Edit File: snow.txt
============================================= Snow Video Codec Specification Draft 20080110 ============================================= Introduction: ============= This specification describes the Snow bitstream syntax and semantics as well as the formal Snow decoding process. The decoding process is described precisely and any compliant decoder MUST produce the exact same output for a spec-conformant Snow stream. For encoding, though, any process which generates a stream compliant to the syntactical and semantic requirements and which is decodable by the process described in this spec shall be considered a conformant Snow encoder. Definitions: ============ MUST the specific part must be done to conform to this standard SHOULD it is recommended to be done that way, but not strictly required ilog2(x) is the rounded down logarithm of x with basis 2 ilog2(0) = 0 Type definitions: ================= b 1-bit range coded u unsigned scalar value range coded s signed scalar value range coded Bitstream syntax: ================= frame: header prediction residual header: keyframe b MID_STATE if(keyframe || always_reset) reset_contexts if(keyframe){ version u header_state always_reset b header_state temporal_decomposition_type u header_state temporal_decomposition_count u header_state spatial_decomposition_count u header_state colorspace_type u header_state if (nb_planes > 2) { chroma_h_shift u header_state chroma_v_shift u header_state } spatial_scalability b header_state max_ref_frames-1 u header_state qlogs } if(!keyframe){ update_mc b header_state if(update_mc){ for(plane=0; plane<nb_plane_types; plane++){ diag_mc b header_state htaps/2-1 u header_state for(i= p->htaps/2; i; i--) |hcoeff[i]| u header_state } } update_qlogs b header_state if(update_qlogs){ spatial_decomposition_count u header_state qlogs } } spatial_decomposition_type s header_state qlog s header_state mv_scale s header_state qbias s header_state block_max_depth s header_state qlogs: for(plane=0; plane<nb_plane_types; plane++){ quant_table[plane][0][0] s header_state for(level=0; level < spatial_decomposition_count; level++){ quant_table[plane][level][1]s header_state quant_table[plane][level][3]s header_state } } reset_contexts *_state[*]= MID_STATE prediction: for(y=0; y<block_count_vertical; y++) for(x=0; x<block_count_horizontal; x++) block(0) block(level): mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0 if(keyframe){ intra=1 }else{ if(level!=max_block_depth){ s_context= 2*left->level + 2*top->level + topleft->level + topright->level leaf b block_state[4 + s_context] } if(level==max_block_depth || leaf){ intra b block_state[1 + left->intra + top->intra] if(intra){ y_diff s block_state[32] cb_diff s block_state[64] cr_diff s block_state[96] }else{ ref_context= ilog2(2*left->ref) + ilog2(2*top->ref) if(ref_frames > 1) ref u block_state[128 + 1024 + 32*ref_context] mx_context= ilog2(2*abs(left->mx - top->mx)) my_context= ilog2(2*abs(left->my - top->my)) mvx_diff s block_state[128 + 32*(mx_context + 16*!!ref)] mvy_diff s block_state[128 + 32*(my_context + 16*!!ref)] } }else{ block(level+1) block(level+1) block(level+1) block(level+1) } } residual: residual2(luma) if (nb_planes > 2) { residual2(chroma_cr) residual2(chroma_cb) } residual2: for(level=0; level<spatial_decomposition_count; level++){ if(level==0) subband(LL, 0) subband(HL, level) subband(LH, level) subband(HH, level) } subband: FIXME nb_plane_types = gray ? 1 : 2; Tag description: ---------------- version 0 this MUST NOT change within a bitstream always_reset if 1 then the range coder contexts will be reset after each frame temporal_decomposition_type 0 temporal_decomposition_count 0 spatial_decomposition_count FIXME colorspace_type 0 unspecified YcbCr 1 Gray 2 Gray + Alpha 3 GBR 4 GBRA this MUST NOT change within a bitstream chroma_h_shift log2(luma.width / chroma.width) this MUST NOT change within a bitstream chroma_v_shift log2(luma.height / chroma.height) this MUST NOT change within a bitstream spatial_scalability 0 max_ref_frames maximum number of reference frames this MUST NOT change within a bitstream update_mc indicates that motion compensation filter parameters are stored in the header diag_mc flag to enable faster diagonal interpolation this SHOULD be 1 unless it turns out to be covered by a valid patent htaps number of half pel interpolation filter taps, MUST be even, >0 and <10 hcoeff half pel interpolation filter coefficients, hcoeff[0] are the 2 middle coefficients [1] are the next outer ones and so on, resulting in a filter like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ... the sign of the coefficients is not explicitly stored but alternates after each coeff and coeff[0] is positive, so ...,+,-,+,-,+,+,-,+,-,+,... hcoeff[0] is not explicitly stored but found by subtracting the sum of all stored coefficients with signs from 32 hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ... a good choice for hcoeff and htaps is htaps= 6 hcoeff={40,-10,2} an alternative which requires more computations at both encoder and decoder side and may or may not be better is htaps= 8 hcoeff={42,-14,6,-2} ref_frames minimum of the number of available reference frames and max_ref_frames for example the first frame after a key frame always has ref_frames=1 spatial_decomposition_type wavelet type 0 is a 9/7 symmetric compact integer wavelet 1 is a 5/3 symmetric compact integer wavelet others are reserved stored as delta from last, last is reset to 0 if always_reset || keyframe qlog quality (logarthmic quantizer scale) stored as delta from last, last is reset to 0 if always_reset || keyframe mv_scale stored as delta from last, last is reset to 0 if always_reset || keyframe FIXME check that everything works fine if this changes between frames qbias dequantization bias stored as delta from last, last is reset to 0 if always_reset || keyframe block_max_depth maximum depth of the block tree stored as delta from last, last is reset to 0 if always_reset || keyframe quant_table quantiztation table Highlevel bitstream structure: ============================= -------------------------------------------- | Header | -------------------------------------------- | ------------------------------------ | | | Block0 | | | | split? | | | | yes no | | | | ......... intra? | | | | : Block01 : yes no | | | | : Block02 : ....... .......... | | | | : Block03 : : y DC : : ref index: | | | | : Block04 : : cb DC : : motion x : | | | | ......... : cr DC : : motion y : | | | | ....... .......... | | | ------------------------------------ | | ------------------------------------ | | | Block1 | | | ... | -------------------------------------------- | ------------ ------------ ------------ | || Y subbands | | Cb subbands| | Cr subbands|| || --- --- | | --- --- | | --- --- || || |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| || || --- --- | | --- --- | | --- --- || || --- --- | | --- --- | | --- --- || || |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| || || --- --- | | --- --- | | --- --- || || --- --- | | --- --- | | --- --- || || |HL1||LH1| | | |HL1||LH1| | | |HL1||LH1| || || --- --- | | --- --- | | --- --- || || --- --- | | --- --- | | --- --- || || |HH1||HL2| | | |HH1||HL2| | | |HH1||HL2| || || ... | | ... | | ... || | ------------ ------------ ------------ | -------------------------------------------- Decoding process: ================= ------------ | | | Subbands | ------------ | | | | ------------ | Intra DC | | | | LL0 subband prediction ------------ | \ Dequantizaton ------------------- \ | | Reference frames | \ IDWT | ------- ------- | Motion \ | ||Frame 0| |Frame 1|| Compensation . OBMC v ------- | ------- ------- | --------------. \------> + --->|Frame n|-->output | ------- ------- | ------- ||Frame 2| |Frame 3||<----------------------------------/ | ... | ------------------- Range Coder: ============ Binary Range Coder: ------------------- The implemented range coder is an adapted version based upon "Range encoding: an algorithm for removing redundancy from a digitised message." by G. N. N. Martin. The symbols encoded by the Snow range coder are bits (0|1). The associated probabilities are not fix but change depending on the symbol mix seen so far. bit seen | new state ---------+----------------------------------------------- 0 | 256 - state_transition_table[256 - old_state]; 1 | state_transition_table[ old_state]; state_transition_table = { 0, 0, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 190, 191, 192, 194, 194, 195, 196, 197, 198, 199, 200, 201, 202, 202, 204, 205, 206, 207, 208, 209, 209, 210, 211, 212, 213, 215, 215, 216, 217, 218, 219, 220, 220, 222, 223, 224, 225, 226, 227, 227, 229, 229, 230, 231, 232, 234, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 248, 0, 0, 0, 0, 0, 0, 0}; FIXME Range Coding of integers: ------------------------- FIXME Neighboring Blocks: =================== left and top are set to the respective blocks unless they are outside of the image in which case they are set to the Null block top-left is set to the top left block unless it is outside of the image in which case it is set to the left block if this block has no larger parent block or it is at the left side of its parent block and the top right block is not outside of the image then the top right block is used for top-right else the top-left block is used Null block y,cb,cr are 128 level, ref, mx and my are 0 Motion Vector Prediction: ========================= 1. the motion vectors of all the neighboring blocks are scaled to compensate for the difference of reference frames scaled_mv= (mv * (256 * (current_reference+1) / (mv.reference+1)) + 128)>>8 2. the median of the scaled left, top and top-right vectors is used as motion vector prediction 3. the used motion vector is the sum of the predictor and (mvx_diff, mvy_diff)*mv_scale Intra DC Predicton: ====================== the luma and chroma values of the left block are used as predictors the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff to reverse this in the decoder apply the following: block[y][x].dc[0] = block[y][x-1].dc[0] + y_diff; block[y][x].dc[1] = block[y][x-1].dc[1] + cb_diff; block[y][x].dc[2] = block[y][x-1].dc[2] + cr_diff; block[*][-1].dc[*]= 128; Motion Compensation: ==================== Halfpel interpolation: ---------------------- halfpel interpolation is done by convolution with the halfpel filter stored in the header: horizontal halfpel samples are found by H1[y][x] = hcoeff[0]*(F[y][x ] + F[y][x+1]) + hcoeff[1]*(F[y][x-1] + F[y][x+2]) + hcoeff[2]*(F[y][x-2] + F[y][x+3]) + ... h1[y][x] = (H1[y][x] + 32)>>6; vertical halfpel samples are found by H2[y][x] = hcoeff[0]*(F[y ][x] + F[y+1][x]) + hcoeff[1]*(F[y-1][x] + F[y+2][x]) + ... h2[y][x] = (H2[y][x] + 32)>>6; vertical+horizontal halfpel samples are found by H3[y][x] = hcoeff[0]*(H2[y][x ] + H2[y][x+1]) + hcoeff[1]*(H2[y][x-1] + H2[y][x+2]) + ... H3[y][x] = hcoeff[0]*(H1[y ][x] + H1[y+1][x]) + hcoeff[1]*(H1[y+1][x] + H1[y+2][x]) + ... h3[y][x] = (H3[y][x] + 2048)>>12; F H1 F | | | | | | | | | F H1 F | | | | | | | | | F-------F-------F-> H1<-F-------F-------F v v v H2 H3 H2 ^ ^ ^ F-------F-------F-> H1<-F-------F-------F | | | | | | | | | F H1 F | | | | | | | | | F H1 F unavailable fullpel samples (outside the picture for example) shall be equal to the closest available fullpel sample Smaller pel interpolation: -------------------------- if diag_mc is set then points which lie on a line between 2 vertically, horiziontally or diagonally adjacent halfpel points shall be interpolated linearls with rounding to nearest and halfway values rounded up. points which lie on 2 diagonals at the same time should only use the one diagonal not containing the fullpel point F-->O---q---O<--h1->O---q---O<--F v \ / v \ / v O O O O O O O | / | \ | q q q q q | / | \ | O O O O O O O ^ / \ ^ / \ ^ h2-->O---q---O<--h3->O---q---O<--h2 v \ / v \ / v O O O O O O O | \ | / | q q q q q | \ | / | O O O O O O O ^ / \ ^ / \ ^ F-->O---q---O<--h1->O---q---O<--F the remaining points shall be bilinearly interpolated from the up to 4 surrounding halfpel and fullpel points, again rounding should be to nearest and halfway values rounded up compliant Snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma interpolation at least Overlapped block motion compensation: ------------------------------------- FIXME LL band prediction: =================== Each sample in the LL0 subband is predicted by the median of the left, top and left+top-topleft samples, samples outside the subband shall be considered to be 0. To reverse this prediction in the decoder apply the following. for(y=0; y<height; y++){ for(x=0; x<width; x++){ sample[y][x] += median(sample[y-1][x], sample[y][x-1], sample[y-1][x]+sample[y][x-1]-sample[y-1][x-1]); } } sample[-1][*]=sample[*][-1]= 0; width,height here are the width and height of the LL0 subband not of the final video Dequantizaton: ============== FIXME Wavelet Transform: ================== Snow supports 2 wavelet transforms, the symmetric biorthogonal 5/3 integer transform and a integer approximation of the symmetric biorthogonal 9/7 daubechies wavelet. 2D IDWT (inverse discrete wavelet transform) -------------------------------------------- The 2D IDWT applies a 2D filter recursively, each time combining the 4 lowest frequency subbands into a single subband until only 1 subband remains. The 2D filter is done by first applying a 1D filter in the vertical direction and then applying it in the horizontal one. --------------- --------------- --------------- --------------- |LL0|HL0| | | | | | | | | | | | |---+---| HL1 | | L0|H0 | HL1 | | LL1 | HL1 | | | | |LH0|HH0| | | | | | | | | | | | |-------+-------|->|-------+-------|->|-------+-------|->| L1 | H1 |->... | | | | | | | | | | | | | LH1 | HH1 | | LH1 | HH1 | | LH1 | HH1 | | | | | | | | | | | | | | | | --------------- --------------- --------------- --------------- 1D Filter: ---------- 1. interleave the samples of the low and high frequency subbands like s={L0, H0, L1, H1, L2, H2, L3, H3, ... } note, this can end with a L or a H, the number of elements shall be w s[-1] shall be considered equivalent to s[1 ] s[w ] shall be considered equivalent to s[w-2] 2. perform the lifting steps in order as described below 5/3 Integer filter: 1. s[i] -= (s[i-1] + s[i+1] + 2)>>2; for all even i < w 2. s[i] += (s[i-1] + s[i+1] )>>1; for all odd i < w \ | /|\ | /|\ | /|\ | /|\ \|/ | \|/ | \|/ | \|/ | + | + | + | + | -1/4 /|\ | /|\ | /|\ | /|\ | / | \|/ | \|/ | \|/ | \|/ | + | + | + | + +1/2 Snow's 9/7 Integer filter: 1. s[i] -= (3*(s[i-1] + s[i+1]) + 4)>>3; for all even i < w 2. s[i] -= s[i-1] + s[i+1] ; for all odd i < w 3. s[i] += ( s[i-1] + s[i+1] + 4*s[i] + 8)>>4; for all even i < w 4. s[i] += (3*(s[i-1] + s[i+1]) )>>1; for all odd i < w \ | /|\ | /|\ | /|\ | /|\ \|/ | \|/ | \|/ | \|/ | + | + | + | + | -3/8 /|\ | /|\ | /|\ | /|\ | / | \|/ | \|/ | \|/ | \|/ (| + (| + (| + (| + -1 \ + /|\ + /|\ + /|\ + /|\ +1/4 \|/ | \|/ | \|/ | \|/ | + | + | + | + | +1/16 /|\ | /|\ | /|\ | /|\ | / | \|/ | \|/ | \|/ | \|/ | + | + | + | + +3/2 optimization tips: following are exactly identical (3a)>>1 == a + (a>>1) (a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2 16bit implementation note: The IDWT can be implemented with 16bits, but this requires some care to prevent overflows, the following list, lists the minimum number of bits needed for some terms 1. lifting step A= s[i-1] + s[i+1] 16bit 3*A + 4 18bit A + (A>>1) + 2 17bit 3. lifting step s[i-1] + s[i+1] 17bit 4. lifiting step 3*(s[i-1] + s[i+1]) 17bit TODO: ===== Important: finetune initial contexts flip wavelet? try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients try the MV length as context for coding the residual coefficients use extradata for stuff which is in the keyframes now? implement per picture halfpel interpolation try different range coder state transition tables for different contexts Not Important: compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality) spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later) Credits: ======== Michael Niedermayer Loren Merritt Copyright: ========== GPL + GFDL + whatever is needed to make this a RFC