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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
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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
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SHOULD  it is recommended to be done that way, but not strictly required
21

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

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

    
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b   1-bit range coded
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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
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    residual
40

    
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header:
42
    keyframe                            b   MID_STATE
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    if(keyframe || always_reset)
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        reset_contexts
45
    if(keyframe){
46
        version                         u   header_state
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        always_reset                    b   header_state
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        temporal_decomposition_type     u   header_state
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        temporal_decomposition_count    u   header_state
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        spatial_decomposition_count     u   header_state
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        colorspace_type                 u   header_state
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        chroma_h_shift                  u   header_state
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        chroma_v_shift                  u   header_state
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        spatial_scalability             b   header_state
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        max_ref_frames-1                u   header_state
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        qlogs
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    }
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    if(!keyframe){
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        update_mc                       b   header_state
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        if(update_mc){
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            for(plane=0; plane<2; plane++){
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                diag_mc                 b   header_state
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                htaps/2-1               u   header_state
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                for(i= p->htaps/2; i; i--)
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                    |hcoeff[i]|         u   header_state
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            }
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        }
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        update_qlogs                    b   header_state
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        if(update_qlogs){
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            spatial_decomposition_count u   header_state
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            qlogs
72
        }
73
    }
74

    
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    spatial_decomposition_type          s   header_state
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    qlog                                s   header_state
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    mv_scale                            s   header_state
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    qbias                               s   header_state
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    block_max_depth                     s   header_state
80

    
81
qlogs:
82
    for(plane=0; plane<2; plane++){
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        quant_table[plane][0][0]        s   header_state
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        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++)
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        for(x=0; x<block_count_horizontal; x++)
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            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{
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        if(level!=max_block_depth){
104
            s_context= 2*left->level + 2*top->level + topleft->level + topright->level
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            leaf                        b   block_state[4 + s_context]
106
        }
107
        if(level==max_block_depth || leaf){
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            intra                       b   block_state[1 + left->intra + top->intra]
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            if(intra){
110
                y_diff                  s   block_state[32]
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                cb_diff                 s   block_state[64]
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                cr_diff                 s   block_state[96]
113
            }else{
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                ref_context= ilog2(2*left->ref) + ilog2(2*top->ref)
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                if(ref_frames > 1)
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                    ref                 u   block_state[128 + 1024 + 32*ref_context]
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                mx_context= ilog2(2*abs(left->mx - top->mx))
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                my_context= ilog2(2*abs(left->my - top->my))
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                mvx_diff                s   block_state[128 + 32*(mx_context + 16*!!ref)]
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                mvy_diff                s   block_state[128 + 32*(my_context + 16*!!ref)]
121
            }
122
        }else{
123
            block(level+1)
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            block(level+1)
125
            block(level+1)
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            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
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    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 ...,+,-,+,-,+,+,-,+,-,+,...
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    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?                 |   |
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|   |     yes              no            |   |
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|   |  .........         intra?          |   |
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|   | : Block01 :    yes         no      |   |
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|   | : Block02 :  .......   ..........  |   |
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|   | : Block03 : :  y DC : : ref index: |   |
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|   | : Block04 : : cb DC : : motion x : |   |
262
|   |  .........  : cr DC : : motion y : |   |
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|   |              .......   ..........  |   |
264
|    ------------------------------------    |
265
|    ------------------------------------    |
266
|   |               Block1               |   |
267
|                    ...                     |
268
 --------------------------------------------
269
| ------------   ------------   ------------ |
270
|| Y subbands | | Cb subbands| | Cr subbands||
271
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
272
|| |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| ||
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||  ---  ---  | |  ---  ---  | |  ---  ---  ||
274
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
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|| |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| ||
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||  ---  ---  | |  ---  ---  | |  ---  ---  ||
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||  ---  ---  | |  ---  ---  | |  ---  ---  ||
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
The implemented range coder is an adapted version based upon "Range encoding:
313
an algorithm for removing redundancy from a digitised message." by G. N. N.
314
Martin.
315
The symbols encoded by the ffmpeg range coder are bits (0|1). The
316
associated probabilities are not fix but change depending on the symbol mix
317
seen so far.
318

    
319

    
320
bit seen | new state
321
---------+--------------------------------------------
322
    0    | 256 - state_transition_table[256 - old_state];
323
    1    |       state_transition_table[      old_state];
324

    
325
state_transition_table = {
326
  0,   0,   0,   0,   0,   0,   0,   0,  20,  21,  22,  23,  24,  25,  26,  27,
327
 28,  29,  30,  31,  32,  33,  34,  35,  36,  37,  37,  38,  39,  40,  41,  42,
328
 43,  44,  45,  46,  47,  48,  49,  50,  51,  52,  53,  54,  55,  56,  56,  57,
329
 58,  59,  60,  61,  62,  63,  64,  65,  66,  67,  68,  69,  70,  71,  72,  73,
330
 74,  75,  75,  76,  77,  78,  79,  80,  81,  82,  83,  84,  85,  86,  87,  88,
331
 89,  90,  91,  92,  93,  94,  94,  95,  96,  97,  98,  99, 100, 101, 102, 103,
332
104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 114, 115, 116, 117, 118,
333
119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 133,
334
134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149,
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150, 151, 152, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164,
336
165, 166, 167, 168, 169, 170, 171, 171, 172, 173, 174, 175, 176, 177, 178, 179,
337
180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 190, 191, 192, 194, 194,
338
195, 196, 197, 198, 199, 200, 201, 202, 202, 204, 205, 206, 207, 208, 209, 209,
339
210, 211, 212, 213, 215, 215, 216, 217, 218, 219, 220, 220, 222, 223, 224, 225,
340
226, 227, 227, 229, 229, 230, 231, 232, 234, 234, 235, 236, 237, 238, 239, 240,
341
241, 242, 243, 244, 245, 246, 247, 248, 248,   0,   0,   0,   0,   0,   0,   0};
342

    
343

    
344
Neighboring Blocks:
345
===================
346
left and top are set to the respective blocks unless they are outside of
347
the image in which case they are set to the Null block
348

    
349
top-left is set to the top left block unless it is outside of the image in
350
which case it is set to the left block
351

    
352
if this block has no larger parent block or it is at the left side of its
353
parent block and the top right block is not outside of the image then the
354
top right block is used for top-right else the top-left block is used
355

    
356
Null block
357
y,cb,cr are 128
358
level, ref, mx and my are 0
359

    
360

    
361
Motion Vector Prediction:
362
=========================
363
1. the motion vectors of all the neighboring blocks are scaled to
364
compensate for the difference of reference frames
365

    
366
scaled_mv= (mv * (256 * (current_reference+1) / (mv.reference+1)) + 128)>>8
367

    
368
2. the median of the scaled left, top and top-right vectors is used as
369
motion vector prediction
370

    
371
3. the used motion vector is the sum of the predictor and
372
   (mvx_diff, mvy_diff)*mv_scale
373

    
374

    
375
Intra DC Predicton:
376
======================
377
the luma and chroma values of the left block are used as predictors
378

    
379
the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff
380
to reverse this in the decoder apply the following:
381
block[y][x].dc[0] = block[y][x-1].dc[0] +  y_diff;
382
block[y][x].dc[1] = block[y][x-1].dc[1] + cb_diff;
383
block[y][x].dc[2] = block[y][x-1].dc[2] + cr_diff;
384
block[*][-1].dc[*]= 128;
385

    
386

    
387
Motion Compensation:
388
====================
389

    
390
Halfpel interpolation:
391
----------------------
392
halfpel interpolation is done by convolution with the halfpel filter stored
393
in the header:
394

    
395
horizontal halfpel samples are found by
396
H1[y][x] =    hcoeff[0]*(F[y][x  ] + F[y][x+1])
397
            + hcoeff[1]*(F[y][x-1] + F[y][x+2])
398
            + hcoeff[2]*(F[y][x-2] + F[y][x+3])
399
            + ...
400
h1[y][x] = (H1[y][x] + 32)>>6;
401

    
402
vertical halfpel samples are found by
403
H2[y][x] =    hcoeff[0]*(F[y  ][x] + F[y+1][x])
404
            + hcoeff[1]*(F[y-1][x] + F[y+2][x])
405
            + ...
406
h2[y][x] = (H2[y][x] + 32)>>6;
407

    
408
vertical+horizontal halfpel samples are found by
409
H3[y][x] =    hcoeff[0]*(H2[y][x  ] + H2[y][x+1])
410
            + hcoeff[1]*(H2[y][x-1] + H2[y][x+2])
411
            + ...
412
H3[y][x] =    hcoeff[0]*(H1[y  ][x] + H1[y+1][x])
413
            + hcoeff[1]*(H1[y+1][x] + H1[y+2][x])
414
            + ...
415
h3[y][x] = (H3[y][x] + 2048)>>12;
416

    
417

    
418
                   F   H1  F
419
                   |   |   |
420
                   |   |   |
421
                   |   |   |
422
                   F   H1  F
423
                   |   |   |
424
                   |   |   |
425
                   |   |   |
426
   F-------F-------F-> H1<-F-------F-------F
427
                   v   v   v
428
                  H2   H3  H2
429
                   ^   ^   ^
430
   F-------F-------F-> H1<-F-------F-------F
431
                   |   |   |
432
                   |   |   |
433
                   |   |   |
434
                   F   H1  F
435
                   |   |   |
436
                   |   |   |
437
                   |   |   |
438
                   F   H1  F
439

    
440

    
441
unavailable fullpel samples (outside the picture for example) shall be equal
442
to the closest available fullpel sample
443

    
444

    
445
Smaller pel interpolation:
446
--------------------------
447
if diag_mc is set then points which lie on a line between 2 vertically,
448
horiziontally or diagonally adjacent halfpel points shall be interpolated
449
linearls with rounding to nearest and halfway values rounded up.
450
points which lie on 2 diagonals at the same time should only use the one
451
diagonal not containing the fullpel point
452

    
453

    
454

    
455
           F-->O---q---O<--h1->O---q---O<--F
456
           v \           / v \           / v
457
           O   O       O   O   O       O   O
458
           |         /     |     \         |
459
           q       q       q       q       q
460
           |     /         |         \     |
461
           O   O       O   O   O       O   O
462
           ^ /           \ ^ /           \ ^
463
          h2-->O---q---O<--h3->O---q---O<--h2
464
           v \           / v \           / v
465
           O   O       O   O   O       O   O
466
           |     \         |         /     |
467
           q       q       q       q       q
468
           |         \     |     /         |
469
           O   O       O   O   O       O   O
470
           ^ /           \ ^ /           \ ^
471
           F-->O---q---O<--h1->O---q---O<--F
472

    
473

    
474

    
475
the remaining points shall be bilinearly interpolated from the
476
up to 4 surrounding halfpel and fullpel points, again rounding should be to
477
nearest and halfway values rounded up
478

    
479
compliant snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma
480
interpolation at least
481

    
482

    
483
Overlapped block motion compensation:
484
-------------------------------------
485
FIXME
486

    
487
LL band prediction:
488
===================
489
Each sample in the LL0 subband is predicted by the median of the left, top and
490
left+top-topleft samples, samples outside the subband shall be considered to
491
be 0. To reverse this prediction in the decoder apply the following.
492
for(y=0; y<height; y++){
493
    for(x=0; x<width; x++){
494
        sample[y][x] += median(sample[y-1][x],
495
                               sample[y][x-1],
496
                               sample[y-1][x]+sample[y][x-1]-sample[y-1][x-1]);
497
    }
498
}
499
sample[-1][*]=sample[*][-1]= 0;
500
width,height here are the width and height of the LL0 subband not of the final
501
video
502

    
503

    
504
Dequantizaton:
505
==============
506
FIXME
507

    
508
Wavelet Transform:
509
==================
510

    
511
Snow supports 2 wavelet transforms, the symmetric biorthogonal 5/3 integer
512
transform and a integer approximation of the symmetric biorthogonal 9/7
513
daubechies wavelet.
514

    
515
2D IDWT (inverse discrete wavelet transform)
516
--------------------------------------------
517
The 2D IDWT applies a 2D filter recursively, each time combining the
518
4 lowest frequency subbands into a single subband until only 1 subband
519
remains.
520
The 2D filter is done by first applying a 1D filter in the vertical direction
521
and then applying it in the horizontal one.
522
 ---------------    ---------------    ---------------    ---------------
523
|LL0|HL0|       |  |   |   |       |  |       |       |  |       |       |
524
|---+---|  HL1  |  | L0|H0 |  HL1  |  |  LL1  |  HL1  |  |       |       |
525
|LH0|HH0|       |  |   |   |       |  |       |       |  |       |       |
526
|-------+-------|->|-------+-------|->|-------+-------|->|   L1  |  H1   |->...
527
|       |       |  |       |       |  |       |       |  |       |       |
528
|  LH1  |  HH1  |  |  LH1  |  HH1  |  |  LH1  |  HH1  |  |       |       |
529
|       |       |  |       |       |  |       |       |  |       |       |
530
 ---------------    ---------------    ---------------    ---------------
531

    
532

    
533
1D Filter:
534
----------
535
1. interleave the samples of the low and high frequency subbands like
536
s={L0, H0, L1, H1, L2, H2, L3, H3, ... }
537
note, this can end with a L or a H, the number of elements shall be w
538
s[-1] shall be considered equivalent to s[1  ]
539
s[w ] shall be considered equivalent to s[w-2]
540

    
541
2. perform the lifting steps in order as described below
542

    
543
5/3 Integer filter:
544
1. s[i] -= (s[i-1] + s[i+1] + 2)>>2; for all even i < w
545
2. s[i] += (s[i-1] + s[i+1]    )>>1; for all odd  i < w
546

    
547
\ | /|\ | /|\ | /|\ | /|\
548
 \|/ | \|/ | \|/ | \|/ |
549
  +  |  +  |  +  |  +  |   -1/4
550
 /|\ | /|\ | /|\ | /|\ |
551
/ | \|/ | \|/ | \|/ | \|/
552
  |  +  |  +  |  +  |  +   +1/2
553

    
554

    
555
snows 9/7 Integer filter:
556
1. s[i] -= (3*(s[i-1] + s[i+1])         + 4)>>3; for all even i < w
557
2. s[i] -=     s[i-1] + s[i+1]                 ; for all odd  i < w
558
3. s[i] += (   s[i-1] + s[i+1] + 4*s[i] + 8)>>4; for all even i < w
559
4. s[i] += (3*(s[i-1] + s[i+1])            )>>1; for all odd  i < w
560

    
561
\ | /|\ | /|\ | /|\ | /|\
562
 \|/ | \|/ | \|/ | \|/ |
563
  +  |  +  |  +  |  +  |   -3/8
564
 /|\ | /|\ | /|\ | /|\ |
565
/ | \|/ | \|/ | \|/ | \|/
566
 (|  + (|  + (|  + (|  +   -1
567
\ + /|\ + /|\ + /|\ + /|\  +1/4
568
 \|/ | \|/ | \|/ | \|/ |
569
  +  |  +  |  +  |  +  |   +1/16
570
 /|\ | /|\ | /|\ | /|\ |
571
/ | \|/ | \|/ | \|/ | \|/
572
  |  +  |  +  |  +  |  +   +3/2
573

    
574
optimization tips:
575
following are exactly identical
576
(3a)>>1 == a + (a>>1)
577
(a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2
578

    
579
16bit implementation note:
580
The IDWT can be implemented with 16bits, but this requires some care to
581
prevent overflows, the following list, lists the minimum number of bits needed
582
for some terms
583
1. lifting step
584
A= s[i-1] + s[i+1]                              16bit
585
3*A + 4                                         18bit
586
A + (A>>1) + 2                                  17bit
587

    
588
3. lifting step
589
s[i-1] + s[i+1]                                 17bit
590

    
591
4. lifiting step
592
3*(s[i-1] + s[i+1])                             17bit
593

    
594

    
595
TODO:
596
=====
597
Important:
598
finetune initial contexts
599
flip wavelet?
600
try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients
601
try the MV length as context for coding the residual coefficients
602
use extradata for stuff which is in the keyframes now?
603
the MV median predictor is patented IIRC
604
implement per picture halfpel interpolation
605
try different range coder state transition tables for different contexts
606

    
607
Not Important:
608
compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality)
609
spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later)
610

    
611

    
612
Credits:
613
========
614
Michael Niedermayer
615
Loren Merritt
616

    
617

    
618
Copyright:
619
==========
620
GPL + GFDL + whatever is needed to make this a RFC