## ffmpeg / doc / snow.txt @ 8787d837

History | View | Annotate | Download (18.9 KB)

1 | 78954a05 | Michael Niedermayer | ============================================= |
---|---|---|---|

2 | SNOW Video Codec Specification Draft 20070103 |
||

3 | ============================================= |
||

4 | |||

5 | 11de04d8 | Michael Niedermayer | 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 | 78954a05 | Michael Niedermayer | |

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 | e9314de6 | Michael Niedermayer | if(!keyframe){ |

59 | b85bf991 | Michael Niedermayer | update_mc b header_state |

60 | if(update_mc){ |
||

61 | e9314de6 | Michael Niedermayer | 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 | bc66275b | Michael Niedermayer | update_qlogs b header_state |

69 | if(update_qlogs){ |
||

70 | spatial_decomposition_count u header_state |
||

71 | qlogs |
||

72 | } |
||

73 | e9314de6 | Michael Niedermayer | } |

74 | 78954a05 | Michael Niedermayer | |

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 | c3922c65 | Michael Niedermayer | mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0 |

100 | 78954a05 | Michael Niedermayer | 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 | 48fe9238 | Michael Niedermayer | 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 | 78954a05 | Michael Niedermayer | 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 | e9314de6 | Michael Niedermayer | 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 | 78954a05 | Michael Niedermayer | 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 | 24dbec7c | Luca Barbato | FIXME check that everything works fine if this changes between frames |

235 | 78954a05 | Michael Niedermayer | |

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 | 8f39b74d | Michael Niedermayer | |

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 | 78954a05 | Michael Niedermayer | 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 | 90b5b51e | Diego Biurrun | top-left is set to the top left block unless it is outside of the image in |

319 | 78954a05 | Michael Niedermayer | which case it is set to the left block |

320 | |||

321 | 90b5b51e | Diego Biurrun | if this block has no larger parent block or it is at the left side of its |

322 | 78954a05 | Michael Niedermayer | 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 | 2cc45470 | Michael Niedermayer | to reverse this in the decoder apply the following: |

350 | c3922c65 | Michael Niedermayer | 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 | 2cc45470 | Michael Niedermayer | block[*][-1].dc[*]= 128; |

354 | 78954a05 | Michael Niedermayer | |

355 | |||

356 | Motion Compensation: |
||

357 | ==================== |
||

358 | e9314de6 | Michael Niedermayer | |

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 | a11dc59a | Michael Niedermayer | up to 4 surrounding halfpel and fullpel points, again rounding should be to |

446 | nearest and halfway values rounded up |
||

447 | e9314de6 | Michael Niedermayer | |

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 | 78954a05 | Michael Niedermayer | FIXME |

455 | |||

456 | LL band prediction: |
||

457 | =================== |
||

458 | 1e37b7e4 | Michael Niedermayer | 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 | 78954a05 | Michael Niedermayer | |

473 | Dequantizaton: |
||

474 | ============== |
||

475 | FIXME |
||

476 | |||

477 | Wavelet Transform: |
||

478 | ================== |
||

479 | fdb99704 | Michael Niedermayer | |

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 | 09671ce7 | Michael Niedermayer | 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 | 7397cf3f | Michael Niedermayer | |---+---| HL1 | | L0|H0 | HL1 | | LL1 | HL1 | | | | |

494 | 09671ce7 | Michael Niedermayer | |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 | fdb99704 | Michael Niedermayer | |

543 | a282102d | Michael Niedermayer | optimization tips: |

544 | following are exactly identical |
||

545 | (3a)>>1 == a + (a>>1) |
||

546 | (a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2 |
||

547 | 78954a05 | Michael Niedermayer | |

548 | 6a1aa752 | Michael Niedermayer | 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 | 78954a05 | Michael Niedermayer | 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 | 2b6134b3 | Michael Niedermayer | implement per picture halfpel interpolation |

574 | c78fc717 | Michael Niedermayer | try different range coder state transition tables for different contexts |

575 | 78954a05 | Michael Niedermayer | |

576 | Not Important: |
||

577 | c64a8712 | Michael Niedermayer | compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality) |

578 | 78954a05 | Michael Niedermayer | 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 |