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

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SNOW Video Codec Specification Draft 20070103 |

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============================================= |

4 | |

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Intro: |

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====== |

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This Specification describes the snow syntax and semmantics as well as |

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how to decode snow. |

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The decoding process is precissely described and any compliant decoder |

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MUST produce the exactly same output for a spec conformant snow stream. |

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For encoding though any process which generates a stream compliant to |

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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 |

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snow encoder. |

15 | |

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Definitions: |

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============ |

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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 |

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ilog2(x) is the rounded down logarithm of x with basis 2 |

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ilog2(0) = 0 |

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Type definitions: |

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================= |

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b 1-bit range coded |

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u unsigned scalar value range coded |

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s signed scalar value range coded |

31 | |

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Bitstream syntax: |

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================= |

35 | |

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frame: |

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header |

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prediction |

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residual |

40 | |

41 |
header: |

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keyframe b MID_STATE |

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if(keyframe || always_reset) |

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reset_contexts |

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if(keyframe){ |

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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 |

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} |

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} |

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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 |

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qlogs: |

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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++){ |

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quant_table[plane][level][1]s header_state |

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quant_table[plane][level][3]s header_state |

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} |

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} |

89 | |

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reset_contexts |

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*_state[*]= MID_STATE |

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prediction: |

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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 | |

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block(level): |

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mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0 |

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if(keyframe){ |

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intra=1 |

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}else{ |

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if(level!=max_block_depth){ |

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s_context= 2*left->level + 2*top->level + topleft->level + topright->level |

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leaf b block_state[4 + s_context] |

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} |

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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){ |

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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] |

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}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)] |

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} |

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}else{ |

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block(level+1) |

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block(level+1) |

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block(level+1) |

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block(level+1) |

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} |

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} |

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residual: |

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residual2(luma) |

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residual2(chroma_cr) |

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residual2(chroma_cb) |

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residual2: |

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for(level=0; level<spatial_decomposition_count; level++){ |

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if(level==0) |

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subband(LL, 0) |

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subband(HL, level) |

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subband(LH, level) |

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subband(HH, level) |

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} |

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subband: |

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FIXME |

147 | |

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149 | |

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Tag description: |

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---------------- |

152 | |

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version |

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0 |

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this MUST NOT change within a bitstream |

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always_reset |

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if 1 then the range coder contexts will be reset after each frame |

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temporal_decomposition_type |

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0 |

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temporal_decomposition_count |

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0 |

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spatial_decomposition_count |

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FIXME |

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colorspace_type |

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0 |

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this MUST NOT change within a bitstream |

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chroma_h_shift |

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log2(luma.width / chroma.width) |

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this MUST NOT change within a bitstream |

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chroma_v_shift |

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log2(luma.height / chroma.height) |

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this MUST NOT change within a bitstream |

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spatial_scalability |

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0 |

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max_ref_frames |

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maximum number of reference frames |

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this MUST NOT change within a bitstream |

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update_mc |

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indicates that motion compensation filter parameters are stored in the |

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header |

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diag_mc |

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flag to enable faster diagonal interpolation |

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this SHOULD be 1 unless it turns out to be covered by a valid patent |

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htaps |

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number of half pel interpolation filter taps, MUST be even, >0 and <10 |

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hcoeff |

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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 |

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like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ... |

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the sign of the coefficients is not explicitly stored but alternates |

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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 |

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of all stored coefficients with signs from 32 |

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hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ... |

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a good choice for hcoeff and htaps is |

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htaps= 6 |

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hcoeff={40,-10,2} |

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an alternative which requires more computations at both encoder and |

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decoder side and may or may not be better is |

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htaps= 8 |

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hcoeff={42,-14,6,-2} |

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ref_frames |

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minimum of the number of available reference frames and max_ref_frames |

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for example the first frame after a key frame always has ref_frames=1 |

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spatial_decomposition_type |

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wavelet type |

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0 is a 9/7 symmetric compact integer wavelet |

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1 is a 5/3 symmetric compact integer wavelet |

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others are reserved |

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stored as delta from last, last is reset to 0 if always_reset || keyframe |

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qlog |

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quality (logarthmic quantizer scale) |

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stored as delta from last, last is reset to 0 if always_reset || keyframe |

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mv_scale |

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stored as delta from last, last is reset to 0 if always_reset || keyframe |

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FIXME check that everything works fine if this changes between frames |

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qbias |

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dequantization bias |

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stored as delta from last, last is reset to 0 if always_reset || keyframe |

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block_max_depth |

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maximum depth of the block tree |

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stored as delta from last, last is reset to 0 if always_reset || keyframe |

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quant_table |

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quantiztation table |

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247 | |

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Highlevel bitstream structure: |

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============================= |

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-------------------------------------------- |

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| Header | |

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-------------------------------------------- |

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| ------------------------------------ | |

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| | Block0 | | |

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| | 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 : | | |

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| | ......... : cr DC : : motion y : | | |

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| | ....... .......... | | |

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| ------------------------------------ | |

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| ------------------------------------ | |

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| | Block1 | | |

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| ... | |

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-------------------------------------------- |

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| ------------ ------------ ------------ | |

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|| Y subbands | | Cb subbands| | Cr subbands|| |

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|| --- --- | | --- --- | | --- --- || |

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|| |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| || |

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|| --- --- | | --- --- | | --- --- || |

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|| --- --- | | --- --- | | --- --- || |

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|| |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| || |

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|| --- --- | | --- --- | | --- --- || |

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|| --- --- | | --- --- | | --- --- || |

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|| |HL1||LH1| | | |HL1||LH1| | | |HL1||LH1| || |

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|| --- --- | | --- --- | | --- --- || |

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|| --- --- | | --- --- | | --- --- || |

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|| |HH1||HL2| | | |HH1||HL2| | | |HH1||HL2| || |

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|| ... | | ... | | ... || |

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| ------------ ------------ ------------ | |

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-------------------------------------------- |

285 | |

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Decoding process: |

287 |
================= |

288 | |

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------------ |

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| | |

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| Subbands | |

292 |
------------ | | |

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| | ------------ |

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| Intra DC | | |

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| | LL0 subband prediction |

296 |
------------ | |

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\ Dequantizaton |

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------------------- \ | |

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| Reference frames | \ IDWT |

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| ------- ------- | Motion \ | |

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||Frame 0| |Frame 1|| Compensation . OBMC v ------- |

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| ------- ------- | --------------. \------> + --->|Frame n|-->output |

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| ------- ------- | ------- |

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||Frame 2| |Frame 3||<----------------------------------/ |

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| ... | |

306 |
------------------- |

307 | |

308 | |

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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 |

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the image in which case they are set to the Null block |

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318 |
top-left is set to the top left block unless it is outside of the image in |

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which case it is set to the left block |

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if this block has no larger parent block or it is at the left side of its |

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parent block and the top right block is not outside of the image then the |

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top right block is used for top-right else the top-left block is used |

324 | |

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Null block |

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y,cb,cr are 128 |

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level, ref, mx and my are 0 |

328 | |

329 | |

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Motion Vector Prediction: |

331 |
========================= |

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1. the motion vectors of all the neighboring blocks are scaled to |

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compensate for the difference of reference frames |

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

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2. the median of the scaled left, top and top-right vectors is used as |

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motion vector prediction |

339 | |

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3. the used motion vector is the sum of the predictor and |

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(mvx_diff, mvy_diff)*mv_scale |

342 | |

343 | |

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Intra DC Predicton: |

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====================== |

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the luma and chroma values of the left block are used as predictors |

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the used luma and chroma is the sum of the predictor and y_diff, cb_diff, cr_diff |

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to reverse this in the decoder apply the following: |

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block[y][x].dc[0] = block[y][x-1].dc[0] + y_diff; |

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block[y][x].dc[1] = block[y][x-1].dc[1] + cb_diff; |

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block[y][x].dc[2] = block[y][x-1].dc[2] + cr_diff; |

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block[*][-1].dc[*]= 128; |

354 | |

355 | |

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Motion Compensation: |

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

358 | |

359 |
Halfpel interpolation: |

360 |
---------------------- |

361 |
halfpel interpolation is done by convolution with the halfpel filter stored |

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in the header: |

363 | |

364 |
horizontal halfpel samples are found by |

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H1[y][x] = hcoeff[0]*(F[y][x ] + F[y][x+1]) |

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+ hcoeff[1]*(F[y][x-1] + F[y][x+2]) |

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+ hcoeff[2]*(F[y][x-2] + F[y][x+3]) |

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+ ... |

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h1[y][x] = (H1[y][x] + 32)>>6; |

370 | |

371 |
vertical halfpel samples are found by |

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H2[y][x] = hcoeff[0]*(F[y ][x] + F[y+1][x]) |

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+ hcoeff[1]*(F[y-1][x] + F[y+2][x]) |

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+ ... |

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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 |
+ ... |

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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 |

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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 |