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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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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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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
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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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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:
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=================
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                                         ------------
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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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Range Coder:
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============
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FIXME
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Neighboring Blocks:
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===================
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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 90b5b51e Diego Biurrun
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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321 90b5b51e Diego Biurrun
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
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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
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Motion Vector Prediction:
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=========================
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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
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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
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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;
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Motion Compensation:
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====================
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359
Halfpel interpolation:
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----------------------
361
halfpel interpolation is done by convolution with the halfpel filter stored
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in the header:
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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;
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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;
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vertical+horizontal halfpel samples are found by
378
H3[y][x] =    hcoeff[0]*(H2[y][x  ] + H2[y][x+1])
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            + hcoeff[1]*(H2[y][x-1] + H2[y][x+2])
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            + ...
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H3[y][x] =    hcoeff[0]*(H1[y  ][x] + H1[y+1][x])
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            + hcoeff[1]*(H1[y+1][x] + H1[y+2][x])
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            + ...
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h3[y][x] = (H3[y][x] + 2048)>>12;
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                   F   H1  F
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                   |   |   |
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                   |   |   |
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                   |   |   |
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                   F   H1  F
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                   |   |   |
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                   |   |   |
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                   |   |   |
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   F-------F-------F-> H1<-F-------F-------F
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                   v   v   v
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                  H2   H3  H2
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                   ^   ^   ^
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   F-------F-------F-> H1<-F-------F-------F
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                   |   |   |
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                   |   |   |
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                   |   |   |
403
                   F   H1  F
404
                   |   |   |
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                   |   |   |
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                   |   |   |
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                   F   H1  F
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unavailable fullpel samples (outside the picture for example) shall be equal
411
to the closest available fullpel sample
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Smaller pel interpolation:
415
--------------------------
416
if diag_mc is set then points which lie on a line between 2 vertically,
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horiziontally or diagonally adjacent halfpel points shall be interpolated
418
linearls with rounding to nearest and halfway values rounded up.
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points which lie on 2 diagonals at the same time should only use the one
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diagonal not containing the fullpel point
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           F-->O---q---O<--h1->O---q---O<--F
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           v \           / v \           / v
426
           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
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           ^ /           \ ^ /           \ ^
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           F-->O---q---O<--h1->O---q---O<--F
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the remaining points shall be bilinearly interpolated from the
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up to 4 surrounding halfpel and fullpel points, again rounding should be to
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nearest and halfway values rounded up
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448
compliant snow decoders MUST support 1-1/8 pel luma and 1/2-1/16 pel chroma
449
interpolation at least
450
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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.
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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
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473
Dequantizaton:
474
==============
475
FIXME
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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
 ---------------    ---------------    ---------------    ---------------
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|LL0|HL0|       |  |   |   |       |  |       |       |  |       |       |
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|---+---|  HL1  |  | L0|H0 |  HL1  |  |  LL1  |  HL1  |  |       |       |
494 09671ce7 Michael Niedermayer
|LH0|HH0|       |  |   |   |       |  |       |       |  |       |       |
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|-------+-------|->|-------+-------|->|-------+-------|->|   L1  |  H1   |->...
496
|       |       |  |       |       |  |       |       |  |       |       |
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|  LH1  |  HH1  |  |  LH1  |  HH1  |  |  LH1  |  HH1  |  |       |       |
498
|       |       |  |       |       |  |       |       |  |       |       |
499
 ---------------    ---------------    ---------------    ---------------
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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
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2. s[i] += (s[i-1] + s[i+1]    )>>1; for all odd  i < w
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\ | /|\ | /|\ | /|\ | /|\
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 \|/ | \|/ | \|/ | \|/ |
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  +  |  +  |  +  |  +  |   -1/4
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 /|\ | /|\ | /|\ | /|\ |
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/ | \|/ | \|/ | \|/ | \|/
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  |  +  |  +  |  +  |  +   +1/2
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523
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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
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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
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\ | /|\ | /|\ | /|\ | /|\
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 \|/ | \|/ | \|/ | \|/ |
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  +  |  +  |  +  |  +  |   -3/8
533
 /|\ | /|\ | /|\ | /|\ |
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/ | \|/ | \|/ | \|/ | \|/
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 (|  + (|  + (|  + (|  +   -1
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\ + /|\ + /|\ + /|\ + /|\  +1/4
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 \|/ | \|/ | \|/ | \|/ |
538
  +  |  +  |  +  |  +  |   +1/16
539
 /|\ | /|\ | /|\ | /|\ |
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/ | \|/ | \|/ | \|/ | \|/
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
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3. lifting step
558
s[i-1] + s[i+1]                                 17bit
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560
4. lifiting step
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3*(s[i-1] + s[i+1])                             17bit
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564 78954a05 Michael Niedermayer
TODO:
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=====
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Important:
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finetune initial contexts
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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
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Credits:
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========
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Michael Niedermayer
584
Loren Merritt
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Copyright:
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==========
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GPL + GFDL + whatever is needed to make this a RFC