]> git.sesse.net Git - vlc/blob - modules/packetizer/mpeg4video.c
Added x264 fourcc to h264 packetizer.
[vlc] / modules / packetizer / mpeg4video.c
1 /*****************************************************************************
2  * mpeg4video.c: mpeg 4 video packetizer
3  *****************************************************************************
4  * Copyright (C) 2001-2006 the VideoLAN team
5  * $Id$
6  *
7  * Authors: Gildas Bazin <gbazin@videolan.org>
8  *          Laurent Aimar <fenrir@via.ecp.fr>
9  *          Eric Petit <titer@videolan.org>
10  *
11  * This program is free software; you can redistribute it and/or modify
12  * it under the terms of the GNU General Public License as published by
13  * the Free Software Foundation; either version 2 of the License, or
14  * (at your option) any later version.
15  *
16  * This program is distributed in the hope that it will be useful,
17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
19  * GNU General Public License for more details.
20  *
21  * You should have received a copy of the GNU General Public License
22  * along with this program; if not, write to the Free Software
23  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
24  *****************************************************************************/
25
26 /*****************************************************************************
27  * Preamble
28  *****************************************************************************/
29
30 #ifdef HAVE_CONFIG_H
31 # include "config.h"
32 #endif
33
34 #include <vlc_common.h>
35 #include <vlc_plugin.h>
36 #include <vlc_sout.h>
37 #include <vlc_codec.h>
38 #include <vlc_block.h>
39 #include <vlc_input.h>                  /* hmmm, just for INPUT_RATE_DEFAULT */
40
41 #include "vlc_bits.h"
42 #include "vlc_block_helper.h"
43
44 /*****************************************************************************
45  * Module descriptor
46  *****************************************************************************/
47 static int  Open ( vlc_object_t * );
48 static void Close( vlc_object_t * );
49
50 vlc_module_begin();
51     set_category( CAT_SOUT );
52     set_subcategory( SUBCAT_SOUT_PACKETIZER );
53     set_description( N_("MPEG4 video packetizer") );
54     set_capability( "packetizer", 50 );
55     set_callbacks( Open, Close );
56 vlc_module_end();
57
58 /****************************************************************************
59  * Local prototypes
60  ****************************************************************************/
61 static block_t *Packetize( decoder_t *, block_t ** );
62
63 struct decoder_sys_t
64 {
65     /*
66      * Input properties
67      */
68     block_bytestream_t bytestream;
69     int i_state;
70     size_t i_offset;
71     uint8_t p_startcode[3];
72
73     /*
74      * Common properties
75      */
76     mtime_t i_interpolated_pts;
77     mtime_t i_interpolated_dts;
78     mtime_t i_last_ref_pts;
79     mtime_t i_last_time_ref;
80     mtime_t i_time_ref;
81     mtime_t i_last_time;
82     mtime_t i_last_timeincr;
83
84     unsigned int i_flags;
85
86     int         i_fps_num;
87     int         i_fps_den;
88     int         i_last_incr;
89     int         i_last_incr_diff;
90
91     bool  b_frame;
92
93     /* Current frame being built */
94     block_t    *p_frame;
95     block_t    **pp_last;
96 };
97
98 enum {
99     STATE_NOSYNC,
100     STATE_NEXT_SYNC
101 };
102
103 static block_t *ParseMPEGBlock( decoder_t *, block_t * );
104 static int ParseVOL( decoder_t *, es_format_t *, uint8_t *, int );
105 static int ParseVOP( decoder_t *, block_t * );
106 static int vlc_log2( unsigned int );
107
108 #define VIDEO_OBJECT_MASK                       0x01f
109 #define VIDEO_OBJECT_LAYER_MASK                 0x00f
110
111 #define VIDEO_OBJECT_START_CODE                 0x100
112 #define VIDEO_OBJECT_LAYER_START_CODE           0x120
113 #define VISUAL_OBJECT_SEQUENCE_START_CODE       0x1b0
114 #define VISUAL_OBJECT_SEQUENCE_END_CODE         0x1b1
115 #define USER_DATA_START_CODE                    0x1b2
116 #define GROUP_OF_VOP_START_CODE                 0x1b3
117 #define VIDEO_SESSION_ERROR_CODE                0x1b4
118 #define VISUAL_OBJECT_START_CODE                0x1b5
119 #define VOP_START_CODE                          0x1b6
120 #define FACE_OBJECT_START_CODE                  0x1ba
121 #define FACE_OBJECT_PLANE_START_CODE            0x1bb
122 #define MESH_OBJECT_START_CODE                  0x1bc
123 #define MESH_OBJECT_PLANE_START_CODE            0x1bd
124 #define STILL_TEXTURE_OBJECT_START_CODE         0x1be
125 #define TEXTURE_SPATIAL_LAYER_START_CODE        0x1bf
126 #define TEXTURE_SNR_LAYER_START_CODE            0x1c0
127
128 /*****************************************************************************
129  * Open: probe the packetizer and return score
130  *****************************************************************************/
131 static int Open( vlc_object_t *p_this )
132 {
133     decoder_t     *p_dec = (decoder_t*)p_this;
134     decoder_sys_t *p_sys;
135
136     switch( p_dec->fmt_in.i_codec )
137     {
138         case VLC_FOURCC( 'm', '4', 's', '2'):
139         case VLC_FOURCC( 'M', '4', 'S', '2'):
140         case VLC_FOURCC( 'm', 'p', '4', 's'):
141         case VLC_FOURCC( 'M', 'P', '4', 'S'):
142         case VLC_FOURCC( 'm', 'p', '4', 'v'):
143         case VLC_FOURCC( 'M', 'P', '4', 'V'):
144         case VLC_FOURCC( 'D', 'I', 'V', 'X'):
145         case VLC_FOURCC( 'd', 'i', 'v', 'x'):
146         case VLC_FOURCC( 'X', 'V', 'I', 'D'):
147         case VLC_FOURCC( 'X', 'v', 'i', 'D'):
148         case VLC_FOURCC( 'x', 'v', 'i', 'd'):
149         case VLC_FOURCC( 'D', 'X', '5', '0'):
150         case VLC_FOURCC( 'd', 'x', '5', '0'):
151         case VLC_FOURCC( 0x04, 0,   0,   0):
152         case VLC_FOURCC( '3', 'I', 'V', '2'):
153         case VLC_FOURCC( 'm', '4', 'c', 'c'):
154         case VLC_FOURCC( 'M', '4', 'C', 'C'):
155             break;
156
157         default:
158             return VLC_EGENERIC;
159     }
160
161     /* Allocate the memory needed to store the decoder's structure */
162     if( ( p_dec->p_sys = p_sys = malloc( sizeof(decoder_sys_t) ) ) == NULL )
163         return VLC_ENOMEM;
164     memset( p_sys, 0, sizeof(decoder_sys_t) );
165
166     /* Misc init */
167     p_sys->i_state = STATE_NOSYNC;
168     p_sys->bytestream = block_BytestreamInit();
169     p_sys->p_startcode[0] = 0;
170     p_sys->p_startcode[1] = 0;
171     p_sys->p_startcode[2] = 1;
172     p_sys->i_offset = 0;
173     p_sys->p_frame = NULL;
174     p_sys->pp_last = &p_sys->p_frame;
175
176     /* Setup properties */
177     es_format_Copy( &p_dec->fmt_out, &p_dec->fmt_in );
178     p_dec->fmt_out.i_codec = VLC_FOURCC( 'm', 'p', '4', 'v' );
179
180     if( p_dec->fmt_in.i_extra )
181     {
182         /* We have a vol */
183         p_dec->fmt_out.i_extra = p_dec->fmt_in.i_extra;
184         p_dec->fmt_out.p_extra = malloc( p_dec->fmt_in.i_extra );
185         memcpy( p_dec->fmt_out.p_extra, p_dec->fmt_in.p_extra,
186                 p_dec->fmt_in.i_extra );
187
188         msg_Dbg( p_dec, "opening with vol size: %d", p_dec->fmt_in.i_extra );
189         ParseVOL( p_dec, &p_dec->fmt_out,
190                   p_dec->fmt_out.p_extra, p_dec->fmt_out.i_extra );
191     }
192     else
193     {
194         /* No vol, we'll have to look for one later on */
195         p_dec->fmt_out.i_extra = 0;
196         p_dec->fmt_out.p_extra = 0;
197     }
198
199     /* Set callback */
200     p_dec->pf_packetize = Packetize;
201
202     return VLC_SUCCESS;
203 }
204
205 /*****************************************************************************
206  * Close: clean up the packetizer
207  *****************************************************************************/
208 static void Close( vlc_object_t *p_this )
209 {
210     decoder_t *p_dec = (decoder_t*)p_this;
211
212     block_BytestreamRelease( &p_dec->p_sys->bytestream );
213     if( p_dec->p_sys->p_frame ) block_ChainRelease( p_dec->p_sys->p_frame );
214     free( p_dec->p_sys );
215 }
216
217 /****************************************************************************
218  * Packetize: the whole thing
219  ****************************************************************************/
220 static block_t *Packetize( decoder_t *p_dec, block_t **pp_block )
221 {
222     decoder_sys_t *p_sys = p_dec->p_sys;
223     block_t       *p_pic;
224     mtime_t       i_pts, i_dts;
225
226     if( pp_block == NULL || *pp_block == NULL ) return NULL;
227
228     if( (*pp_block)->i_flags&(BLOCK_FLAG_DISCONTINUITY|BLOCK_FLAG_CORRUPTED) )
229     {
230         if( (*pp_block)->i_flags&BLOCK_FLAG_CORRUPTED )
231         {
232             p_sys->i_state = STATE_NOSYNC;
233             block_BytestreamFlush( &p_sys->bytestream );
234
235             if( p_sys->p_frame )
236                 block_ChainRelease( p_sys->p_frame );
237             p_sys->p_frame = NULL;
238             p_sys->pp_last = &p_sys->p_frame;
239         }
240 //        p_sys->i_interpolated_pts =
241 //        p_sys->i_interpolated_dts =
242 //        p_sys->i_last_ref_pts =
243 //        p_sys->i_last_time_ref =
244 //        p_sys->i_time_ref =
245 //        p_sys->i_last_time =
246 //        p_sys->i_last_timeincr = 0;
247
248         block_Release( *pp_block );
249         return NULL;
250     }
251
252     block_BytestreamPush( &p_sys->bytestream, *pp_block );
253
254     while( 1 )
255     {
256         switch( p_sys->i_state )
257         {
258
259         case STATE_NOSYNC:
260             if( block_FindStartcodeFromOffset( &p_sys->bytestream,
261                     &p_sys->i_offset, p_sys->p_startcode, 3 ) == VLC_SUCCESS )
262             {
263                 p_sys->i_state = STATE_NEXT_SYNC;
264             }
265
266             if( p_sys->i_offset )
267             {
268                 block_SkipBytes( &p_sys->bytestream, p_sys->i_offset );
269                 p_sys->i_offset = 0;
270                 block_BytestreamFlush( &p_sys->bytestream );
271             }
272
273             if( p_sys->i_state != STATE_NEXT_SYNC )
274             {
275                 /* Need more data */
276                 return NULL;
277             }
278
279             p_sys->i_offset = 1; /* To find next startcode */
280
281         case STATE_NEXT_SYNC:
282             /* TODO: If p_block == NULL, flush the buffer without checking the
283              * next sync word */
284
285             /* Find the next startcode */
286             if( block_FindStartcodeFromOffset( &p_sys->bytestream,
287                     &p_sys->i_offset, p_sys->p_startcode, 3 ) != VLC_SUCCESS )
288             {
289                 /* Need more data */
290                 return NULL;
291             }
292
293             /* Get the new fragment and set the pts/dts */
294             p_pic = block_New( p_dec, p_sys->i_offset );
295             block_BytestreamFlush( &p_sys->bytestream );
296             p_pic->i_pts = i_pts = p_sys->bytestream.p_block->i_pts;
297             p_pic->i_dts = i_dts = p_sys->bytestream.p_block->i_dts;
298             p_pic->i_rate = p_sys->bytestream.p_block->i_rate;
299
300             block_GetBytes( &p_sys->bytestream, p_pic->p_buffer,
301                             p_pic->i_buffer );
302
303             p_sys->i_offset = 0;
304
305             /* Get picture if any */
306             if( !( p_pic = ParseMPEGBlock( p_dec, p_pic ) ) )
307             {
308                 p_sys->i_state = STATE_NOSYNC;
309                 break;
310             }
311
312             /* don't reuse the same timestamps several times */
313             if( i_pts == p_sys->bytestream.p_block->i_pts &&
314                 i_dts == p_sys->bytestream.p_block->i_dts )
315             {
316                 p_sys->bytestream.p_block->i_pts = 0;
317                 p_sys->bytestream.p_block->i_dts = 0;
318             }
319
320             /* We've just started the stream, wait for the first PTS.
321              * We discard here so we can still get the sequence header. */
322             if( p_sys->i_interpolated_pts <= 0 &&
323                 p_sys->i_interpolated_dts <= 0 )
324             {
325                 msg_Dbg( p_dec, "need a starting pts/dts" );
326                 p_sys->i_state = STATE_NOSYNC;
327                 block_Release( p_pic );
328                 break;
329             }
330
331             /* When starting the stream we can have the first frame with
332              * a null DTS (i_interpolated_pts is initialized to 0) */
333             if( !p_pic->i_dts ) p_pic->i_dts = p_pic->i_pts;
334
335             /* So p_block doesn't get re-added several times */
336             *pp_block = block_BytestreamPop( &p_sys->bytestream );
337
338             p_sys->i_state = STATE_NOSYNC;
339
340             return p_pic;
341         }
342     }
343 }
344
345 /*****************************************************************************
346  * ParseMPEGBlock: Re-assemble fragments into a block containing a picture
347  *****************************************************************************/
348 static block_t *ParseMPEGBlock( decoder_t *p_dec, block_t *p_frag )
349 {
350     decoder_sys_t *p_sys = p_dec->p_sys;
351     block_t *p_pic = NULL;
352
353     if( p_frag->p_buffer[3] == 0xB0 || p_frag->p_buffer[3] == 0xB1 || p_frag->p_buffer[3] == 0xB2 )
354     {   /* VOS and USERDATA */
355 #if 0
356         /* Remove VOS start/end code from the original stream */
357         block_Release( p_frag );
358 #else
359         /* Append the block for now since ts/ps muxers rely on VOL
360          * being present in the stream */
361         block_ChainLastAppend( &p_sys->pp_last, p_frag );
362 #endif
363         return NULL;
364     }
365     if( p_frag->p_buffer[3] >= 0x20 && p_frag->p_buffer[3] <= 0x2f )
366     {
367         /* Copy the complete VOL */
368         if( (size_t)p_dec->fmt_out.i_extra != p_frag->i_buffer )
369         {
370             p_dec->fmt_out.p_extra =
371                 realloc( p_dec->fmt_out.p_extra, p_frag->i_buffer );
372             p_dec->fmt_out.i_extra = p_frag->i_buffer;
373         }
374         memcpy( p_dec->fmt_out.p_extra, p_frag->p_buffer, p_frag->i_buffer );
375         ParseVOL( p_dec, &p_dec->fmt_out,
376                   p_dec->fmt_out.p_extra, p_dec->fmt_out.i_extra );
377
378 #if 0
379         /* Remove from the original stream */
380         block_Release( p_frag );
381 #else
382         /* Append the block for now since ts/ps muxers rely on VOL
383          * being present in the stream */
384         block_ChainLastAppend( &p_sys->pp_last, p_frag );
385 #endif
386         return NULL;
387     }
388     else
389     {
390         if( !p_dec->fmt_out.i_extra )
391         {
392             msg_Warn( p_dec, "waiting for VOL" );
393             block_Release( p_frag );
394             return NULL;
395         }
396
397         /* Append the block */
398         block_ChainLastAppend( &p_sys->pp_last, p_frag );
399     }
400
401     if( p_frag->p_buffer[3] == 0xb6 &&
402         ParseVOP( p_dec, p_frag ) == VLC_SUCCESS )
403     {
404         /* We are dealing with a VOP */
405         p_pic = block_ChainGather( p_sys->p_frame );
406         p_pic->i_flags = p_sys->i_flags;
407         p_pic->i_pts = p_sys->i_interpolated_pts;
408         p_pic->i_dts = p_sys->i_interpolated_dts;
409
410         /* Reset context */
411         p_sys->p_frame = NULL;
412         p_sys->pp_last = &p_sys->p_frame;
413     }
414
415     return p_pic;
416 }
417
418 /* ParseVOL:
419  *  TODO:
420  *      - support aspect ratio
421  */
422 static int ParseVOL( decoder_t *p_dec, es_format_t *fmt,
423                      uint8_t *p_vol, int i_vol )
424 {
425     decoder_sys_t *p_sys = p_dec->p_sys;
426     int i_vo_type, i_vo_ver_id, i_ar, i_shape;
427     bs_t s;
428
429     for( ;; )
430     {
431         if( p_vol[0] == 0x00 && p_vol[1] == 0x00 && p_vol[2] == 0x01 &&
432             p_vol[3] >= 0x20 && p_vol[3] <= 0x2f ) break;
433
434         p_vol++; i_vol--;
435         if( i_vol <= 4 ) return VLC_EGENERIC;
436     }
437
438     bs_init( &s, &p_vol[4], i_vol - 4 );
439
440     bs_skip( &s, 1 );   /* random access */
441     i_vo_type = bs_read( &s, 8 );
442     if( bs_read1( &s ) )
443     {
444         i_vo_ver_id = bs_read( &s, 4 );
445         bs_skip( &s, 3 );
446     }
447     else
448     {
449         i_vo_ver_id = 1;
450     }
451     i_ar = bs_read( &s, 4 );
452     if( i_ar == 0xf )
453     {
454         int i_ar_width, i_ar_height;
455
456         i_ar_width = bs_read( &s, 8 );
457         i_ar_height= bs_read( &s, 8 );
458     }
459     if( bs_read1( &s ) )
460     {
461         int i_chroma_format;
462         int i_low_delay;
463
464         /* vol control parameter */
465         i_chroma_format = bs_read( &s, 2 );
466         i_low_delay = bs_read1( &s );
467
468         if( bs_read1( &s ) )
469         {
470             bs_skip( &s, 16 );
471             bs_skip( &s, 16 );
472             bs_skip( &s, 16 );
473             bs_skip( &s, 3 );
474             bs_skip( &s, 11 );
475             bs_skip( &s, 1 );
476             bs_skip( &s, 16 );
477         }
478     }
479     /* shape 0->RECT, 1->BIN, 2->BIN_ONLY, 3->GRAY */
480     i_shape = bs_read( &s, 2 );
481     if( i_shape == 3 && i_vo_ver_id != 1 )
482     {
483         bs_skip( &s, 4 );
484     }
485
486     if( !bs_read1( &s ) ) return VLC_EGENERIC; /* Marker */
487
488     p_sys->i_fps_num = bs_read( &s, 16 ); /* Time increment resolution*/
489     if( !p_sys->i_fps_num ) p_sys->i_fps_num = 1;
490
491     if( !bs_read1( &s ) ) return VLC_EGENERIC; /* Marker */
492
493     if( bs_read1( &s ) )
494     {
495         int i_time_increment_bits = vlc_log2( p_sys->i_fps_num - 1 ) + 1;
496
497         if( i_time_increment_bits < 1 ) i_time_increment_bits = 1;
498
499         p_sys->i_fps_den = bs_read( &s, i_time_increment_bits );
500     }
501     if( i_shape == 0 )
502     {
503         bs_skip( &s, 1 );
504         fmt->video.i_width = bs_read( &s, 13 );
505         bs_skip( &s, 1 );
506         fmt->video.i_height= bs_read( &s, 13 );
507         bs_skip( &s, 1 );
508     }
509
510     return VLC_SUCCESS;
511 }
512
513 static int ParseVOP( decoder_t *p_dec, block_t *p_vop )
514 {
515     decoder_sys_t *p_sys = p_dec->p_sys;
516     int64_t i_time_increment, i_time_ref;
517     int i_modulo_time_base = 0, i_time_increment_bits;
518     bs_t s;
519
520     bs_init( &s, &p_vop->p_buffer[4], p_vop->i_buffer - 4 );
521
522     switch( bs_read( &s, 2 ) )
523     {
524     case 0:
525         p_sys->i_flags = BLOCK_FLAG_TYPE_I;
526         break;
527     case 1:
528         p_sys->i_flags = BLOCK_FLAG_TYPE_P;
529         break;
530     case 2:
531         p_sys->i_flags = BLOCK_FLAG_TYPE_B;
532         p_sys->b_frame = true;
533         break;
534     case 3: /* gni ? */
535         p_sys->i_flags = BLOCK_FLAG_TYPE_PB;
536         break;
537     }
538
539     while( bs_read( &s, 1 ) ) i_modulo_time_base++;
540     if( !bs_read1( &s ) ) return VLC_EGENERIC; /* Marker */
541
542     /* VOP time increment */
543     i_time_increment_bits = vlc_log2(p_dec->p_sys->i_fps_num - 1) + 1;
544     if( i_time_increment_bits < 1 ) i_time_increment_bits = 1;
545     i_time_increment = bs_read( &s, i_time_increment_bits );
546
547     /* Interpolate PTS/DTS */
548     if( !(p_sys->i_flags & BLOCK_FLAG_TYPE_B) )
549     {
550         p_sys->i_last_time_ref = p_sys->i_time_ref;
551         p_sys->i_time_ref +=
552             (i_modulo_time_base * p_dec->p_sys->i_fps_num);
553         i_time_ref = p_sys->i_time_ref;
554     }
555     else
556     {
557         i_time_ref = p_sys->i_last_time_ref +
558             (i_modulo_time_base * p_dec->p_sys->i_fps_num);
559     }
560
561 #if 0
562     msg_Err( p_dec, "interp pts/dts (%lli,%lli), pts/dts (%lli,%lli)",
563              p_sys->i_interpolated_pts, p_sys->i_interpolated_dts,
564              p_vop->i_pts, p_vop->i_dts );
565 #endif
566
567     if( p_dec->p_sys->i_fps_num < 5 && /* Work-around buggy streams */
568         p_dec->fmt_in.video.i_frame_rate > 0 &&
569         p_dec->fmt_in.video.i_frame_rate_base > 0 )
570     {
571         p_sys->i_interpolated_pts += INT64_C(1000000) *
572         p_dec->fmt_in.video.i_frame_rate_base *
573         p_vop->i_rate / INPUT_RATE_DEFAULT /
574         p_dec->fmt_in.video.i_frame_rate;
575     }
576     else if( p_dec->p_sys->i_fps_num )
577         p_sys->i_interpolated_pts +=
578             ( INT64_C(1000000) * (i_time_ref + i_time_increment -
579               p_sys->i_last_time - p_sys->i_last_timeincr) *
580               p_vop->i_rate / INPUT_RATE_DEFAULT /
581               p_dec->p_sys->i_fps_num );
582
583     p_sys->i_last_time = i_time_ref;
584     p_sys->i_last_timeincr = i_time_increment;
585
586     /* Correct interpolated dts when we receive a new pts/dts */
587     if( p_vop->i_pts > 0 )
588         p_sys->i_interpolated_pts = p_vop->i_pts;
589     if( p_vop->i_dts > 0 )
590         p_sys->i_interpolated_dts = p_vop->i_dts;
591
592     if( (p_sys->i_flags & BLOCK_FLAG_TYPE_B) || !p_sys->b_frame )
593     {
594         /* Trivial case (DTS == PTS) */
595
596         p_sys->i_interpolated_dts = p_sys->i_interpolated_pts;
597
598         if( p_vop->i_pts > 0 )
599             p_sys->i_interpolated_dts = p_vop->i_pts;
600         if( p_vop->i_dts > 0 )
601             p_sys->i_interpolated_dts = p_vop->i_dts;
602
603         p_sys->i_interpolated_pts = p_sys->i_interpolated_dts;
604     }
605     else
606     {
607         if( p_sys->i_last_ref_pts > 0 )
608             p_sys->i_interpolated_dts = p_sys->i_last_ref_pts;
609
610         p_sys->i_last_ref_pts = p_sys->i_interpolated_pts;
611     }
612
613     return VLC_SUCCESS;
614 }
615
616 /* look at ffmpeg av_log2 ;) */
617 static int vlc_log2( unsigned int v )
618 {
619     int n = 0;
620     static const int vlc_log2_table[16] =
621     {
622         0,0,1,1,2,2,2,2, 3,3,3,3,3,3,3,3
623     };
624
625     if( v&0xffff0000 )
626     {
627         v >>= 16;
628         n += 16;
629     }
630     if( v&0xff00 )
631     {
632         v >>= 8;
633         n += 8;
634     }
635     if( v&0xf0 )
636     {
637         v >>= 4;
638         n += 4;
639     }
640     n += vlc_log2_table[v];
641
642     return n;
643 }