git.sesse.net Git - vlc/blob - modules/packetizer/mpegvideo.c

   1 /*****************************************************************************
   2  * mpegvideo.c: parse and packetize an MPEG1/2 video stream
   3  *****************************************************************************
   4  * Copyright (C) 2001-2005 VideoLAN (Centrale Réseaux) and its contributors
   5  * $Id$
   6  *
   7  * Authors: Laurent Aimar <fenrir@via.ecp.fr>
   8  *          Eric Petit <titer@videolan.org>
   9  *          Gildas Bazin <gbazin@videolan.org>
  10  *          Jean-Paul Saman <jpsaman #_at_# m2x dot nl>
  11  *
  12  * This program is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU General Public License as published by
  14  * the Free Software Foundation; either version 2 of the License, or
  15  * (at your option) any later version.
  16  *
  17  * This program is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU General Public License
  23  * along with this program; if not, write to the Free Software
  24  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111, USA.
  25  *****************************************************************************/
  26
  27 /*****************************************************************************
  28  * Problem with this implementation:
  29  *
  30  * Although we should time-stamp each picture with a PTS, this isn't possible
  31  * with the current implementation.
  32  * The problem comes from the fact that for non-low-delay streams we can't
  33  * calculate the PTS of pictures used as backward reference. Even the temporal
  34  * reference number doesn't help here because all the pictures don't
  35  * necessarily have the same duration (eg. 3:2 pulldown).
  36  *
  37  * However this doesn't really matter as far as the MPEG muxers are concerned
  38  * because they allow having empty PTS fields. --gibalou
  39  *****************************************************************************/
  40
  41 /*****************************************************************************
  42  * Preamble
  43  *****************************************************************************/
  44 #include <stdlib.h>                                      /* malloc(), free() */
  45
  46 #include <vlc/vlc.h>
  47 #include <vlc/decoder.h>
  48 #include <vlc/input.h>
  49
  50 #include "vlc_block_helper.h"
  51
  52 #define SYNC_INTRAFRAME_TEXT N_("Sync on intraframe")
  53 #define SYNC_INTRAFRAME_LONGTEXT N_("Normally the packetizer would " \
  54     "sync on the next full frame. This flags instructs the packetizer " \
  55     "to sync on the first intraframe found.")
  56
  57 /*****************************************************************************
  58  * Module descriptor
  59  *****************************************************************************/
  60 static int  Open ( vlc_object_t * );
  61 static void Close( vlc_object_t * );
  62
  63 vlc_module_begin();
  64     set_category( CAT_SOUT );
  65     set_subcategory( SUBCAT_SOUT_PACKETIZER );
  66     set_description( _("MPEG-I/II video packetizer") );
  67     set_capability( "packetizer", 50 );
  68     set_callbacks( Open, Close );
  69
  70     add_bool( "packetizer-mpegvideo-sync-iframe", 0, NULL, SYNC_INTRAFRAME_TEXT,
  71               SYNC_INTRAFRAME_LONGTEXT, VLC_TRUE );
  72 vlc_module_end();
  73
  74 /*****************************************************************************
  75  * Local prototypes
  76  *****************************************************************************/
  77 static block_t *Packetize( decoder_t *, block_t ** );
  78 static block_t *ParseMPEGBlock( decoder_t *, block_t * );
  79
  80 struct decoder_sys_t
  81 {
  82     /*
  83      * Input properties
  84      */
  85     block_bytestream_t bytestream;
  86     int i_state;
  87     int i_offset;
  88     uint8_t p_startcode[3];
  89
  90     /* Sequence header and extension */
  91     block_t *p_seq;
  92     block_t *p_ext;
  93
  94     /* Current frame being built */
  95     block_t    *p_frame;
  96     block_t    **pp_last;
  97
  98     vlc_bool_t b_frame_slice;
  99     mtime_t i_pts;
 100     mtime_t i_dts;
 101
 102     /* Sequence properties */
 103     int         i_frame_rate;
 104     int         i_frame_rate_base;
 105     vlc_bool_t  b_seq_progressive;
 106     vlc_bool_t  b_low_delay;
 107     int         i_aspect_ratio_info;
 108     vlc_bool_t  b_inited;
 109
 110     /* Picture properties */
 111     int i_temporal_ref;
 112     int i_picture_type;
 113     int i_picture_structure;
 114     int i_top_field_first;
 115     int i_repeat_first_field;
 116     int i_progressive_frame;
 117
 118     mtime_t i_interpolated_dts;
 119     mtime_t i_old_duration;
 120     mtime_t i_last_ref_pts;
 121
 122     /* Number of pictures since last sequence header */
 123     int i_seq_old;
 124
 125     /* Sync behaviour */
 126     vlc_bool_t  b_sync_on_intra_frame;
 127     vlc_bool_t  b_discontinuity;
 128 };
 129
 130 enum {
 131     STATE_NOSYNC,
 132     STATE_NEXT_SYNC
 133 };
 134
 135 /*****************************************************************************
 136  * Open:
 137  *****************************************************************************/
 138 static int Open( vlc_object_t *p_this )
 139 {
 140     decoder_t *p_dec = (decoder_t*)p_this;
 141     decoder_sys_t *p_sys;
 142
 143     if( p_dec->fmt_in.i_codec != VLC_FOURCC( 'm', 'p', 'g', '1' ) &&
 144         p_dec->fmt_in.i_codec != VLC_FOURCC( 'm', 'p', 'g', '2' ) &&
 145         p_dec->fmt_in.i_codec != VLC_FOURCC( 'm', 'p', 'g', 'v' ) )
 146     {
 147         return VLC_EGENERIC;
 148     }
 149
 150     es_format_Init( &p_dec->fmt_out, VIDEO_ES, VLC_FOURCC('m','p','g','v') );
 151     p_dec->pf_packetize = Packetize;
 152
 153     p_dec->p_sys = p_sys = malloc( sizeof( decoder_sys_t ) );
 154
 155     /* Misc init */
 156     p_sys->i_state = STATE_NOSYNC;
 157     p_sys->bytestream = block_BytestreamInit( p_dec );
 158     p_sys->p_startcode[0] = 0;
 159     p_sys->p_startcode[1] = 0;
 160     p_sys->p_startcode[2] = 1;
 161     p_sys->i_offset = 0;
 162
 163     p_sys->p_seq = NULL;
 164     p_sys->p_ext = NULL;
 165     p_sys->p_frame = NULL;
 166     p_sys->pp_last = &p_sys->p_frame;
 167     p_sys->b_frame_slice = VLC_FALSE;
 168
 169     p_sys->i_dts = p_sys->i_pts = 0;
 170
 171     p_sys->i_frame_rate = 1;
 172     p_sys->i_frame_rate_base = 1;
 173     p_sys->b_seq_progressive = VLC_TRUE;
 174     p_sys->b_low_delay = VLC_TRUE;
 175     p_sys->i_seq_old = 0;
 176
 177     p_sys->i_temporal_ref = 0;
 178     p_sys->i_picture_type = 0;
 179     p_sys->i_picture_structure = 0x03; /* frame */
 180     p_sys->i_top_field_first = 0;
 181     p_sys->i_repeat_first_field = 0;
 182     p_sys->i_progressive_frame = 0;
 183     p_sys->b_inited = 0;
 184
 185     p_sys->i_interpolated_dts = 0;
 186     p_sys->i_old_duration = 0;
 187     p_sys->i_last_ref_pts = 0;
 188
 189     p_sys->b_discontinuity = VLC_FALSE;
 190     p_sys->b_sync_on_intra_frame = var_CreateGetBool( p_dec, "packetizer-mpegvideo-sync-iframe" );
 191     if( p_sys->b_sync_on_intra_frame )
 192         msg_Dbg( p_dec, "syncing happens on intraframe now." );
 193
 194     return VLC_SUCCESS;
 195 }
 196
 197 /*****************************************************************************
 198  * Close:
 199  *****************************************************************************/
 200 static void Close( vlc_object_t *p_this )
 201 {
 202     decoder_t     *p_dec = (decoder_t*)p_this;
 203     decoder_sys_t *p_sys = p_dec->p_sys;
 204
 205     block_BytestreamRelease( &p_sys->bytestream );
 206
 207     if( p_sys->p_seq )
 208     {
 209         block_Release( p_sys->p_seq );
 210     }
 211     if( p_sys->p_ext )
 212     {
 213         block_Release( p_sys->p_ext );
 214     }
 215     if( p_sys->p_frame )
 216     {
 217         block_ChainRelease( p_sys->p_frame );
 218     }
 219
 220     var_Destroy( p_dec, "packetizer-mpegvideo-sync-iframe" );
 221
 222     free( p_sys );
 223 }
 224
 225 /*****************************************************************************
 226  * Packetize:
 227  *****************************************************************************/
 228 static block_t *Packetize( decoder_t *p_dec, block_t **pp_block )
 229 {
 230     decoder_sys_t *p_sys = p_dec->p_sys;
 231     block_t       *p_pic;
 232
 233     if( pp_block == NULL || *pp_block == NULL )
 234     {
 235         return NULL;
 236     }
 237
 238     if( (*pp_block)->i_flags & (BLOCK_FLAG_DISCONTINUITY|BLOCK_FLAG_CORRUPTED) )
 239     {
 240         p_sys->i_state = STATE_NOSYNC;
 241         p_sys->b_discontinuity = VLC_TRUE;
 242         if( p_sys->p_frame )
 243             block_ChainRelease( p_sys->p_frame );
 244         p_sys->p_frame = NULL;
 245         p_sys->pp_last = &p_sys->p_frame;
 246         p_sys->b_frame_slice = VLC_FALSE;
 247         block_Release( *pp_block );
 248         return NULL;
 249     }
 250
 251     block_BytestreamPush( &p_sys->bytestream, *pp_block );
 252
 253     while( 1 )
 254     {
 255         switch( p_sys->i_state )
 256         {
 257
 258         case STATE_NOSYNC:
 259             if( block_FindStartcodeFromOffset( &p_sys->bytestream,
 260                     &p_sys->i_offset, p_sys->p_startcode, 3 ) == VLC_SUCCESS )
 261             {
 262                 p_sys->i_state = STATE_NEXT_SYNC;
 263             }
 264
 265             if( p_sys->i_offset )
 266             {
 267                 block_SkipBytes( &p_sys->bytestream, p_sys->i_offset );
 268                 p_sys->i_offset = 0;
 269                 block_BytestreamFlush( &p_sys->bytestream );
 270             }
 271
 272             if( p_sys->i_state != STATE_NEXT_SYNC )
 273             {
 274                 /* Need more data */
 275                 return NULL;
 276             }
 277
 278             p_sys->i_offset = 1; /* To find next startcode */
 279
 280         case STATE_NEXT_SYNC:
 281             /* TODO: If p_block == NULL, flush the buffer without checking the
 282              * next sync word */
 283
 284             /* Find the next startcode */
 285             if( block_FindStartcodeFromOffset( &p_sys->bytestream,
 286                     &p_sys->i_offset, p_sys->p_startcode, 3 ) != VLC_SUCCESS )
 287             {
 288                 /* Need more data */
 289                 return NULL;
 290             }
 291
 292             /* Get the new fragment and set the pts/dts */
 293             p_pic = block_New( p_dec, p_sys->i_offset );
 294             block_BytestreamFlush( &p_sys->bytestream );
 295             p_pic->i_pts = p_sys->bytestream.p_block->i_pts;
 296             p_pic->i_dts = p_sys->bytestream.p_block->i_dts;
 297
 298             block_GetBytes( &p_sys->bytestream, p_pic->p_buffer,
 299                             p_pic->i_buffer );
 300
 301             /* don't reuse the same timestamps several times */
 302             if( p_pic->i_buffer >= 4 && p_pic->p_buffer[3] == 0x00 )
 303             {
 304                 /* We have a picture start code */
 305                 p_sys->bytestream.p_block->i_pts = 0;
 306                 p_sys->bytestream.p_block->i_dts = 0;
 307             }
 308
 309             p_sys->i_offset = 0;
 310
 311             /* Get picture if any */
 312             if( !( p_pic = ParseMPEGBlock( p_dec, p_pic ) ) )
 313             {
 314                 p_sys->i_state = STATE_NOSYNC;
 315                 break;
 316             }
 317
 318             /* If a discontinuity has been encountered, then wait till
 319              * the next Intra frame before continuing with packetizing */
 320             if( p_sys->b_discontinuity &&
 321                 p_sys->b_sync_on_intra_frame )
 322             {
 323                 if( p_pic->i_flags & BLOCK_FLAG_TYPE_I )
 324                 {
 325                     msg_Dbg( p_dec, "synced on Intra frame" );
 326                     p_sys->b_discontinuity = VLC_FALSE;
 327                     p_pic->i_flags |= BLOCK_FLAG_DISCONTINUITY;
 328                 }
 329                 else
 330                 {
 331                     p_sys->i_state = STATE_NOSYNC;
 332                     break;
 333                 }
 334             }
 335
 336             /* We've just started the stream, wait for the first PTS.
 337              * We discard here so we can still get the sequence header. */
 338             if( p_sys->i_dts <= 0 && p_sys->i_pts <= 0 &&
 339                 p_sys->i_interpolated_dts <= 0 )
 340             {
 341                 msg_Dbg( p_dec, "need a starting pts/dts" );
 342                 p_sys->i_state = STATE_NOSYNC;
 343                 block_Release( p_pic );
 344                 break;
 345             }
 346
 347             /* When starting the stream we can have the first frame with
 348              * a null DTS (i_interpolated_pts is initialized to 0) */
 349             if( !p_pic->i_dts ) p_pic->i_dts = p_pic->i_pts;
 350
 351             /* So p_block doesn't get re-added several times */
 352             *pp_block = block_BytestreamPop( &p_sys->bytestream );
 353
 354             p_sys->i_state = STATE_NOSYNC;
 355
 356             return p_pic;
 357         }
 358     }
 359 }
 360
 361 /*****************************************************************************
 362  * ParseMPEGBlock: Re-assemble fragments into a block containing a picture
 363  *****************************************************************************/
 364 static block_t *ParseMPEGBlock( decoder_t *p_dec, block_t *p_frag )
 365 {
 366     decoder_sys_t *p_sys = p_dec->p_sys;
 367     block_t *p_pic = NULL;
 368
 369     /*
 370      * Check if previous picture is finished
 371      */
 372     if( ( p_sys->b_frame_slice &&
 373           (p_frag->p_buffer[3] == 0x00 || p_frag->p_buffer[3] > 0xaf) ) &&
 374           p_sys->p_seq == NULL )
 375     {
 376         /* We have a picture but without a sequence header we can't
 377          * do anything */
 378         msg_Dbg( p_dec, "waiting for sequence start" );
 379         if( p_sys->p_frame ) block_ChainRelease( p_sys->p_frame );
 380         p_sys->p_frame = NULL;
 381         p_sys->pp_last = &p_sys->p_frame;
 382         p_sys->b_frame_slice = VLC_FALSE;
 383
 384     }
 385     else if( p_sys->b_frame_slice &&
 386              (p_frag->p_buffer[3] == 0x00 || p_frag->p_buffer[3] > 0xaf) )
 387     {
 388         mtime_t i_duration;
 389
 390         p_pic = block_ChainGather( p_sys->p_frame );
 391
 392         i_duration = (mtime_t)( 1000000 * p_sys->i_frame_rate_base /
 393                                 p_sys->i_frame_rate );
 394
 395         if( !p_sys->b_seq_progressive && p_sys->i_picture_structure != 0x03 )
 396         {
 397             i_duration /= 2;
 398         }
 399
 400         if( p_sys->b_seq_progressive )
 401         {
 402             if( p_sys->i_top_field_first == 0 &&
 403                 p_sys->i_repeat_first_field == 1 )
 404             {
 405                 i_duration *= 2;
 406             }
 407             else if( p_sys->i_top_field_first == 1 &&
 408                      p_sys->i_repeat_first_field == 1 )
 409             {
 410                 i_duration *= 3;
 411             }
 412         }
 413         else
 414         {
 415             if( p_sys->i_picture_structure == 0x03 )
 416             {
 417                 if( p_sys->i_progressive_frame && p_sys->i_repeat_first_field )
 418                 {
 419                     i_duration += i_duration / 2;
 420                 }
 421             }
 422         }
 423
 424         if( p_sys->b_low_delay || p_sys->i_picture_type == 0x03 )
 425         {
 426             /* Trivial case (DTS == PTS) */
 427             /* Correct interpolated dts when we receive a new pts/dts */
 428             if( p_sys->i_pts > 0 ) p_sys->i_interpolated_dts = p_sys->i_pts;
 429             if( p_sys->i_dts > 0 ) p_sys->i_interpolated_dts = p_sys->i_dts;
 430         }
 431         else
 432         {
 433             /* Correct interpolated dts when we receive a new pts/dts */
 434             if( p_sys->i_last_ref_pts > 0 )
 435                 p_sys->i_interpolated_dts = p_sys->i_last_ref_pts;
 436             if( p_sys->i_dts > 0 ) p_sys->i_interpolated_dts = p_sys->i_dts;
 437
 438             p_sys->i_last_ref_pts = p_sys->i_pts;
 439         }
 440
 441         p_pic->i_dts = p_sys->i_interpolated_dts;
 442
 443         /* Set PTS only if we have a B frame or if it comes from the stream */
 444         if( p_sys->i_pts > 0 )
 445         {
 446             p_pic->i_pts = p_sys->i_pts;
 447         }
 448         else if( p_sys->i_picture_type == 0x03 )
 449         {
 450             p_pic->i_pts = p_pic->i_dts;
 451         }
 452         else
 453         {
 454             p_pic->i_pts = 0;
 455         }
 456
 457         if( p_sys->b_low_delay || p_sys->i_picture_type == 0x03 )
 458         {
 459             /* Trivial case (DTS == PTS) */
 460             p_sys->i_interpolated_dts += i_duration;
 461         }
 462         else
 463         {
 464             p_sys->i_interpolated_dts += p_sys->i_old_duration;
 465             p_sys->i_old_duration = i_duration;
 466         }
 467
 468         switch ( p_sys->i_picture_type )
 469         {
 470         case 0x01:
 471             p_pic->i_flags |= BLOCK_FLAG_TYPE_I;
 472             break;
 473         case 0x02:
 474             p_pic->i_flags |= BLOCK_FLAG_TYPE_P;
 475             break;
 476         case 0x03:
 477             p_pic->i_flags |= BLOCK_FLAG_TYPE_B;
 478             break;
 479         }
 480
 481         p_pic->i_length = p_sys->i_interpolated_dts - p_pic->i_dts;
 482
 483 #if 0
 484         msg_Dbg( p_dec, "pic: type=%d dts="I64Fd" pts-dts="I64Fd,
 485         p_sys->i_picture_type, p_pic->i_dts, p_pic->i_pts - p_pic->i_dts);
 486 #endif
 487
 488         /* Reset context */
 489         p_sys->p_frame = NULL;
 490         p_sys->pp_last = &p_sys->p_frame;
 491         p_sys->b_frame_slice = VLC_FALSE;
 492     }
 493
 494     /*
 495      * Check info of current fragment
 496      */
 497     if( p_frag->p_buffer[3] == 0xb8 )
 498     {
 499         /* Group start code */
 500         if( p_sys->p_seq &&
 501             p_sys->i_seq_old > p_sys->i_frame_rate/p_sys->i_frame_rate_base )
 502         {
 503             /* Usefull for mpeg1: repeat sequence header every second */
 504             block_ChainLastAppend( &p_sys->pp_last, block_Duplicate( p_sys->p_seq ) );
 505             if( p_sys->p_ext )
 506             {
 507                 block_ChainLastAppend( &p_sys->pp_last, block_Duplicate( p_sys->p_ext ) );
 508             }
 509
 510             p_sys->i_seq_old = 0;
 511         }
 512     }
 513     else if( p_frag->p_buffer[3] == 0xb3 && p_frag->i_buffer >= 8 )
 514     {
 515         /* Sequence header code */
 516         static const int code_to_frame_rate[16][2] =
 517         {
 518             { 1, 1 },  /* invalid */
 519             { 24000, 1001 }, { 24, 1 }, { 25, 1 },       { 30000, 1001 },
 520             { 30, 1 },       { 50, 1 }, { 60000, 1001 }, { 60, 1 },
 521             /* Unofficial 15fps from Xing*/
 522             { 15, 1001 },
 523             /* Unofficial economy rates from libmpeg3 */
 524             { 5, 1001 }, { 10, 1001 }, { 12, 1001 }, { 15, 1001 },
 525             { 1, 1 },  { 1, 1 }  /* invalid */
 526         };
 527
 528         if( p_sys->p_seq ) block_Release( p_sys->p_seq );
 529         if( p_sys->p_ext ) block_Release( p_sys->p_ext );
 530
 531         p_sys->p_seq = block_Duplicate( p_frag );
 532         p_sys->i_seq_old = 0;
 533         p_sys->p_ext = NULL;
 534
 535         p_dec->fmt_out.video.i_width =
 536             ( p_frag->p_buffer[4] << 4)|(p_frag->p_buffer[5] >> 4 );
 537         p_dec->fmt_out.video.i_height =
 538             ( (p_frag->p_buffer[5]&0x0f) << 8 )|p_frag->p_buffer[6];
 539         p_sys->i_aspect_ratio_info = p_frag->p_buffer[7] >> 4;
 540
 541         /* TODO: MPEG1 aspect ratio */
 542
 543         p_sys->i_frame_rate = code_to_frame_rate[p_frag->p_buffer[7]&0x0f][0];
 544         p_sys->i_frame_rate_base =
 545             code_to_frame_rate[p_frag->p_buffer[7]&0x0f][1];
 546
 547         p_dec->fmt_out.video.i_frame_rate = p_sys->i_frame_rate;
 548         p_dec->fmt_out.video.i_frame_rate_base = p_sys->i_frame_rate_base;
 549
 550         p_sys->b_seq_progressive = VLC_TRUE;
 551         p_sys->b_low_delay = VLC_TRUE;
 552
 553         if ( !p_sys->b_inited )
 554         {
 555             msg_Dbg( p_dec, "Size %dx%d fps=%.3f",
 556                  p_dec->fmt_out.video.i_width, p_dec->fmt_out.video.i_height,
 557                  p_sys->i_frame_rate / (float)p_sys->i_frame_rate_base );
 558             p_sys->b_inited = 1;
 559         }
 560     }
 561     else if( p_frag->p_buffer[3] == 0xb5 )
 562     {
 563         int i_type = p_frag->p_buffer[4] >> 4;
 564
 565         /* Extention start code */
 566         if( i_type == 0x01 )
 567         {
 568 #if 0
 569             static const int mpeg2_aspect[16][2] =
 570             {
 571                 {0,1}, {1,1}, {4,3}, {16,9}, {221,100},
 572                 {0,1}, {0,1}, {0,1}, {0,1}, {0,1}, {0,1}, {0,1}, {0,1}, {0,1},
 573                 {0,1}, {0,1}
 574             };
 575 #endif
 576
 577             /* sequence extention */
 578             if( p_sys->p_ext) block_Release( p_sys->p_ext );
 579             p_sys->p_ext = block_Duplicate( p_frag );
 580
 581             if( p_frag->i_buffer >= 10 )
 582             {
 583                 p_sys->b_seq_progressive =
 584                     p_frag->p_buffer[5]&0x08 ? VLC_TRUE : VLC_FALSE;
 585                 p_sys->b_low_delay =
 586                     p_frag->p_buffer[9]&0x80 ? VLC_TRUE : VLC_FALSE;
 587             }
 588
 589             /* Do not set aspect ratio : in case we're transcoding,
 590              * transcode will take our fmt_out as a fmt_in to libmpeg2.
 591              * libmpeg2.c will then believe that the user has requested
 592              * a specific aspect ratio, which she hasn't. Thus in case
 593              * of aspect ratio change, we're screwed. --Meuuh
 594              */
 595 #if 0
 596             p_dec->fmt_out.video.i_aspect =
 597                 mpeg2_aspect[p_sys->i_aspect_ratio_info][0] *
 598                 VOUT_ASPECT_FACTOR /
 599                 mpeg2_aspect[p_sys->i_aspect_ratio_info][1];
 600 #endif
 601
 602         }
 603         else if( i_type == 0x08 )
 604         {
 605             /* picture extention */
 606             p_sys->i_picture_structure = p_frag->p_buffer[6]&0x03;
 607             p_sys->i_top_field_first   = p_frag->p_buffer[7] >> 7;
 608             p_sys->i_repeat_first_field= (p_frag->p_buffer[7]>>1)&0x01;
 609             p_sys->i_progressive_frame = p_frag->p_buffer[8] >> 7;
 610         }
 611     }
 612     else if( p_frag->p_buffer[3] == 0x00 )
 613     {
 614         /* Picture start code */
 615         p_sys->i_seq_old++;
 616
 617         if( p_frag->i_buffer >= 6 )
 618         {
 619             p_sys->i_temporal_ref =
 620                 ( p_frag->p_buffer[4] << 2 )|(p_frag->p_buffer[5] >> 6);
 621             p_sys->i_picture_type = ( p_frag->p_buffer[5] >> 3 ) & 0x03;
 622         }
 623
 624         p_sys->i_dts = p_frag->i_dts;
 625         p_sys->i_pts = p_frag->i_pts;
 626     }
 627     else if( p_frag->p_buffer[3] >= 0x01 && p_frag->p_buffer[3] <= 0xaf )
 628     {
 629         /* Slice start code */
 630         p_sys->b_frame_slice = VLC_TRUE;
 631     }
 632
 633     /* Append the block */
 634     block_ChainLastAppend( &p_sys->pp_last, p_frag );
 635
 636     return p_pic;
 637 }