         s->mv[0][0][0] = h->current_mv_x * 2;//gets divided by 2 in motion compensation
 373         s->mv[0][0][1] = h->current_mv_y * 2;
 374     }
 375     else{
 376         h->current_mv_x = s->mv[0][0][0] = 0;
 377         h->current_mv_x = s->mv[0][0][1] = 0;
 378     }
 379
 380 intra:
 381     /* decode each block */
 382     if(s->mb_intra || HAS_CBP(h->mtype)){
 383         for (i = 0; i < 6; i++) {
 384             if (h261_decode_block(h, s->block[i], i, cbp&32) < 0){
 385                 return SLICE_ERROR;
 386             }
 387             cbp+=cbp;
 388         }
 389     }
 390
 391     MPV_decode_mb(s, s->block);
 392
 393     if(IS_FIL (h->mtype)){
 394         ff_h261_loop_filter(h);
 395     }
 396
 397     return SLICE_OK;
 398 }
 399
 400 /**
 401  * decodes a macroblock
 402  * @return <0 if an error occured
 403  */
 404 static int h261_decode_block(H261Context * h, DCTELEM * block,
 405                              int n, int coded)
 406 {
 407     MpegEncContext * const s = &h->s;
 408     int code, level, i, j, run;
 409     RLTable *rl = &h261_rl_tcoeff;
 410     const uint8_t *scan_table;
 411
 412     // For the variable length encoding there are two code tables, one being used for
 413     // the first transmitted LEVEL in INTER, INTER+MC and INTER+MC+FIL blocks, the second
 414     // for all other LEVELs except the first one in INTRA blocks which is fixed length
 415     // coded with 8 bits.
 416     // NOTE: the two code tables only differ in one VLC so we handle that manually.
 417     scan_table = s->intra_scantable.permutated;
 418     if (s->mb_intra){
 419         /* DC coef */
 420         level = get_bits(&s->gb, 8);
 421         // 0 (00000000b) and -128 (10000000b) are FORBIDDEN
 422         if((level&0x7F) == 0){
 423             av_log(s->avctx, AV_LOG_ERROR, "illegal dc %d at %d %d\n", level, s->mb_x, s->mb_y);
 424             return -1;
 425         }
 426         // The code 1000 0000 is not used, the reconstruction level of 1024 being coded as 1111 1111.
 427         if (level == 255)
 428             level = 128;
 429         block[0] = level;
 430         i = 1;
 431     }else if(coded){
 432         // Run  Level   Code
 433         // EOB                  Not possible for first level when cbp is available (that's why the table is different)
 434         // 0    1               1s
 435         // *    *               0*
 436         int check = show_bits(&s->gb, 2);
 437         i = 0;
 438         if ( check & 0x2 ){
 439             skip_bits(&s->gb, 2);
 440             block[0] = ( check & 0x1 ) ? -1 : 1;
 441             i = 1;
 442         }
 443     }else{
 444         i = 0;
 445     }
 446     if(!coded){
 447         s->block_last_index[n] = i - 1;
 448         return 0;
 449     }
 450     for(;;){
 451         code = get_vlc2(&s->gb, rl->vlc.table, TCOEFF_VLC_BITS, 2);
 452         if (code < 0){
 453             av_log(s->avctx, AV_LOG_ERROR, "illegal ac vlc code at %dx%d\n", s->mb_x, s->mb_y);
 454             return -1;
 455         }
 456         if (code == rl->n) {
 457             /* escape */
 458             // 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.
 459             run = get_bits(&s->gb, 6);
 460             level = (int8_t)get_bits(&s->gb, 8);
 461         }else if(code == 0){
 462             break;
 463         }else{
 464             run = rl->table_run[code];
 465             level = rl->table_level[code];
 466             if (get_bits1(&s->gb))
 467                 level = -level;
 468         }
 469         i += run;
 470         if (i >= 64){
 471             av_log(s->avctx, AV_LOG_ERROR, "run overflow at %dx%d\n", s->mb_x, s->mb_y);
 472             return -1;
 473         }
 474         j = scan_table[i];
 475         block[j] = level;
 476         i++;
 477     }
 478     s->block_last_index[n] = i-1;
 479     return 0;
 480 }
 481
 482 /**
 483  * decodes the H261 picture header.
 484  * @return <0 if no startcode found
 485  */
 486 int h261_decode_picture_header(H261Context *h){
 487     MpegEncContext * const s = &h->s;
 488     int format, i;
 489     uint32_t startcode;
 490     align_get_bits(&s->gb);
 491
 492     startcode = (h->last_bits << (12 - (8-h->bits_left))) | get_bits(&s->gb, 20-8 - (8- h->bits_left));
 493
 494     for(i= s->gb.size_in_bits - get_bits_count(&s->gb); i>24; i-=1){
 495         startcode = ((startcode << 1) | get_bits(&s->gb, 1)) & 0x000FFFFF;
 496
 497         if(startcode == 0x10)
 498             break;
 499     }
 500
 501     if (startcode != 0x10){
 502         av_log(s->avctx, AV_LOG_ERROR, "Bad picture start code\n");
 503         return -1;
 504     }
 505
 506     /* temporal reference */
 507     s->picture_number = get_bits(&s->gb, 5); /* picture timestamp */
 508
 509     /* PTYPE starts here */
 510     skip_bits1(&s->gb); /* split screen off */
 511     skip_bits1(&s->gb); /* camera  off */
 512     skip_bits1(&s->gb); /* freeze picture release off */
 513
 514     format = get_bits1(&s->gb);
 515
 516     //only 2 formats possible
 517     if (format == 0){//QCIF
 518         s->width = 176;
 519         s->height = 144;
 520         s->mb_width = 11;
 521         s->mb_height = 9;
 522     }else{//CIF
 523         s->width = 352;
 524         s->height = 288;
 525         s->mb_width = 22;
 526         s->mb_height = 18;
 527     }
 528
 529     s->mb_num = s->mb_width * s->mb_height;
 530
 531     skip_bits1(&s->gb); /* still image mode off */
 532     skip_bits1(&s->gb); /* Reserved */
 533
 534     /* PEI */
 535     while (get_bits1(&s->gb) != 0){
 536         skip_bits(&s->gb, 8);
 537     }
 538
 539     // h261 has no I-FRAMES, but if we pass I_TYPE for the first frame, the codec crashes if it does
 540     // not contain all I-blocks (e.g. when a packet is lost)
 541     s->pict_type = P_TYPE;
 542
 543     h->gob_number = 0;
 544     return 0;
 545 }
 546
 547 static int h261_decode_gob(H261Context *h){
 548     MpegEncContext * const s = &h->s;
 549
 550     ff_set_qscale(s, s->qscale);
 551
 552     /* decode mb's */
 553     while(h->current_mba <= MBA_STUFFING)
 554     {
 555         int ret;
 556         /* DCT & quantize */
 557         ret= h261_decode_mb(h);
 558         if(ret<0){
 559             if(ret==SLICE_END){
 560                 h261_decode_mb_skipped(h, h->current_mba, 33);
 561                 return 0;
 562             }
 563             av_log(s->avctx, AV_LOG_ERROR, "Error at MB: %d\n", s->mb_x + s->mb_y*s->mb_stride);
 564             return -1;
 565         }
 566
 567         h261_decode_mb_skipped(h, h->current_mba-h->mba_diff, h->current_mba-1);
 568     }
 569
 570     return -1;
 571 }
 572
 573 static int h261_find_frame_end(ParseContext *pc, AVCodecContext* avctx, const uint8_t *buf, int buf_size){
 574     int vop_found, i, j, bits_left, last_bits;
 575     uint32_t state;
 576
 577     H261Context *h = avctx->priv_data;
 578
 579     if(h){
 580         bits_left = h->bits_left;
 581         last_bits = h->last_bits;
 582     }
 583     else{
 584         bits_left = 0;
 585         last_bits = 0;
 586     }
 587
 588     vop_found= pc->frame_start_found;
 589     state= pc->state;
 590     if(bits_left!=0 && !vop_found)
 591         state = state << (8-bits_left) | last_bits;
 592     i=0;
 593     if(!vop_found){
 594         for(i=0; i<buf_size; i++){
 595             state= (state<<8) | buf[i];
 596             for(j=0; j<8; j++){
 597                 if(( (  (state<<j)  |  (buf[i]>>(8-j))  )>>(32-20) == 0x10 )&&(((state >> (17-j)) & 0x4000) == 0x0)){
 598                     i++;
 599                     vop_found=1;
 600                     break;
 601                 }
 602             }
 603             if(vop_found)
 604                     break;
 605         }
 606     }
 607     if(vop_found){
 608         for(; i<buf_size; i++){
 609             if(avctx->flags & CODEC_FLAG_TRUNCATED)//XXX ffplay workaround, someone a better solution?
 610                 state= (state<<8) | buf[i];
 611             for(j=0; j<8; j++){
 612                 if(( (  (state<<j)  |  (buf[i]>>(8-j))  )>>(32-20) == 0x10 )&&(((state >> (17-j)) & 0x4000) == 0x0)){
 613                     pc->frame_start_found=0;
 614                     pc->state=-1;
 615                     return i-3;
 616                 }
 617             }
 618         }
 619     }
 620
 621     pc->frame_start_found= vop_found;
 622     pc->state= state;
 623     return END_NOT_FOUND;
 624 }
 625
 626 static int h261_parse(AVCodecParserContext *s,
 627                       AVCodecContext *avctx,
 628                       uint8_t **poutbuf, int *poutbuf_size,
 629                       const uint8_t *buf, int buf_size)
 630 {
 631     ParseContext *pc = s->priv_data;
 632     int next;
 633
 634     next= h261_find_frame_end(pc,avctx, buf, buf_size);
 635     if (ff_combine_frame(pc, next, (uint8_t **)&buf, &buf_size) < 0) {
 636         *poutbuf = NULL;
 637         *poutbuf_size = 0;
 638         return buf_size;
 639     }
 640     *poutbuf = (uint8_t *)buf;
 641     *poutbuf_size = buf_size;
 642     return next;
 643 }
 644
 645 /**
 646  * returns the number of bytes consumed for building the current frame
 647  */
 648 static int get_consumed_bytes(MpegEncContext *s, int buf_size){
 649     int pos= (get_bits_count(&s->gb)+7)>>3;
 650
 651     if(s->flags&CODEC_FLAG_TRUNCATED){
 652         pos -= s->parse_context.last_index;
 653         if(pos<0) pos=0;// padding is not really read so this might be -1
 654         return pos;
 655     }else{
 656         if(pos==0) pos=1; //avoid infinite loops (i doubt thats needed but ...)
 657         if(pos+10>buf_size) pos=buf_size; // oops ;)
 658
 659         return pos;
 660     }
 661 }
 662
 663 static int h261_decode_frame(AVCodecContext *avctx,
 664                              void *data, int *data_size,
 665                              uint8_t *buf, int buf_size)
 666 {
 667     H261Context *h= avctx->priv_data;
 668     MpegEncContext *s = &h->s;
 669     int ret;
 670     AVFrame *pict = data;
 671
 672 #ifdef DEBUG
 673     printf("*****frame %d size=%d\n", avctx->frame_number, buf_size);
 674     printf("bytes=%x %x %x %x\n", buf[0], buf[1], buf[2], buf[3]);
 675 #endif
 676     s->flags= avctx->flags;
 677     s->flags2= avctx->flags2;
 678
 679     /* no supplementary picture */
 680     if (buf_size == 0) {
 681
 682         return 0;
 683     }
 684
 685     if(s->flags&CODEC_FLAG_TRUNCATED){
 686         int next;
 687
 688         next= h261_find_frame_end(&s->parse_context,avctx, buf, buf_size);
 689
 690         if( ff_combine_frame(&s->parse_context, next, &buf, &buf_size) < 0 )
 691             return buf_size;
 692     }
 693
 694
 695 retry:
 696
 697     init_get_bits(&s->gb, buf, buf_size*8);
 698
 699     if(!s->context_initialized){
 700         if (MPV_common_init(s) < 0) //we need the idct permutaton for reading a custom matrix
 701             return -1;
 702     }
 703
 704     //we need to set current_picture_ptr before reading the header, otherwise we cant store anyting im there
 705     if(s->current_picture_ptr==NULL || s->current_picture_ptr->data[0]){
 706         int i= ff_find_unused_picture(s, 0);
 707         s->current_picture_ptr= &s->picture[i];
 708     }
 709
 710     ret = h261_decode_picture_header(h);
 711
 712     /* skip if the header was thrashed */
 713     if (ret < 0){
 714         av_log(s->avctx, AV_LOG_ERROR, "header damaged\n");
 715         return -1;
 716     }
 717
 718     if (s->width != avctx->coded_width || s->height != avctx->coded_height){
 719         ParseContext pc= s->parse_context; //FIXME move these demuxng hack to avformat
 720         s->parse_context.buffer=0;
 721         MPV_common_end(s);
 722         s->parse_context= pc;
 723     }
 724     if (!s->context_initialized) {
 725         avcodec_set_dimensions(avctx, s->width, s->height);
 726
 727         goto retry;
 728     }
 729
 730     // for hurry_up==5
 731     s->current_picture.pict_type= s->pict_type;
 732     s->current_picture.key_frame= s->pict_type == I_TYPE;
 733
 734     /* skip everything if we are in a hurry>=5 */
 735     if(avctx->hurry_up>=5) return get_consumed_bytes(s, buf_size);
 736
 737     if(MPV_frame_start(s, avctx) < 0)
 738         return -1;
 739
 740     ff_er_frame_start(s);
 741
 742     /* decode each macroblock */
 743     s->mb_x=0;
 744     s->mb_y=0;
 745
 746     while(h->gob_number < (s->mb_height==18 ? 12 : 5)){
 747         if(ff_h261_resync(h)<0)
 748             break;
 749         h261_decode_gob(h);
 750     }
 751     MPV_frame_end(s);
 752
 753     // h261 doesn't have byte aligned codes
 754     // store the bits of the next frame that are left in the last byte
 755     // in the H261Context and remember the number of stored bits
 756     {
 757         int bitsleft;
 758         int current_pos= get_bits_count(&s->gb)>>3;
 759         bitsleft =  (current_pos<<3) - get_bits_count(&s->gb);
 760         h->bits_left = - bitsleft;
 761         if(bitsleft > 0)
 762             h->last_bits= get_bits(&s->gb, 8 - h->bits_left);
 763         else
 764             h->last_bits = 0;
 765     }
 766
 767 assert(s->current_picture.pict_type == s->current_picture_ptr->pict_type);
 768 assert(s->current_picture.pict_type == s->pict_type);
 769     *pict= *(AVFrame*)&s->current_picture;
 770     ff_print_debug_info(s, pict);
 771
 772     /* Return the Picture timestamp as the frame number */
 773     /* we substract 1 because it is added on utils.c    */
 774     avctx->frame_number = s->picture_number - 1;
 775
 776     *data_size = sizeof(AVFrame);
 777
 778     return get_consumed_bytes(s, buf_size);
 779 }
 780
 781 static int h261_decode_end(AVCodecContext *avctx)
 782 {
 783     H261Context *h= avctx->priv_data;
 784     MpegEncContext *s = &h->s;
 785
 786     MPV_common_end(s);
 787     return 0;
 788 }
 789
 790 AVCodec h261_decoder = {
 791     "h261",
 792     CODEC_TYPE_VIDEO,
 793     CODEC_ID_H261,
 794     sizeof(H261Context),
 795     h261_decode_init,
 796     NULL,
 797     h261_decode_end,
 798     h261_decode_frame,
 799     CODEC_CAP_TRUNCATED,
 800 };
 801
 802 AVCodecParser h261_parser = {
 803     { CODEC_ID_H261 },
 804     sizeof(ParseContext),
 805     NULL,
 806     h261_parse,
 807     ff_parse_close,
 808 };