picture-dec.tex

   1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   2 % - This chapter defines the overall process  - %
   3 % - for decoding a picture                    - %
   4 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   5
   6 \label{picturedec}
   7
   8 This section defines the processes for decoding a picture from a Dirac stream.
   9
  10 Picture decoding depends upon correctly parsing the stream, and decoding operations
  11  are dependent on decoding the sequence header and picture metadata
  12 (Section \ref{sequenceheader} and \ref{picturesyntax}) and unpacking the coefficient
  13 and motion data (Sections \ref{wltunpacking} and \ref{motiondec}).
  14
  15 \subsection{Overall picture decoding process}
  16 \label{overallpicturedec}
  17
  18 Picture data from the current picture being decoded is stored in the $\CurrentPicture$ state
  19 variable, which is a map with labels $\PicNum$, and $Y$, $C1$ and $C2$ representing
  20 luma and chroma data.
  21
  22 After decoding the decoded picture is returned to the decoding application.
  23
  24 The $picture\_decode()$ process shall be invoked after parsing the $picture\_parse()$ process and shall be defined as follows:
  25
  26 \begin{pseudo}{picture\_decode}{}
  27 \bsCODE{\CurrentPicture=\{\}}
  28 \bsCODE{\CurrentPicture[\PicNum]=\PictureNumber}
  29 \bsIF{is\_ref(()}
  30     \bsCODE{ref\_picture\_remove()}{\ref{refbuffer}}
  31 \bsEND
  32 \bsIF{\ZeroResidual==\false}
  33     \bsCODE{inverse\_wavelet\_transform()}{\ref{idwt}}
  34 \bsELSE
  35     \bsCODE{\CurrentPicture[Y]=\bf{0}}
  36     \bsCODE{\CurrentPicture[C1]=\bf{0}}
  37     \bsCODE{\CurrentPicture[C2]=\bf{0}}
  38 \bsEND
  39 \bsIF{is\_inter()}
  40     \bsCODE{ref1=get\_ref(\RefOneNum)}{\ref{refbuffer}}
  41     \bsIF{num\_refs()==2}{\ref{parsecodevalues}}
  42         \bsCODE{ref2=get\_ref(\RefTwoNum)}{\ref{refbuffer}}
  43     \bsEND
  44     \bsCODE{motion\_compensate(ref1[Y], ref2[Y],  \CurrentPicture[Y], Y)}{\ref{motioncompensate}}
  45     \bsCODE{motion\_compensate(ref1[C1], ref2[C1],  \CurrentPicture[C1], C1)}{\ref{motioncompensate}}
  46     \bsCODE{motion\_compensate(ref1[C2], ref2[C2],  \CurrentPicture[C2], C2)}{\ref{motioncompensate}}
  47 \bsEND
  48 \bsCODE{clip\_picture()}{\ref{pictureclip}}
  49 \bsIF{is\_ref()}
  50     \bsCODE{ref\_picture\_add()}{\ref{refbuffer}}
  51 \bsEND
  52 \bsCODE{offset\_output\_picture(\CurrentPicture)}{\ref{videooutput}}
  53 \bsRET{\CurrentPicture}
  54 \end{pseudo}
  55
  56
  57 \subsection{Picture reordering}
  58 Picture numbers within the stream may not be in numerical order, and
  59 subsequent reordering may be required: the size
  60 of the decoded picture buffer required to perform any such reordering may be
  61 specified as part of the application profile and level (Annex~\ref{profilelevel}).
  62
  63 \subsection{Random access}
  64 \label{randomaccess}
  65 Sequence headers represent safe entry points for decoding a sequence.
  66
  67 An accessible picture (with reference to a given sequence header)
  68 shall be defined as a picture decodeable without dependence on
  69 to data prior to the sequence header in coded order.
  70
  71 Accessibility should normally imply that each accessible picture has
  72 no reference picture prior to the sequence header, and no chain of
  73 references leading to a reference picture prior to the sequence header.
  74 A given level may allow this condition to
  75 be relaxed (for example, in P-only coding where unavailable references
  76 may be substituted for by zero pictures), but where no specific provision
  77 to the contrary is specified in an applicable level or profile, it shall
  78 apply.
  79
  80 The first picture data unit after a sequence header shall be called the
  81 access picture and shall be accessible with respect to the sequence header.
  82 It should normally be an intra picture. If the sequence contains inter pictures
  83 it should normally be an intra reference picture.
  84
  85 All picture data units subsequent to the sequence header in coded order
  86 shall also be accessible with respect to the sequence header
  87 if their picture numbers are greater than
  88 or equal to that of the access picture. The access picture therefore represents
  89 a temporal access point into the sequence.
  90
  91 \begin{informative}
  92
  93 If a sequence satisfies a maximum reordering depth constraint
  94 (Annex~\ref{picturereordering})
  95 of size $N$ all pictures more than $N$ pictures later than the sequence header will
  96 have larger picture numbers than the first picture after the sequence header, and hence will be accessible. A reordering depth constraint thus implies that after a
  97 sequence header at most $N$ pictures will need to be discarded before all pictures are decodeable.
  98 \end{informative}
  99 \subsection{Reference picture buffer management}
 100 \label{refbuffer}
 101
 102 This section specifies how the Dirac stream data shall be used to manage the reference
 103 picture buffer $\RefBuffer$. The reference picture buffer has a maximum size of
 104 $\RefBufferSize$ elements, as set in the applicable level (Annex~\ref{profilelevel}).
 105
 106 The $ref\_picture\_remove()$ process shall be defined as
 107 follows:
 108
 109 \begin{pseudo}{ref\_picture\_remove}{}
 110 \bsCODE{n=\RetiredPicture}
 111 \bsFOR{k=0}{\RefBufferSize-1}
 112    \bsIF{\RefBuffer[k][\PicNum]==n}
 113         \bsFOR{j=k}{\RefBufferSize-2}
 114             \bsCODE{\RefBuffer[j]=\RefBuffer[j+1]}
 115         \bsEND
 116         \bsCODE{\RefBufferSize -= 1}
 117     \bsEND
 118 \bsEND
 119 \end{pseudo}
 120
 121 The $get\_ref(n)$ function shall returns the reference picture in the buffer with
 122 picture number $n$.  If there is no such picture it shall return an all-zero picture.
 123
 124 The $ref\_picture\_add()$ process for adding pictures to the reference picture
 125 buffer shall proceed according to the following rules:
 126
 127 {\bf Case 1.} If the reference picture buffer is not full i.e. has fewer than $\MaxRefBufferSize$ elements,
 128 then add $\CurrentPicture$ to the end of the buffer.
 129
 130 {\bf Case 2.} If the reference picture is full i.e. it has $\MaxRefBufferSize$ elements, then remove the
 131 first (i.e. oldest) element of the buffer, $\RefBuffer[0]$, set
 132 \[\RefBuffer[i] = \RefBuffer[i+1] \]
 133 for $i=0$ to $\RefBufferSize-2$, and set the last element $\RefBuffer[\RefBufferSize-1]$ equal to
 134 a copy of $\CurrentPicture$.
 135
 136 \input{idwt}
 137
 138 \subsection{Motion compensation}
 139 \input{mc}
 140
 141 \subsection{Clipping}
 142 \label{pictureclip}
 143
 144 Picture data must be clipped prior to being output or being
 145 used as a reference:
 146
 147 \begin{pseudo}{clip\_picture}{}
 148 \bsFOREACH{c}{Y,C1,C2}
 149     \bsCODE{clip\_component(\CurrentPicture[c])}
 150 \bsEND
 151 \end{pseudo}
 152
 153
 154 \begin{pseudo}{clip\_component}{comp\_data,c}
 155 \bsIF{c==Y}
 156     \bsCODE{bit\_depth=\LumaDepth}
 157 \bsELSE
 158     \bsCODE{bit\_depth=\ChromaDepth}
 159 \bsEND
 160 \bsFOR{y=0}{\height(comp\_data)-1}
 161     \bsFOR{x=0}{\width(comp\_data)-1}
 162         \bsCODE{data = \clip(comp\_data[y][x], -2^{bit\_depth-1}, 2^{bit\_depth-1}-1)}
 163      \bsEND
 164 \bsEND
 165 \end{pseudo}
 166
 167 \begin{informative}
 168 Note that clipping is incorporated into motion compensation, so that strictly speaking additional
 169 clipping is only required for intra pictures.
 170 \end{informative}
 171
 172 \subsection{Video output ranges}
 173 \label{videooutput}
 174
 175 Video output data ranges are deemed to be non-negative, so that the offset and excursion
 176  values may be applied by subsequent processing. Since decoded video data is bipolar, it must be suitably offset before output:
 177
 178 \begin{pseudo}{offset\_output\_data}{picture\_data}
 179 \bsFOREACH{c}{Y, C1, C2}
 180     \bsIF{c==Y}
 181         \bsCODE{bit\_depth=\LumaDepth}
 182     \bsELSE
 183         \bsCODE{bit\_depth=\ChromaDepth}
 184     \bsEND
 185     \bsCODE{comp=picture\_data[c]}
 186     \bsFOR{y=0}{\height(comp)-1}
 187         \bsFOR{x=0}{\width(comp)-1}
 188             \bsCODE{comp[y][x]+=2^{bit\_depth-1}}
 189         \bsEND
 190     \bsEND
 191 \bsEND
 192 \end{pseudo}