snow.txt

ffmpeg移植到symbian的全部源代码

=============================================
Snow Video Codec Specification Draft 20080110
=============================================

Introduction:
=============
This specification describes the Snow bitstream syntax and semantics as
well as the formal Snow decoding process.

The decoding process is described precisely and any compliant decoder
MUST produce the exact same output for a spec-conformant Snow stream.
For encoding, though, any process which generates a stream compliant to
the syntactical and semantic requirements and which is decodable by
the process described in this spec shall be considered a conformant
Snow encoder.

Definitions:
============

MUST    the specific part must be done to conform to this standard
SHOULD  it is recommended to be done that way, but not strictly required

ilog2(x) is the rounded down logarithm of x with basis 2
ilog2(0) = 0

Type definitions:
=================

b   1-bit range coded
u   unsigned scalar value range coded
s   signed scalar value range coded


Bitstream syntax:
=================

frame:
    header
    prediction
    residual

header:
    keyframe                            b   MID_STATE
    if(keyframe || always_reset)
        reset_contexts
    if(keyframe){
        version                         u   header_state
        always_reset                    b   header_state
        temporal_decomposition_type     u   header_state
        temporal_decomposition_count    u   header_state
        spatial_decomposition_count     u   header_state
        colorspace_type                 u   header_state
        chroma_h_shift                  u   header_state
        chroma_v_shift                  u   header_state
        spatial_scalability             b   header_state
        max_ref_frames-1                u   header_state
        qlogs
    }
    if(!keyframe){
        update_mc                       b   header_state
        if(update_mc){
            for(plane=0; plane<2; plane++){
                diag_mc                 b   header_state
                htaps/2-1               u   header_state
                for(i= p->htaps/2; i; i--)
                    |hcoeff[i]|         u   header_state
            }
        }
        update_qlogs                    b   header_state
        if(update_qlogs){
            spatial_decomposition_count u   header_state
            qlogs
        }
    }

    spatial_decomposition_type          s   header_state
    qlog                                s   header_state
    mv_scale                            s   header_state
    qbias                               s   header_state
    block_max_depth                     s   header_state

qlogs:
    for(plane=0; plane<2; plane++){
        quant_table[plane][0][0]        s   header_state
        for(level=0; level < spatial_decomposition_count; level++){
            quant_table[plane][level][1]s   header_state
            quant_table[plane][level][3]s   header_state
        }
    }

reset_contexts
    *_state[*]= MID_STATE

prediction:
    for(y=0; y<block_count_vertical; y++)
        for(x=0; x<block_count_horizontal; x++)
            block(0)

block(level):
    mvx_diff=mvy_diff=y_diff=cb_diff=cr_diff=0
    if(keyframe){
        intra=1
    }else{
        if(level!=max_block_depth){
            s_context= 2*left->level + 2*top->level + topleft->level + topright->level
            leaf                        b   block_state[4 + s_context]
        }
        if(level==max_block_depth || leaf){
            intra                       b   block_state[1 + left->intra + top->intra]
            if(intra){
                y_diff                  s   block_state[32]
                cb_diff                 s   block_state[64]
                cr_diff                 s   block_state[96]
            }else{
                ref_context= ilog2(2*left->ref) + ilog2(2*top->ref)
                if(ref_frames > 1)
                    ref                 u   block_state[128 + 1024 + 32*ref_context]
                mx_context= ilog2(2*abs(left->mx - top->mx))
                my_context= ilog2(2*abs(left->my - top->my))
                mvx_diff                s   block_state[128 + 32*(mx_context + 16*!!ref)]
                mvy_diff                s   block_state[128 + 32*(my_context + 16*!!ref)]
            }
        }else{
            block(level+1)
            block(level+1)
            block(level+1)
            block(level+1)
        }
    }


residual:
    residual2(luma)
    residual2(chroma_cr)
    residual2(chroma_cb)

residual2:
    for(level=0; level<spatial_decomposition_count; level++){
        if(level==0)
            subband(LL, 0)
        subband(HL, level)
        subband(LH, level)
        subband(HH, level)
    }

subband:
    FIXME



Tag description:
----------------

version
    0
    this MUST NOT change within a bitstream

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

temporal_decomposition_type
    0

temporal_decomposition_count
    0

spatial_decomposition_count
    FIXME

colorspace_type
    0
    this MUST NOT change within a bitstream

chroma_h_shift
    log2(luma.width / chroma.width)
    this MUST NOT change within a bitstream

chroma_v_shift
    log2(luma.height / chroma.height)
    this MUST NOT change within a bitstream

spatial_scalability
    0

max_ref_frames
    maximum number of reference frames
    this MUST NOT change within a bitstream

update_mc
    indicates that motion compensation filter parameters are stored in the
    header

diag_mc
    flag to enable faster diagonal interpolation
    this SHOULD be 1 unless it turns out to be covered by a valid patent

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

hcoeff
    half pel interpolation filter coefficients, hcoeff[0] are the 2 middle
    coefficients [1] are the next outer ones and so on, resulting in a filter
    like: ...eff[2], hcoeff[1], hcoeff[0], hcoeff[0], hcoeff[1], hcoeff[2] ...
    the sign of the coefficients is not explicitly stored but alternates
    after each coeff and coeff[0] is positive, so ...,+,-,+,-,+,+,-,+,-,+,...
    hcoeff[0] is not explicitly stored but found by subtracting the sum
    of all stored coefficients with signs from 32
    hcoeff[0]= 32 - hcoeff[1] - hcoeff[2] - ...
    a good choice for hcoeff and htaps is
    htaps= 6
    hcoeff={40,-10,2}
    an alternative which requires more computations at both encoder and
    decoder side and may or may not be better is
    htaps= 8
    hcoeff={42,-14,6,-2}


ref_frames
    minimum of the number of available reference frames and max_ref_frames
    for example the first frame after a key frame always has ref_frames=1

spatial_decomposition_type
    wavelet type
    0 is a 9/7 symmetric compact integer wavelet
    1 is a 5/3 symmetric compact integer wavelet
    others are reserved
    stored as delta from last, last is reset to 0 if always_reset || keyframe

qlog
    quality (logarthmic quantizer scale)
    stored as delta from last, last is reset to 0 if always_reset || keyframe

mv_scale
    stored as delta from last, last is reset to 0 if always_reset || keyframe
    FIXME check that everything works fine if this changes between frames

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

block_max_depth
    maximum depth of the block tree
    stored as delta from last, last is reset to 0 if always_reset || keyframe

quant_table
    quantiztation table


Highlevel bitstream structure:
=============================
 --------------------------------------------
|                   Header                   |
 --------------------------------------------
|    ------------------------------------    |
|   |               Block0               |   |
|   |             split?                 |   |
|   |     yes              no            |   |
|   |  .........         intra?          |   |
|   | : Block01 :    yes         no      |   |
|   | : Block02 :  .......   ..........  |   |
|   | : Block03 : :  y DC : : ref index: |   |
|   | : Block04 : : cb DC : : motion x : |   |
|   |  .........  : cr DC : : motion y : |   |
|   |              .......   ..........  |   |
|    ------------------------------------    |
|    ------------------------------------    |
|   |               Block1               |   |
|                    ...                     |
 --------------------------------------------
| ------------   ------------   ------------ |
|| Y subbands | | Cb subbands| | Cr subbands||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
|| |LL0||HL0| | | |LL0||HL0| | | |LL0||HL0| ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
|| |LH0||HH0| | | |LH0||HH0| | | |LH0||HH0| ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
|| |HL1||LH1| | | |HL1||LH1| | | |HL1||LH1| ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
||  ---  ---  | |  ---  ---  | |  ---  ---  ||
|| |HH1||HL2| | | |HH1||HL2| | | |HH1||HL2| ||
||    ...     | |    ...     | |    ...     ||
| ------------   ------------   ------------ |
 --------------------------------------------

Decoding process:
=================

                                         ------------
                                        |            |
                                        |  Subbands  |
                   ------------         |            |
                  |            |         ------------
                  |  Intra DC  |               |
                  |            |    LL0 subband prediction
                   ------------                |
                                \        Dequantizaton
 -------------------             \             |
|  Reference frames |             \           IDWT
| -------   ------- |    Motion    \           |
||Frame 0| |Frame 1|| Compensation  .   OBMC   v      -------
| -------   ------- | --------------. \------> + --->|Frame n|-->output
| -------   ------- |                                 -------
||Frame 2| |Frame 3||<----------------------------------/
|        ...        |
 -------------------


Range Coder:
============

Binary Range Coder:
-------------------
The implemented range coder is an adapted version based upon "Range encoding:
an algorithm for removing redundancy from a digitised message." by G. N. N.