Statistics
| Branch: | Revision:

ffmpeg / doc / snow.txt @ 8787d837

History | View | Annotate | Download (18.9 KB)

1
=============================================
2
SNOW Video Codec Specification Draft 20070103
3
=============================================
4

    
5
Intro:
6
======
7
This Specification describes the snow syntax and semmantics as well as
8
how to decode snow.
9
The decoding process is precissely described and any compliant decoder
10
MUST produce the exactly same output for a spec conformant snow stream.
11
For encoding though any process which generates a stream compliant to
12
the syntactical and semmantical requirements and which is decodeable by
13
the process described in this spec shall be considered a conformant
14
snow encoder.
15

    
16
Definitions:
17
============
18

    
19
MUST    the specific part must be done to conform to this standard
20
SHOULD  it is recommended to be done that way, but not strictly required
21

    
22
ilog2(x) is the rounded down logarithm of x with basis 2
23
ilog2(0) = 0
24

    
25
Type definitions:
26
=================
27

    
28
b   1-bit range coded
29
u   unsigned scalar value range coded
30
s   signed scalar value range coded
31

    
32

    
33
Bitstream syntax:
34
=================
35

    
36
frame:
37
    header
38
    prediction
39
    residual
40

    
41
header:
42
    keyframe                            b   MID_STATE
43
    if(keyframe || always_reset)
44
        reset_contexts
45
    if(keyframe){
46
        version                         u   header_state
47
        always_reset                    b   header_state
48
        temporal_decomposition_type     u   header_state
49
        temporal_decomposition_count    u   header_state
50
        spatial_decomposition_count     u   header_state
51
        colorspace_type                 u   header_state
52
        chroma_h_shift                  u   header_state
53
        chroma_v_shift                  u   header_state
54
        spatial_scalability             b   header_state
55
        max_ref_frames-1                u   header_state
56
        qlogs
57
    }
58
    if(!keyframe){
59
        update_mc                       b   header_state
60
        if(update_mc){
61
            for(plane=0; plane<2; plane++){
62
                diag_mc                 b   header_state
63
                htaps/2-1               u   header_state
64
                for(i= p->htaps/2; i; i--)
65
                    |hcoeff[i]|         u   header_state
66
            }
67
        }
68
        update_qlogs                    b   header_state
69
        if(update_qlogs){
70
            spatial_decomposition_count u   header_state
71
            qlogs
72
        }
73
    }
74

    
75
    spatial_decomposition_type          s   header_state
76
    qlog                                s   header_state
77
    mv_scale                            s   header_state
78
    qbias                               s   header_state
79
    block_max_depth                     s   header_state
80

    
81
qlogs:
82
    for(plane=0; plane<2; plane++){
83
        quant_table[plane][0][0]        s   header_state
84
        for(level=0; level < spatial_decomposition_count; level++){
85
            quant_table[plane][level][1]s   header_state
86
            quant_table[plane][level][3]s   header_state
87
        }
88
    }
89

    
90
reset_contexts
91
    *_state[*]= MID_STATE
92

    
93
prediction:
94
    for(y=0; y<block_count_vertical; y++)
95
        for(x=0; x<block_count_horizontal; x++)
96
            block(0)
97

    
98
block(level):
99
    mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0
100
    if(keyframe){
101
        intra=1
102
    }else{
103
        if(level!=max_block_depth){
104
            s_context= 2*left->level + 2*top->level + topleft->level + topright->level
105
            leaf                        b   block_state[4 + s_context]
106
        }
107
        if(level==max_block_depth || leaf){
108
            intra                       b   block_state[1 + left->intra + top->intra]
109
            if(intra){
110
                y_diff                  s   block_state[32]
111
                cb_diff                 s   block_state[64]
112
                cr_diff                 s   block_state[96]
113
            }else{
114
                ref_context= ilog2(2*left->ref) + ilog2(2*top->ref)
115
                if(ref_frames > 1)
116
                    ref                 u   block_state[128 + 1024 + 32*ref_context]
117
                mx_context= ilog2(2*abs(left->mx - top->mx))
118
                my_context= ilog2(2*abs(left->my - top->my))
119
                mvx_diff                s   block_state[128 + 32*(mx_context + 16*!!ref)]
120
                mvy_diff                s   block_state[128 + 32*(my_context + 16*!!ref)]
121
            }
122
        }else{
123
            block(level+1)
124
            block(level+1)
125
            block(level+1)
126
            block(level+1)
127
        }
128
    }
129

    
130

    
131
residual:
132
    residual2(luma)
133
    residual2(chroma_cr)
134
    residual2(chroma_cb)
135

    
136
residual2:
137
    for(level=0; level<spatial_decomposition_count; level++){
138
        if(level==0)
139
            subband(LL, 0)
140
        subband(HL, level)
141
        subband(LH, level)
142
        subband(HH, level)
143
    }
144

    
145
subband:
146
    FIXME
147

    
148

    
149

    
150
Tag description:
151
----------------
152

    
153
version
154
    0
155
    this MUST NOT change within a bitstream
156

    
157
always_reset
158
    if 1 then the range coder contexts will be reset after each frame
159

    
160
temporal_decomposition_type
161
    0
162

    
163
temporal_decomposition_count
164
    0
165

    
166
spatial_decomposition_count
167
    FIXME
168

    
169
colorspace_type
170
    0
171
    this MUST NOT change within a bitstream
172

    
173
chroma_h_shift
174
    log2(luma.width / chroma.width)
175
    this MUST NOT change within a bitstream
176

    
177
chroma_v_shift
178
    log2(luma.height / chroma.height)
179
    this MUST NOT change within a bitstream
180

    
181
spatial_scalability
182
    0
183

    
184
max_ref_frames
185
    maximum number of reference frames
186
    this MUST NOT change within a bitstream
187

    
188
update_mc
189
    indicates that motion compensation filter parameters are stored in the
190
    header
191

    
192
diag_mc
193
    flag to enable faster diagonal interpolation
194
    this SHOULD be 1 unless it turns out to be covered by a valid patent
195

    
196
htaps
197
    number of half pel interpolation filter taps, MUST be even, >0 and <10
198

    
199
hcoeff
200
    half pel interpolation filter coefficients, hcoeff[0] are the 2 middle
201
    coefficients [1] are the next outer ones and so on, resulting in a filter
202
    like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ...
203
    the sign of the coefficients is not explicitly stored but alternates
204
    after each coeff and coeff[0] is positive, so ...,+,-,+,-,+,+,-,+,-,+,...
205
    hcoeff[0] is not explicitly stored but found by subtracting the sum
206
    of all stored coefficients with signs from 32
207
    hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ...
208
    a good choice for hcoeff and htaps is
209
    htaps= 6
210
    hcoeff={40,-10,2}
211
    an alternative which requires more computations at both encoder and
212
    decoder side and may or may not be better is
213
    htaps= 8
214
    hcoeff={42,-14,6,-2}
215

    
216

    
217
ref_frames
218
    minimum of the number of available reference frames and max_ref_frames
219
    for example the first frame after a key frame always has ref_frames=1
220

    
221
spatial_decomposition_type
222
    wavelet type
223
    0 is a 9/7 symmetric compact integer wavelet
224
    1 is a 5/3 symmetric compact integer wavelet
225
    others are reserved
226
    stored as delta from last, last is reset to 0 if always_reset || keyframe
227

    
228
qlog
229
    quality (logarthmic quantizer scale)
230
    stored as delta from last, last is reset to 0 if always_reset || keyframe
231

    
232
mv_scale
233
    stored as delta from last, last is reset to 0 if always_reset || keyframe
234
    FIXME check that everything works fine if this changes between frames
235

    
236
qbias
237
    dequantization bias
238
    stored as delta from last, last is reset to 0 if always_reset || keyframe
239

    
240
block_max_depth
241
    maximum depth of the block tree
242
    stored as delta from last, last is reset to 0 if always_reset || keyframe
243

    
244
quant_table
245
    quantiztation table
246

    
247

    
248
Highlevel bitstream structure:
249
=============================
250
 --------------------------------------------
251
|                   Header                   |
252
 --------------------------------------------
253
|    ------------------------------------    |
254
|   |               Block0               |   |
255
|   |             split?                 |   |
256
|   |     yes              no            |   |
257
|   |  .........         intra?          |   |
258
|   | : Block01 :    yes         no      |   |
259
|   | : Block02 :  .......   ..........  |   |
260
|   | : Block03 : :  y DC : : ref index: |   |
261
|   | : Block04 : : cb DC : : motion x : |   |
262
|   |  .........  : cr DC : : motion y : |   |
263
|   |              .......   ..........  |   |
264
|    ------------------------------------    |
265
|    ------------------------------------    |
266
|   |               Block1               |   |
267
|                    ...                     |
268
 --------------------------------------------
269
| ------------   ------------   ------------ |
270
|| Y subbands | | Cb subbands| | Cr subbands||
271
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
272
|| |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| ||
273
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
274
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
275
|| |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| ||
276
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
277
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
278
|| |HL1||LH1| | | |HL1||LH1| | | |HL1||LH1| ||
279
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
280
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
281
|| |HH1||HL2| | | |HH1||HL2| | | |HH1||HL2| ||
282
||    ...     | |    ...     | |    ...     ||
283
| ------------   ------------   ------------ |
284
 --------------------------------------------
285

    
286
Decoding process:
287
=================
288

    
289
                                         ------------
290
                                        |            |
291
                                        |  Subbands  |
292
                   ------------         |            |
293
                  |            |         ------------
294
                  |  Intra DC  |               |
295
                  |            |    LL0 subband prediction
296
                   ------------                |
297
                                \        Dequantizaton
298
 -------------------             \             |
299
|  Reference frames |             \           IDWT
300
| -------   ------- |    Motion    \           |
301
||Frame 0| |Frame 1|| Compensation  .   OBMC   v      -------
302
| -------   ------- | --------------. \------> + --->|Frame n|-->output
303
| -------   ------- |                                 -------
304
||Frame 2| |Frame 3||<----------------------------------/
305
|        ...        |
306
 -------------------
307

    
308

    
309
Range Coder:
310
============
311
FIXME
312

    
313
Neighboring Blocks:
314
===================
315
left and top are set to the respective blocks unless they are outside of
316
the image in which case they are set to the Null block
317

    
318
top-left is set to the top left block unless it is outside of the image in
319
which case it is set to the left block
320

    
321
if this block has no larger parent block or it is at the left side of its
322
parent block and the top right block is not outside of the image then the
323
top right block is used for top-right else the top-left block is used
324

    
325
Null block
326
y,cb,cr are 128
327
level, ref, mx and my are 0
328

    
329

    
330
Motion Vector Prediction:
331
=========================
332
1. the motion vectors of all the neighboring blocks are scaled to
333
compensate for the difference of reference frames
334

    
335
scaled_mv= (mv * (256 * (current_reference+1) / (mv.reference+1)) + 128)>>8
336

    
337
2. the median of the scaled left, top and top-right vectors is used as
338
motion vector prediction
339

    
340
3. the used motion vector is the sum of the predictor and
341
   (mvx_diff, mvy_diff)*mv_scale
342

    
343

    
344
Intra DC Predicton:
345
======================
346
the luma and chroma values of the left block are used as predictors
347

    
348
the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff
349
to reverse this in the decoder apply the following:
350
block[y][x].dc[0] = block[y][x-1].dc[0] +  y_diff;
351
block[y][x].dc[1] = block[y][x-1].dc[1] + cb_diff;
352
block[y][x].dc[2] = block[y][x-1].dc[2] + cr_diff;
353
block[*][-1].dc[*]= 128;
354

    
355

    
356
Motion Compensation:
357
====================
358

    
359
Halfpel interpolation:
360
----------------------
361
halfpel interpolation is done by convolution with the halfpel filter stored
362
in the header:
363

    
364
horizontal halfpel samples are found by
365
H1[y][x] =    hcoeff[0]*(F[y][x  ] + F[y][x+1])
366
            + hcoeff[1]*(F[y][x-1] + F[y][x+2])
367
            + hcoeff[2]*(F[y][x-2] + F[y][x+3])
368
            + ...
369
h1[y][x] = (H1[y][x] + 32)>>6;
370

    
371
vertical halfpel samples are found by
372
H2[y][x] =    hcoeff[0]*(F[y  ][x] + F[y+1][x])
373
            + hcoeff[1]*(F[y-1][x] + F[y+2][x])
374
            + ...
375
h2[y][x] = (H2[y][x] + 32)>>6;
376

    
377
vertical+horizontal halfpel samples are found by
378
H3[y][x] =    hcoeff[0]*(H2[y][x  ] + H2[y][x+1])
379
            + hcoeff[1]*(H2[y][x-1] + H2[y][x+2])
380
            + ...
381
H3[y][x] =    hcoeff[0]*(H1[y  ][x] + H1[y+1][x])
382
            + hcoeff[1]*(H1[y+1][x] + H1[y+2][x])
383
            + ...
384
h3[y][x] = (H3[y][x] + 2048)>>12;
385

    
386

    
387
                   F   H1  F
388
                   |   |   |
389
                   |   |   |
390
                   |   |   |
391
                   F   H1  F
392
                   |   |   |
393
                   |   |   |
394
                   |   |   |
395
   F-------F-------F-> H1<-F-------F-------F
396
                   v   v   v
397
                  H2   H3  H2
398
                   ^   ^   ^
399
   F-------F-------F-> H1<-F-------F-------F
400
                   |   |   |
401
                   |   |   |
402
                   |   |   |
403
                   F   H1  F
404
                   |   |   |
405
                   |   |   |
406
                   |   |   |
407
                   F   H1  F
408

    
409

    
410
unavailable fullpel samples (outside the picture for example) shall be equal
411
to the closest available fullpel sample
412

    
413

    
414
Smaller pel interpolation:
415
--------------------------
416
if diag_mc is set then points which lie on a line between 2 vertically,
417
horiziontally or diagonally adjacent halfpel points shall be interpolated
418
linearls with rounding to nearest and halfway values rounded up.
419
points which lie on 2 diagonals at the same time should only use the one
420
diagonal not containing the fullpel point
421

    
422

    
423

    
424
           F-->O---q---O<--h1->O---q---O<--F
425
           v \           / v \           / v
426
           O   O       O   O   O       O   O
427
           |         /     |     \         |
428
           q       q       q       q       q
429
           |     /         |         \     |
430
           O   O       O   O   O       O   O
431
           ^ /           \ ^ /           \ ^
432
          h2-->O---q---O<--h3->O---q---O<--h2
433
           v \           / v \           / v
434
           O   O       O   O   O       O   O
435
           |     \         |         /     |
436
           q       q       q       q       q
437
           |         \     |     /         |
438
           O   O       O   O   O       O   O
439
           ^ /           \ ^ /           \ ^
440
           F-->O---q---O<--h1->O---q---O<--F
441

    
442

    
443

    
444
the remaining points shall be bilinearly interpolated from the
445
up to 4 surrounding halfpel and fullpel points, again rounding should be to
446
nearest and halfway values rounded up
447

    
448
compliant snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma
449
interpolation at least
450

    
451

    
452
Overlapped block motion compensation:
453
-------------------------------------
454
FIXME
455

    
456
LL band prediction:
457
===================
458
Each sample in the LL0 subband is predicted by the median of the left, top and
459
left+top-topleft samples, samples outside the subband shall be considered to
460
be 0. To reverse this prediction in the decoder apply the following.
461
for(y=0; y<height; y++){
462
    for(x=0; x<width; x++){
463
        sample[y][x] += median(sample[y-1][x],
464
                               sample[y][x-1],
465
                               sample[y-1][x]+sample[y][x-1]-sample[y-1][x-1]);
466
    }
467
}
468
sample[-1][*]=sample[*][-1]= 0;
469
width,height here are the width and height of the LL0 subband not of the final
470
video
471

    
472

    
473
Dequantizaton:
474
==============
475
FIXME
476

    
477
Wavelet Transform:
478
==================
479

    
480
Snow supports 2 wavelet transforms, the symmetric biorthogonal 5/3 integer
481
transform and a integer approximation of the symmetric biorthogonal 9/7
482
daubechies wavelet.
483

    
484
2D IDWT (inverse discrete wavelet transform)
485
--------------------------------------------
486
The 2D IDWT applies a 2D filter recursively, each time combining the
487
4 lowest frequency subbands into a single subband until only 1 subband
488
remains.
489
The 2D filter is done by first applying a 1D filter in the vertical direction
490
and then applying it in the horizontal one.
491
 ---------------    ---------------    ---------------    ---------------
492
|LL0|HL0|       |  |   |   |       |  |       |       |  |       |       |
493
|---+---|  HL1  |  | L0|H0 |  HL1  |  |  LL1  |  HL1  |  |       |       |
494
|LH0|HH0|       |  |   |   |       |  |       |       |  |       |       |
495
|-------+-------|->|-------+-------|->|-------+-------|->|   L1  |  H1   |->...
496
|       |       |  |       |       |  |       |       |  |       |       |
497
|  LH1  |  HH1  |  |  LH1  |  HH1  |  |  LH1  |  HH1  |  |       |       |
498
|       |       |  |       |       |  |       |       |  |       |       |
499
 ---------------    ---------------    ---------------    ---------------
500

    
501

    
502
1D Filter:
503
----------
504
1. interleave the samples of the low and high frequency subbands like
505
s={L0, H0, L1, H1, L2, H2, L3, H3, ... }
506
note, this can end with a L or a H, the number of elements shall be w
507
s[-1] shall be considered equivalent to s[1  ]
508
s[w ] shall be considered equivalent to s[w-2]
509

    
510
2. perform the lifting steps in order as described below
511

    
512
5/3 Integer filter:
513
1. s[i] -= (s[i-1] + s[i+1] + 2)>>2; for all even i < w
514
2. s[i] += (s[i-1] + s[i+1]    )>>1; for all odd  i < w
515

    
516
\ | /|\ | /|\ | /|\ | /|\
517
 \|/ | \|/ | \|/ | \|/ |
518
  +  |  +  |  +  |  +  |   -1/4
519
 /|\ | /|\ | /|\ | /|\ |
520
/ | \|/ | \|/ | \|/ | \|/
521
  |  +  |  +  |  +  |  +   +1/2
522

    
523

    
524
snows 9/7 Integer filter:
525
1. s[i] -= (3*(s[i-1] + s[i+1])         + 4)>>3; for all even i < w
526
2. s[i] -=     s[i-1] + s[i+1]                 ; for all odd  i < w
527
3. s[i] += (   s[i-1] + s[i+1] + 4*s[i] + 8)>>4; for all even i < w
528
4. s[i] += (3*(s[i-1] + s[i+1])            )>>1; for all odd  i < w
529

    
530
\ | /|\ | /|\ | /|\ | /|\
531
 \|/ | \|/ | \|/ | \|/ |
532
  +  |  +  |  +  |  +  |   -3/8
533
 /|\ | /|\ | /|\ | /|\ |
534
/ | \|/ | \|/ | \|/ | \|/
535
 (|  + (|  + (|  + (|  +   -1
536
\ + /|\ + /|\ + /|\ + /|\  +1/4
537
 \|/ | \|/ | \|/ | \|/ |
538
  +  |  +  |  +  |  +  |   +1/16
539
 /|\ | /|\ | /|\ | /|\ |
540
/ | \|/ | \|/ | \|/ | \|/
541
  |  +  |  +  |  +  |  +   +3/2
542

    
543
optimization tips:
544
following are exactly identical
545
(3a)>>1 == a + (a>>1)
546
(a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2
547

    
548
16bit implementation note:
549
The IDWT can be implemented with 16bits, but this requires some care to
550
prevent overflows, the following list, lists the minimum number of bits needed
551
for some terms
552
1. lifting step
553
A= s[i-1] + s[i+1]                              16bit
554
3*A + 4                                         18bit
555
A + (A>>1) + 2                                  17bit
556

    
557
3. lifting step
558
s[i-1] + s[i+1]                                 17bit
559

    
560
4. lifiting step
561
3*(s[i-1] + s[i+1])                             17bit
562

    
563

    
564
TODO:
565
=====
566
Important:
567
finetune initial contexts
568
flip wavelet?
569
try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients
570
try the MV length as context for coding the residual coefficients
571
use extradata for stuff which is in the keyframes now?
572
the MV median predictor is patented IIRC
573
implement per picture halfpel interpolation
574
try different range coder state transition tables for different contexts
575

    
576
Not Important:
577
compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality)
578
spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later)
579

    
580

    
581
Credits:
582
========
583
Michael Niedermayer
584
Loren Merritt
585

    
586

    
587
Copyright:
588
==========
589
GPL + GFDL + whatever is needed to make this a RFC