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1 | ============================================= |
2 | Snow Video Codec Specification Draft 20080110 | |
3 | ============================================= | |
4 | ||
5 | Introduction: | |
6 | ============= | |
7 | This specification describes the Snow bitstream syntax and semantics as | |
8 | well as the formal Snow decoding process. | |
9 | ||
10 | The decoding process is described precisely and any compliant decoder | |
11 | MUST produce the exact same output for a spec-conformant Snow stream. | |
12 | For encoding, though, any process which generates a stream compliant to | |
13 | the syntactical and semantic requirements and which is decodable by | |
14 | the process described in this spec shall be considered a conformant | |
15 | Snow encoder. | |
16 | ||
17 | Definitions: | |
18 | ============ | |
19 | ||
20 | MUST the specific part must be done to conform to this standard | |
21 | SHOULD it is recommended to be done that way, but not strictly required | |
22 | ||
23 | ilog2(x) is the rounded down logarithm of x with basis 2 | |
24 | ilog2(0) = 0 | |
25 | ||
26 | Type definitions: | |
27 | ================= | |
28 | ||
29 | b 1-bit range coded | |
30 | u unsigned scalar value range coded | |
31 | s signed scalar value range coded | |
32 | ||
33 | ||
34 | Bitstream syntax: | |
35 | ================= | |
36 | ||
37 | frame: | |
38 | header | |
39 | prediction | |
40 | residual | |
41 | ||
42 | header: | |
43 | keyframe b MID_STATE | |
44 | if(keyframe || always_reset) | |
45 | reset_contexts | |
46 | if(keyframe){ | |
47 | version u header_state | |
48 | always_reset b header_state | |
49 | temporal_decomposition_type u header_state | |
50 | temporal_decomposition_count u header_state | |
51 | spatial_decomposition_count u header_state | |
52 | colorspace_type u header_state | |
53 | if (nb_planes > 2) { | |
54 | chroma_h_shift u header_state | |
55 | chroma_v_shift u header_state | |
56 | } | |
57 | spatial_scalability b header_state | |
58 | max_ref_frames-1 u header_state | |
59 | qlogs | |
60 | } | |
61 | if(!keyframe){ | |
62 | update_mc b header_state | |
63 | if(update_mc){ | |
64 | for(plane=0; plane<nb_plane_types; plane++){ | |
65 | diag_mc b header_state | |
66 | htaps/2-1 u header_state | |
67 | for(i= p->htaps/2; i; i--) | |
68 | |hcoeff[i]| u header_state | |
69 | } | |
70 | } | |
71 | update_qlogs b header_state | |
72 | if(update_qlogs){ | |
73 | spatial_decomposition_count u header_state | |
74 | qlogs | |
75 | } | |
76 | } | |
77 | ||
78 | spatial_decomposition_type s header_state | |
79 | qlog s header_state | |
80 | mv_scale s header_state | |
81 | qbias s header_state | |
82 | block_max_depth s header_state | |
83 | ||
84 | qlogs: | |
85 | for(plane=0; plane<nb_plane_types; plane++){ | |
86 | quant_table[plane][0][0] s header_state | |
87 | for(level=0; level < spatial_decomposition_count; level++){ | |
88 | quant_table[plane][level][1]s header_state | |
89 | quant_table[plane][level][3]s header_state | |
90 | } | |
91 | } | |
92 | ||
93 | reset_contexts | |
94 | *_state[*]= MID_STATE | |
95 | ||
96 | prediction: | |
97 | for(y=0; y<block_count_vertical; y++) | |
98 | for(x=0; x<block_count_horizontal; x++) | |
99 | block(0) | |
100 | ||
101 | block(level): | |
102 | mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0 | |
103 | if(keyframe){ | |
104 | intra=1 | |
105 | }else{ | |
106 | if(level!=max_block_depth){ | |
107 | s_context= 2*left->level + 2*top->level + topleft->level + topright->level | |
108 | leaf b block_state[4 + s_context] | |
109 | } | |
110 | if(level==max_block_depth || leaf){ | |
111 | intra b block_state[1 + left->intra + top->intra] | |
112 | if(intra){ | |
113 | y_diff s block_state[32] | |
114 | cb_diff s block_state[64] | |
115 | cr_diff s block_state[96] | |
116 | }else{ | |
117 | ref_context= ilog2(2*left->ref) + ilog2(2*top->ref) | |
118 | if(ref_frames > 1) | |
119 | ref u block_state[128 + 1024 + 32*ref_context] | |
120 | mx_context= ilog2(2*abs(left->mx - top->mx)) | |
121 | my_context= ilog2(2*abs(left->my - top->my)) | |
122 | mvx_diff s block_state[128 + 32*(mx_context + 16*!!ref)] | |
123 | mvy_diff s block_state[128 + 32*(my_context + 16*!!ref)] | |
124 | } | |
125 | }else{ | |
126 | block(level+1) | |
127 | block(level+1) | |
128 | block(level+1) | |
129 | block(level+1) | |
130 | } | |
131 | } | |
132 | ||
133 | ||
134 | residual: | |
135 | residual2(luma) | |
136 | if (nb_planes > 2) { | |
137 | residual2(chroma_cr) | |
138 | residual2(chroma_cb) | |
139 | } | |
140 | ||
141 | residual2: | |
142 | for(level=0; level<spatial_decomposition_count; level++){ | |
143 | if(level==0) | |
144 | subband(LL, 0) | |
145 | subband(HL, level) | |
146 | subband(LH, level) | |
147 | subband(HH, level) | |
148 | } | |
149 | ||
150 | subband: | |
151 | FIXME | |
152 | ||
153 | nb_plane_types = gray ? 1 : 2; | |
154 | ||
155 | Tag description: | |
156 | ---------------- | |
157 | ||
158 | version | |
159 | 0 | |
160 | this MUST NOT change within a bitstream | |
161 | ||
162 | always_reset | |
163 | if 1 then the range coder contexts will be reset after each frame | |
164 | ||
165 | temporal_decomposition_type | |
166 | 0 | |
167 | ||
168 | temporal_decomposition_count | |
169 | 0 | |
170 | ||
171 | spatial_decomposition_count | |
172 | FIXME | |
173 | ||
174 | colorspace_type | |
175 | 0 unspecified YcbCr | |
176 | 1 Gray | |
177 | 2 Gray + Alpha | |
178 | 3 GBR | |
179 | 4 GBRA | |
180 | this MUST NOT change within a bitstream | |
181 | ||
182 | chroma_h_shift | |
183 | log2(luma.width / chroma.width) | |
184 | this MUST NOT change within a bitstream | |
185 | ||
186 | chroma_v_shift | |
187 | log2(luma.height / chroma.height) | |
188 | this MUST NOT change within a bitstream | |
189 | ||
190 | spatial_scalability | |
191 | 0 | |
192 | ||
193 | max_ref_frames | |
194 | maximum number of reference frames | |
195 | this MUST NOT change within a bitstream | |
196 | ||
197 | update_mc | |
198 | indicates that motion compensation filter parameters are stored in the | |
199 | header | |
200 | ||
201 | diag_mc | |
202 | flag to enable faster diagonal interpolation | |
203 | this SHOULD be 1 unless it turns out to be covered by a valid patent | |
204 | ||
205 | htaps | |
206 | number of half pel interpolation filter taps, MUST be even, >0 and <10 | |
207 | ||
208 | hcoeff | |
209 | half pel interpolation filter coefficients, hcoeff[0] are the 2 middle | |
210 | coefficients [1] are the next outer ones and so on, resulting in a filter | |
211 | like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ... | |
212 | the sign of the coefficients is not explicitly stored but alternates | |
213 | after each coeff and coeff[0] is positive, so ...,+,-,+,-,+,+,-,+,-,+,... | |
214 | hcoeff[0] is not explicitly stored but found by subtracting the sum | |
215 | of all stored coefficients with signs from 32 | |
216 | hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ... | |
217 | a good choice for hcoeff and htaps is | |
218 | htaps= 6 | |
219 | hcoeff={40,-10,2} | |
220 | an alternative which requires more computations at both encoder and | |
221 | decoder side and may or may not be better is | |
222 | htaps= 8 | |
223 | hcoeff={42,-14,6,-2} | |
224 | ||
225 | ||
226 | ref_frames | |
227 | minimum of the number of available reference frames and max_ref_frames | |
228 | for example the first frame after a key frame always has ref_frames=1 | |
229 | ||
230 | spatial_decomposition_type | |
231 | wavelet type | |
232 | 0 is a 9/7 symmetric compact integer wavelet | |
233 | 1 is a 5/3 symmetric compact integer wavelet | |
234 | others are reserved | |
235 | stored as delta from last, last is reset to 0 if always_reset || keyframe | |
236 | ||
237 | qlog | |
238 | quality (logarthmic quantizer scale) | |
239 | stored as delta from last, last is reset to 0 if always_reset || keyframe | |
240 | ||
241 | mv_scale | |
242 | stored as delta from last, last is reset to 0 if always_reset || keyframe | |
243 | FIXME check that everything works fine if this changes between frames | |
244 | ||
245 | qbias | |
246 | dequantization bias | |
247 | stored as delta from last, last is reset to 0 if always_reset || keyframe | |
248 | ||
249 | block_max_depth | |
250 | maximum depth of the block tree | |
251 | stored as delta from last, last is reset to 0 if always_reset || keyframe | |
252 | ||
253 | quant_table | |
254 | quantiztation table | |
255 | ||
256 | ||
257 | Highlevel bitstream structure: | |
258 | ============================= | |
259 | -------------------------------------------- | |
260 | | Header | | |
261 | -------------------------------------------- | |
262 | | ------------------------------------ | | |
263 | | | Block0 | | | |
264 | | | split? | | | |
265 | | | yes no | | | |
266 | | | ......... intra? | | | |
267 | | | : Block01 : yes no | | | |
268 | | | : Block02 : ....... .......... | | | |
269 | | | : Block03 : : y DC : : ref index: | | | |
270 | | | : Block04 : : cb DC : : motion x : | | | |
271 | | | ......... : cr DC : : motion y : | | | |
272 | | | ....... .......... | | | |
273 | | ------------------------------------ | | |
274 | | ------------------------------------ | | |
275 | | | Block1 | | | |
276 | | ... | | |
277 | -------------------------------------------- | |
278 | | ------------ ------------ ------------ | | |
279 | || Y subbands | | Cb subbands| | Cr subbands|| | |
280 | || --- --- | | --- --- | | --- --- || | |
281 | || |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| || | |
282 | || --- --- | | --- --- | | --- --- || | |
283 | || --- --- | | --- --- | | --- --- || | |
284 | || |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| || | |
285 | || --- --- | | --- --- | | --- --- || | |
286 | || --- --- | | --- --- | | --- --- || | |
287 | || |HL1||LH1| | | |HL1||LH1| | | |HL1||LH1| || | |
288 | || --- --- | | --- --- | | --- --- || | |
289 | || --- --- | | --- --- | | --- --- || | |
290 | || |HH1||HL2| | | |HH1||HL2| | | |HH1||HL2| || | |
291 | || ... | | ... | | ... || | |
292 | | ------------ ------------ ------------ | | |
293 | -------------------------------------------- | |
294 | ||
295 | Decoding process: | |
296 | ================= | |
297 | ||
298 | ------------ | |
299 | | | | |
300 | | Subbands | | |
301 | ------------ | | | |
302 | | | ------------ | |
303 | | Intra DC | | | |
304 | | | LL0 subband prediction | |
305 | ------------ | | |
306 | \ Dequantizaton | |
307 | ------------------- \ | | |
308 | | Reference frames | \ IDWT | |
309 | | ------- ------- | Motion \ | | |
310 | ||Frame 0| |Frame 1|| Compensation . OBMC v ------- | |
311 | | ------- ------- | --------------. \------> + --->|Frame n|-->output | |
312 | | ------- ------- | ------- | |
313 | ||Frame 2| |Frame 3||<----------------------------------/ | |
314 | | ... | | |
315 | ------------------- | |
316 | ||
317 | ||
318 | Range Coder: | |
319 | ============ | |
320 | ||
321 | Binary Range Coder: | |
322 | ------------------- | |
323 | The implemented range coder is an adapted version based upon "Range encoding: | |
324 | an algorithm for removing redundancy from a digitised message." by G. N. N. | |
325 | Martin. | |
326 | The symbols encoded by the Snow range coder are bits (0|1). The | |
327 | associated probabilities are not fix but change depending on the symbol mix | |
328 | seen so far. | |
329 | ||
330 | ||
331 | bit seen | new state | |
332 | ---------+----------------------------------------------- | |
333 | 0 | 256 - state_transition_table[256 - old_state]; | |
334 | 1 | state_transition_table[ old_state]; | |
335 | ||
336 | state_transition_table = { | |
337 | 0, 0, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, | |
338 | 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 37, 38, 39, 40, 41, 42, | |
339 | 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57, | |
340 | 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, | |
341 | 74, 75, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, | |
342 | 89, 90, 91, 92, 93, 94, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, | |
343 | 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 114, 115, 116, 117, 118, | |
344 | 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 133, | |
345 | 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, | |
346 | 150, 151, 152, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, | |
347 | 165, 166, 167, 168, 169, 170, 171, 171, 172, 173, 174, 175, 176, 177, 178, 179, | |
348 | 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 190, 191, 192, 194, 194, | |
349 | 195, 196, 197, 198, 199, 200, 201, 202, 202, 204, 205, 206, 207, 208, 209, 209, | |
350 | 210, 211, 212, 213, 215, 215, 216, 217, 218, 219, 220, 220, 222, 223, 224, 225, | |
351 | 226, 227, 227, 229, 229, 230, 231, 232, 234, 234, 235, 236, 237, 238, 239, 240, | |
352 | 241, 242, 243, 244, 245, 246, 247, 248, 248, 0, 0, 0, 0, 0, 0, 0}; | |
353 | ||
354 | FIXME | |
355 | ||
356 | ||
357 | Range Coding of integers: | |
358 | ------------------------- | |
359 | FIXME | |
360 | ||
361 | ||
362 | Neighboring Blocks: | |
363 | =================== | |
364 | left and top are set to the respective blocks unless they are outside of | |
365 | the image in which case they are set to the Null block | |
366 | ||
367 | top-left is set to the top left block unless it is outside of the image in | |
368 | which case it is set to the left block | |
369 | ||
370 | if this block has no larger parent block or it is at the left side of its | |
371 | parent block and the top right block is not outside of the image then the | |
372 | top right block is used for top-right else the top-left block is used | |
373 | ||
374 | Null block | |
375 | y,cb,cr are 128 | |
376 | level, ref, mx and my are 0 | |
377 | ||
378 | ||
379 | Motion Vector Prediction: | |
380 | ========================= | |
381 | 1. the motion vectors of all the neighboring blocks are scaled to | |
382 | compensate for the difference of reference frames | |
383 | ||
384 | scaled_mv= (mv * (256 * (current_reference+1) / (mv.reference+1)) + 128)>>8 | |
385 | ||
386 | 2. the median of the scaled left, top and top-right vectors is used as | |
387 | motion vector prediction | |
388 | ||
389 | 3. the used motion vector is the sum of the predictor and | |
390 | (mvx_diff, mvy_diff)*mv_scale | |
391 | ||
392 | ||
393 | Intra DC Predicton: | |
394 | ====================== | |
395 | the luma and chroma values of the left block are used as predictors | |
396 | ||
397 | the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff | |
398 | to reverse this in the decoder apply the following: | |
399 | block[y][x].dc[0] = block[y][x-1].dc[0] + y_diff; | |
400 | block[y][x].dc[1] = block[y][x-1].dc[1] + cb_diff; | |
401 | block[y][x].dc[2] = block[y][x-1].dc[2] + cr_diff; | |
402 | block[*][-1].dc[*]= 128; | |
403 | ||
404 | ||
405 | Motion Compensation: | |
406 | ==================== | |
407 | ||
408 | Halfpel interpolation: | |
409 | ---------------------- | |
410 | halfpel interpolation is done by convolution with the halfpel filter stored | |
411 | in the header: | |
412 | ||
413 | horizontal halfpel samples are found by | |
414 | H1[y][x] = hcoeff[0]*(F[y][x ] + F[y][x+1]) | |
415 | + hcoeff[1]*(F[y][x-1] + F[y][x+2]) | |
416 | + hcoeff[2]*(F[y][x-2] + F[y][x+3]) | |
417 | + ... | |
418 | h1[y][x] = (H1[y][x] + 32)>>6; | |
419 | ||
420 | vertical halfpel samples are found by | |
421 | H2[y][x] = hcoeff[0]*(F[y ][x] + F[y+1][x]) | |
422 | + hcoeff[1]*(F[y-1][x] + F[y+2][x]) | |
423 | + ... | |
424 | h2[y][x] = (H2[y][x] + 32)>>6; | |
425 | ||
426 | vertical+horizontal halfpel samples are found by | |
427 | H3[y][x] = hcoeff[0]*(H2[y][x ] + H2[y][x+1]) | |
428 | + hcoeff[1]*(H2[y][x-1] + H2[y][x+2]) | |
429 | + ... | |
430 | H3[y][x] = hcoeff[0]*(H1[y ][x] + H1[y+1][x]) | |
431 | + hcoeff[1]*(H1[y+1][x] + H1[y+2][x]) | |
432 | + ... | |
433 | h3[y][x] = (H3[y][x] + 2048)>>12; | |
434 | ||
435 | ||
436 | F H1 F | |
437 | | | | | |
438 | | | | | |
439 | | | | | |
440 | F H1 F | |
441 | | | | | |
442 | | | | | |
443 | | | | | |
444 | F-------F-------F-> H1<-F-------F-------F | |
445 | v v v | |
446 | H2 H3 H2 | |
447 | ^ ^ ^ | |
448 | F-------F-------F-> H1<-F-------F-------F | |
449 | | | | | |
450 | | | | | |
451 | | | | | |
452 | F H1 F | |
453 | | | | | |
454 | | | | | |
455 | | | | | |
456 | F H1 F | |
457 | ||
458 | ||
459 | unavailable fullpel samples (outside the picture for example) shall be equal | |
460 | to the closest available fullpel sample | |
461 | ||
462 | ||
463 | Smaller pel interpolation: | |
464 | -------------------------- | |
465 | if diag_mc is set then points which lie on a line between 2 vertically, | |
466 | horiziontally or diagonally adjacent halfpel points shall be interpolated | |
467 | linearls with rounding to nearest and halfway values rounded up. | |
468 | points which lie on 2 diagonals at the same time should only use the one | |
469 | diagonal not containing the fullpel point | |
470 | ||
471 | ||
472 | ||
473 | F-->O---q---O<--h1->O---q---O<--F | |
474 | v \ / v \ / v | |
475 | O O O O O O O | |
476 | | / | \ | | |
477 | q q q q q | |
478 | | / | \ | | |
479 | O O O O O O O | |
480 | ^ / \ ^ / \ ^ | |
481 | h2-->O---q---O<--h3->O---q---O<--h2 | |
482 | v \ / v \ / v | |
483 | O O O O O O O | |
484 | | \ | / | | |
485 | q q q q q | |
486 | | \ | / | | |
487 | O O O O O O O | |
488 | ^ / \ ^ / \ ^ | |
489 | F-->O---q---O<--h1->O---q---O<--F | |
490 | ||
491 | ||
492 | ||
493 | the remaining points shall be bilinearly interpolated from the | |
494 | up to 4 surrounding halfpel and fullpel points, again rounding should be to | |
495 | nearest and halfway values rounded up | |
496 | ||
497 | compliant Snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma | |
498 | interpolation at least | |
499 | ||
500 | ||
501 | Overlapped block motion compensation: | |
502 | ------------------------------------- | |
503 | FIXME | |
504 | ||
505 | LL band prediction: | |
506 | =================== | |
507 | Each sample in the LL0 subband is predicted by the median of the left, top and | |
508 | left+top-topleft samples, samples outside the subband shall be considered to | |
509 | be 0. To reverse this prediction in the decoder apply the following. | |
510 | for(y=0; y<height; y++){ | |
511 | for(x=0; x<width; x++){ | |
512 | sample[y][x] += median(sample[y-1][x], | |
513 | sample[y][x-1], | |
514 | sample[y-1][x]+sample[y][x-1]-sample[y-1][x-1]); | |
515 | } | |
516 | } | |
517 | sample[-1][*]=sample[*][-1]= 0; | |
518 | width,height here are the width and height of the LL0 subband not of the final | |
519 | video | |
520 | ||
521 | ||
522 | Dequantizaton: | |
523 | ============== | |
524 | FIXME | |
525 | ||
526 | Wavelet Transform: | |
527 | ================== | |
528 | ||
529 | Snow supports 2 wavelet transforms, the symmetric biorthogonal 5/3 integer | |
530 | transform and a integer approximation of the symmetric biorthogonal 9/7 | |
531 | daubechies wavelet. | |
532 | ||
533 | 2D IDWT (inverse discrete wavelet transform) | |
534 | -------------------------------------------- | |
535 | The 2D IDWT applies a 2D filter recursively, each time combining the | |
536 | 4 lowest frequency subbands into a single subband until only 1 subband | |
537 | remains. | |
538 | The 2D filter is done by first applying a 1D filter in the vertical direction | |
539 | and then applying it in the horizontal one. | |
540 | --------------- --------------- --------------- --------------- | |
541 | |LL0|HL0| | | | | | | | | | | | | |
542 | |---+---| HL1 | | L0|H0 | HL1 | | LL1 | HL1 | | | | | |
543 | |LH0|HH0| | | | | | | | | | | | | |
544 | |-------+-------|->|-------+-------|->|-------+-------|->| L1 | H1 |->... | |
545 | | | | | | | | | | | | | | |
546 | | LH1 | HH1 | | LH1 | HH1 | | LH1 | HH1 | | | | | |
547 | | | | | | | | | | | | | | |
548 | --------------- --------------- --------------- --------------- | |
549 | ||
550 | ||
551 | 1D Filter: | |
552 | ---------- | |
553 | 1. interleave the samples of the low and high frequency subbands like | |
554 | s={L0, H0, L1, H1, L2, H2, L3, H3, ... } | |
555 | note, this can end with a L or a H, the number of elements shall be w | |
556 | s[-1] shall be considered equivalent to s[1 ] | |
557 | s[w ] shall be considered equivalent to s[w-2] | |
558 | ||
559 | 2. perform the lifting steps in order as described below | |
560 | ||
561 | 5/3 Integer filter: | |
562 | 1. s[i] -= (s[i-1] + s[i+1] + 2)>>2; for all even i < w | |
563 | 2. s[i] += (s[i-1] + s[i+1] )>>1; for all odd i < w | |
564 | ||
565 | \ | /|\ | /|\ | /|\ | /|\ | |
566 | \|/ | \|/ | \|/ | \|/ | | |
567 | + | + | + | + | -1/4 | |
568 | /|\ | /|\ | /|\ | /|\ | | |
569 | / | \|/ | \|/ | \|/ | \|/ | |
570 | | + | + | + | + +1/2 | |
571 | ||
572 | ||
573 | Snow's 9/7 Integer filter: | |
574 | 1. s[i] -= (3*(s[i-1] + s[i+1]) + 4)>>3; for all even i < w | |
575 | 2. s[i] -= s[i-1] + s[i+1] ; for all odd i < w | |
576 | 3. s[i] += ( s[i-1] + s[i+1] + 4*s[i] + 8)>>4; for all even i < w | |
577 | 4. s[i] += (3*(s[i-1] + s[i+1]) )>>1; for all odd i < w | |
578 | ||
579 | \ | /|\ | /|\ | /|\ | /|\ | |
580 | \|/ | \|/ | \|/ | \|/ | | |
581 | + | + | + | + | -3/8 | |
582 | /|\ | /|\ | /|\ | /|\ | | |
583 | / | \|/ | \|/ | \|/ | \|/ | |
584 | (| + (| + (| + (| + -1 | |
585 | \ + /|\ + /|\ + /|\ + /|\ +1/4 | |
586 | \|/ | \|/ | \|/ | \|/ | | |
587 | + | + | + | + | +1/16 | |
588 | /|\ | /|\ | /|\ | /|\ | | |
589 | / | \|/ | \|/ | \|/ | \|/ | |
590 | | + | + | + | + +3/2 | |
591 | ||
592 | optimization tips: | |
593 | following are exactly identical | |
594 | (3a)>>1 == a + (a>>1) | |
595 | (a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2 | |
596 | ||
597 | 16bit implementation note: | |
598 | The IDWT can be implemented with 16bits, but this requires some care to | |
599 | prevent overflows, the following list, lists the minimum number of bits needed | |
600 | for some terms | |
601 | 1. lifting step | |
602 | A= s[i-1] + s[i+1] 16bit | |
603 | 3*A + 4 18bit | |
604 | A + (A>>1) + 2 17bit | |
605 | ||
606 | 3. lifting step | |
607 | s[i-1] + s[i+1] 17bit | |
608 | ||
609 | 4. lifiting step | |
610 | 3*(s[i-1] + s[i+1]) 17bit | |
611 | ||
612 | ||
613 | TODO: | |
614 | ===== | |
615 | Important: | |
616 | finetune initial contexts | |
617 | flip wavelet? | |
618 | try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients | |
619 | try the MV length as context for coding the residual coefficients | |
620 | use extradata for stuff which is in the keyframes now? | |
621 | implement per picture halfpel interpolation | |
622 | try different range coder state transition tables for different contexts | |
623 | ||
624 | Not Important: | |
625 | compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality) | |
626 | spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later) | |
627 | ||
628 | ||
629 | Credits: | |
630 | ======== | |
631 | Michael Niedermayer | |
632 | Loren Merritt | |
633 | ||
634 | ||
635 | Copyright: | |
636 | ========== | |
637 | GPL + GFDL + whatever is needed to make this a RFC |