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1 78954a05 Michael Niedermayer
=============================================
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SNOW Video Codec Specification Draft 20070103
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=============================================
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5 11de04d8 Michael Niedermayer
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.
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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
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32
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Bitstream syntax:
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=================
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frame:
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    header
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    prediction
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    residual
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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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    }
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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)
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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
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Tag description:
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----------------
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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
234 24dbec7c Luca Barbato
    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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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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 --------------------------------------------
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Decoding process:
287
=================
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289
                                         ------------
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                                        |            |
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                                        |  Subbands  |
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                   ------------         |            |
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                  |            |         ------------
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                  |  Intra DC  |               |
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                  |            |    LL0 subband prediction
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                   ------------                |
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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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|        ...        |
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 -------------------
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309 78954a05 Michael Niedermayer
Range Coder:
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============
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FIXME
312 e5635270 lars.taeuber@web.de
The implemented range coder is an adapted version based upon "Range encoding:
313
an algorithm for removing redundancy from a digitised message." by G. N. N.
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Martin.
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The symbols encoded by the ffmpeg range coder are bits (0|1). The
316 e5635270 lars.taeuber@web.de
associated probabilities are not fix but change depending on the symbol mix
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seen so far.
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bit seen | new state
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---------+--------------------------------------------
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    0    | 256 - state_transition_table[256 - old_state];
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    1    |       state_transition_table[      old_state];
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state_transition_table = {
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  0,   0,   0,   0,   0,   0,   0,   0,  20,  21,  22,  23,  24,  25,  26,  27,
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 28,  29,  30,  31,  32,  33,  34,  35,  36,  37,  37,  38,  39,  40,  41,  42,
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 43,  44,  45,  46,  47,  48,  49,  50,  51,  52,  53,  54,  55,  56,  56,  57,
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 58,  59,  60,  61,  62,  63,  64,  65,  66,  67,  68,  69,  70,  71,  72,  73,
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 74,  75,  75,  76,  77,  78,  79,  80,  81,  82,  83,  84,  85,  86,  87,  88,
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 89,  90,  91,  92,  93,  94,  94,  95,  96,  97,  98,  99, 100, 101, 102, 103,
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104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 114, 115, 116, 117, 118,
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119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 133,
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134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149,
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150, 151, 152, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164,
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165, 166, 167, 168, 169, 170, 171, 171, 172, 173, 174, 175, 176, 177, 178, 179,
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180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 190, 191, 192, 194, 194,
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195, 196, 197, 198, 199, 200, 201, 202, 202, 204, 205, 206, 207, 208, 209, 209,
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210, 211, 212, 213, 215, 215, 216, 217, 218, 219, 220, 220, 222, 223, 224, 225,
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226, 227, 227, 229, 229, 230, 231, 232, 234, 234, 235, 236, 237, 238, 239, 240,
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241, 242, 243, 244, 245, 246, 247, 248, 248,   0,   0,   0,   0,   0,   0,   0};
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343
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Neighboring Blocks:
345
===================
346
left and top are set to the respective blocks unless they are outside of
347
the image in which case they are set to the Null block
348
349 90b5b51e Diego Biurrun
top-left is set to the top left block unless it is outside of the image in
350 78954a05 Michael Niedermayer
which case it is set to the left block
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352 90b5b51e Diego Biurrun
if this block has no larger parent block or it is at the left side of its
353 78954a05 Michael Niedermayer
parent block and the top right block is not outside of the image then the
354
top right block is used for top-right else the top-left block is used
355
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Null block
357
y,cb,cr are 128
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level, ref, mx and my are 0
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360
361
Motion Vector Prediction:
362
=========================
363
1. the motion vectors of all the neighboring blocks are scaled to
364
compensate for the difference of reference frames
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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
369
motion vector prediction
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3. the used motion vector is the sum of the predictor and
372
   (mvx_diff, mvy_diff)*mv_scale
373
374
375
Intra DC Predicton:
376
======================
377
the luma and chroma values of the left block are used as predictors
378
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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;
382
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;
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386
387
Motion Compensation:
388
====================
389 e9314de6 Michael Niedermayer
390
Halfpel interpolation:
391
----------------------
392
halfpel interpolation is done by convolution with the halfpel filter stored
393
in the header:
394
395
horizontal halfpel samples are found by
396
H1[y][x] =    hcoeff[0]*(F[y][x  ] + F[y][x+1])
397
            + hcoeff[1]*(F[y][x-1] + F[y][x+2])
398
            + hcoeff[2]*(F[y][x-2] + F[y][x+3])
399
            + ...
400
h1[y][x] = (H1[y][x] + 32)>>6;
401
402
vertical halfpel samples are found by
403
H2[y][x] =    hcoeff[0]*(F[y  ][x] + F[y+1][x])
404
            + hcoeff[1]*(F[y-1][x] + F[y+2][x])
405
            + ...
406
h2[y][x] = (H2[y][x] + 32)>>6;
407
408
vertical+horizontal halfpel samples are found by
409
H3[y][x] =    hcoeff[0]*(H2[y][x  ] + H2[y][x+1])
410
            + hcoeff[1]*(H2[y][x-1] + H2[y][x+2])
411
            + ...
412
H3[y][x] =    hcoeff[0]*(H1[y  ][x] + H1[y+1][x])
413
            + hcoeff[1]*(H1[y+1][x] + H1[y+2][x])
414
            + ...
415
h3[y][x] = (H3[y][x] + 2048)>>12;
416
417
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                   F   H1  F
419
                   |   |   |
420
                   |   |   |
421
                   |   |   |
422
                   F   H1  F
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                   |   |   |
424
                   |   |   |
425
                   |   |   |
426
   F-------F-------F-> H1<-F-------F-------F
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                   v   v   v
428
                  H2   H3  H2
429
                   ^   ^   ^
430
   F-------F-------F-> H1<-F-------F-------F
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                   |   |   |
432
                   |   |   |
433
                   |   |   |
434
                   F   H1  F
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                   |   |   |
436
                   |   |   |
437
                   |   |   |
438
                   F   H1  F
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440
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unavailable fullpel samples (outside the picture for example) shall be equal
442
to the closest available fullpel sample
443
444
445
Smaller pel interpolation:
446
--------------------------
447
if diag_mc is set then points which lie on a line between 2 vertically,
448
horiziontally or diagonally adjacent halfpel points shall be interpolated
449
linearls with rounding to nearest and halfway values rounded up.
450
points which lie on 2 diagonals at the same time should only use the one
451
diagonal not containing the fullpel point
452
453
454
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           F-->O---q---O<--h1->O---q---O<--F
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           v \           / v \           / v
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           O   O       O   O   O       O   O
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           |         /     |     \         |
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           q       q       q       q       q
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           |     /         |         \     |
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           O   O       O   O   O       O   O
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           ^ /           \ ^ /           \ ^
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          h2-->O---q---O<--h3->O---q---O<--h2
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           v \           / v \           / v
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           O   O       O   O   O       O   O
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           |     \         |         /     |
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           q       q       q       q       q
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           |         \     |     /         |
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           O   O       O   O   O       O   O
470
           ^ /           \ ^ /           \ ^
471
           F-->O---q---O<--h1->O---q---O<--F
472
473
474
475
the remaining points shall be bilinearly interpolated from the
476 a11dc59a Michael Niedermayer
up to 4 surrounding halfpel and fullpel points, again rounding should be to
477
nearest and halfway values rounded up
478 e9314de6 Michael Niedermayer
479
compliant snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma
480
interpolation at least
481
482
483
Overlapped block motion compensation:
484
-------------------------------------
485 78954a05 Michael Niedermayer
FIXME
486
487
LL band prediction:
488
===================
489 1e37b7e4 Michael Niedermayer
Each sample in the LL0 subband is predicted by the median of the left, top and
490
left+top-topleft samples, samples outside the subband shall be considered to
491
be 0. To reverse this prediction in the decoder apply the following.
492
for(y=0; y<height; y++){
493
    for(x=0; x<width; x++){
494
        sample[y][x] += median(sample[y-1][x],
495
                               sample[y][x-1],
496
                               sample[y-1][x]+sample[y][x-1]-sample[y-1][x-1]);
497
    }
498
}
499
sample[-1][*]=sample[*][-1]= 0;
500
width,height here are the width and height of the LL0 subband not of the final
501
video
502
503 78954a05 Michael Niedermayer
504
Dequantizaton:
505
==============
506
FIXME
507
508
Wavelet Transform:
509
==================
510 fdb99704 Michael Niedermayer
511
Snow supports 2 wavelet transforms, the symmetric biorthogonal 5/3 integer
512
transform and a integer approximation of the symmetric biorthogonal 9/7
513
daubechies wavelet.
514
515 09671ce7 Michael Niedermayer
2D IDWT (inverse discrete wavelet transform)
516
--------------------------------------------
517
The 2D IDWT applies a 2D filter recursively, each time combining the
518
4 lowest frequency subbands into a single subband until only 1 subband
519
remains.
520
The 2D filter is done by first applying a 1D filter in the vertical direction
521
and then applying it in the horizontal one.
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 ---------------    ---------------    ---------------    ---------------
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|LL0|HL0|       |  |   |   |       |  |       |       |  |       |       |
524 7397cf3f Michael Niedermayer
|---+---|  HL1  |  | L0|H0 |  HL1  |  |  LL1  |  HL1  |  |       |       |
525 09671ce7 Michael Niedermayer
|LH0|HH0|       |  |   |   |       |  |       |       |  |       |       |
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|-------+-------|->|-------+-------|->|-------+-------|->|   L1  |  H1   |->...
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|       |       |  |       |       |  |       |       |  |       |       |
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|  LH1  |  HH1  |  |  LH1  |  HH1  |  |  LH1  |  HH1  |  |       |       |
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|       |       |  |       |       |  |       |       |  |       |       |
530
 ---------------    ---------------    ---------------    ---------------
531
532
533
1D Filter:
534
----------
535
1. interleave the samples of the low and high frequency subbands like
536
s={L0, H0, L1, H1, L2, H2, L3, H3, ... }
537
note, this can end with a L or a H, the number of elements shall be w
538
s[-1] shall be considered equivalent to s[1  ]
539
s[w ] shall be considered equivalent to s[w-2]
540
541
2. perform the lifting steps in order as described below
542
543
5/3 Integer filter:
544
1. s[i] -= (s[i-1] + s[i+1] + 2)>>2; for all even i < w
545
2. s[i] += (s[i-1] + s[i+1]    )>>1; for all odd  i < w
546
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\ | /|\ | /|\ | /|\ | /|\
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 \|/ | \|/ | \|/ | \|/ |
549
  +  |  +  |  +  |  +  |   -1/4
550
 /|\ | /|\ | /|\ | /|\ |
551
/ | \|/ | \|/ | \|/ | \|/
552
  |  +  |  +  |  +  |  +   +1/2
553
554
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snows 9/7 Integer filter:
556
1. s[i] -= (3*(s[i-1] + s[i+1])         + 4)>>3; for all even i < w
557
2. s[i] -=     s[i-1] + s[i+1]                 ; for all odd  i < w
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3. s[i] += (   s[i-1] + s[i+1] + 4*s[i] + 8)>>4; for all even i < w
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4. s[i] += (3*(s[i-1] + s[i+1])            )>>1; for all odd  i < w
560
561
\ | /|\ | /|\ | /|\ | /|\
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 \|/ | \|/ | \|/ | \|/ |
563
  +  |  +  |  +  |  +  |   -3/8
564
 /|\ | /|\ | /|\ | /|\ |
565
/ | \|/ | \|/ | \|/ | \|/
566
 (|  + (|  + (|  + (|  +   -1
567
\ + /|\ + /|\ + /|\ + /|\  +1/4
568
 \|/ | \|/ | \|/ | \|/ |
569
  +  |  +  |  +  |  +  |   +1/16
570
 /|\ | /|\ | /|\ | /|\ |
571
/ | \|/ | \|/ | \|/ | \|/
572
  |  +  |  +  |  +  |  +   +3/2
573 fdb99704 Michael Niedermayer
574 a282102d Michael Niedermayer
optimization tips:
575
following are exactly identical
576
(3a)>>1 == a + (a>>1)
577
(a + 4b + 8)>>4 == ((a>>2) + b + 2)>>2
578 78954a05 Michael Niedermayer
579 6a1aa752 Michael Niedermayer
16bit implementation note:
580
The IDWT can be implemented with 16bits, but this requires some care to
581
prevent overflows, the following list, lists the minimum number of bits needed
582
for some terms
583
1. lifting step
584
A= s[i-1] + s[i+1]                              16bit
585
3*A + 4                                         18bit
586
A + (A>>1) + 2                                  17bit
587
588
3. lifting step
589
s[i-1] + s[i+1]                                 17bit
590
591
4. lifiting step
592
3*(s[i-1] + s[i+1])                             17bit
593
594
595 78954a05 Michael Niedermayer
TODO:
596
=====
597
Important:
598
finetune initial contexts
599
flip wavelet?
600
try to use the wavelet transformed predicted image (motion compensated image) as context for coding the residual coefficients
601
try the MV length as context for coding the residual coefficients
602
use extradata for stuff which is in the keyframes now?
603
the MV median predictor is patented IIRC
604 2b6134b3 Michael Niedermayer
implement per picture halfpel interpolation
605 c78fc717 Michael Niedermayer
try different range coder state transition tables for different contexts
606 78954a05 Michael Niedermayer
607
Not Important:
608 c64a8712 Michael Niedermayer
compare the 6 tap and 8 tap hpel filters (psnr/bitrate and subjective quality)
609 78954a05 Michael Niedermayer
spatial_scalability b vs u (!= 0 breaks syntax anyway so we can add a u later)
610
611
612
Credits:
613
========
614
Michael Niedermayer
615
Loren Merritt
616
617
618
Copyright:
619
==========
620
GPL + GFDL + whatever is needed to make this a RFC