7. Parsing Process

Questa sezione conserva il testo RFC su APV, includendo bitstream syntax, syntax element processing, decoding and parsing processes, metadata, profiles, levels, bands, raw bitstream format e implementation references.

Testo RFC originale

7.  Parsing Process

7.1.  Process for Syntax Element Type h(v)

   This process is invoked for the parsing of syntax elements with
   descriptor h(v) in Section 5.3.15 and Section 5.3.16.

7.1.1.  Process for abs_dc_coeff_diff

   Inputs to this process are bits for the abs_dc_coeff_diff syntax
   element.  Output of this process is a value of the abs_dc_coeff_diff
   syntax element.  The variable kParam is derived as follows:

      kParam = clip(0, 5, PrevDcDiff >> 1)

   The value of syntax element abs_dc_coeff_diff is obtained by invoking
   the parsing process for variable-length codes as specified in
   Section 7.1.4 with kParam.

7.1.2.  Process for coeff_zero_run

   Inputs to this process are bits for the coeff_zero_run syntax
   element.

   Output of this process is a value of the coeff_zero_run syntax
   element.

   The variable kParam is derived as follows:

      kParam = clip(0, 2, PrevRun >> 2)

   The value of syntax element coeff_zero_run is obtained by invoking
   the parsing process for variable-length codes as specified in
   Section 7.1.4 with kParam.

7.1.3.  Process for abs_ac_coeff_minus1

   Inputs to this process are bits for the abs_ac_coeff_minus1 syntax
   element.

   Output of this process is a value of the abs_ac_coeff_minus1 syntax
   element.

   The variable kParam is derived as follows:

      kParam = clip(0, 4, PrevLevel >> 2)

   The value of syntax element abs_ac_coeff_minus1 is obtained by
   invoking the parsing process for variable-length codes as specified
   in Section 7.1.4 with kParam.

7.1.4.  Process for Variable-Length Codes

   Input to this process is kParam.

   Output of this process is a value, symbolValue, of a syntax element.

   The symbolValue is derived as follows:

   symbolValue = 0
   parseExpGolomb = 1
   k = kParam
   stopLoop = 0

   if(read_bits(1) == 1){
     parseExpGolomb = 0
   }
   else{
     if(read_bits (1) == 0){
       symbolValue += (1 << k)
       parseExpGolomb = 0
     }
     else{
       symbolValue += (2 << k)
       parseExpGolomb = 1
     }
   }

   if(parseExpGolomb){
     do{
       if(read_bits(1) == 1){
         stopLoop = 1
       }
       else{
         symbolValue += (1 << k)
         k++
       }
     } while(!stopLoop)
   }

   if(k > 0)
     symbolValue += read_bits(k)

                 Figure 26: Parsing process of symbolValue

   where the value returned from read_bits(n) is interpreted as a binary
   representation of an n-bit unsigned integer with the most significant
   bit written first.

7.2.  Codeword Generation Process for h(v) (Informative)

   This process specifies the code generation process for syntax
   elements with descriptor h(v).

7.2.1.  Process for abs_dc_coeff_diff

   Input to this process is a symbol value of the abs_dc_coeff_diff
   syntax element.

   Output of this process is a codeword of the abs_dc_coeff_diff syntax
   element.

   The variable kParam is derived as follows:

      kParam = clip(0, 5, PrevDcDiff >> 1)

   The codeword of syntax element abs_dc_coeff_diff is obtained by
   invoking the generation process for variable-length codes as
   specified in Section 7.2.4 with the symbol value symbolValue and
   kParam.

7.2.2.  Process for coeff_zero_run

   Input to this process is a symbol value of the coeff_zero_run syntax
   element.

   Output of this process is a codeword of the coeff_zero_run syntax
   element.

   The variable kParam is derived as follows:

      kParam = clip(0, 2, PrevRun >> 2)

   The codeword of syntax element coeff_zero_run is obtained by invoking
   the generation process for variable-length codes as specified in
   Section 7.2.4 with the symbol value symbolValue and kParam.

7.2.3.  Process for abs_ac_coeff_minus1

   Input to this process is a symbol value of the abs_ac_coeff_minus1
   syntax element.

   Output of this process is a codeword of the abs_ac_coeff_minus1
   syntax element.

   The variable kParam is derived as follows:

      kParam = clip(0, 4, PrevLevel >> 2)

   The codeword of syntax element abs_ac_coeff_minus1 is obtained by
   invoking the generation for variable-length codes as specified in
   Section 7.2.4 with the symbol value symbolValue and kParam.

7.2.4.  Process for Variable-Length Codes

   Inputs to this process are symbolVal and kParam

   Output of this process is a codeword of a syntax element.

   The codeword is derived as follows:

   PrefixVLCTable[3][2] = {{1, 0}, {0, 0}, {0, 1}}

   symbolValue = symbolVal
   valPrefixVLC = clip(0, 2, symbolVal >> kParam)
   bitCount = 0
   k = kParam

   while(symbolValue >= (1 << k)){
     symbolValue -= (1 << k)
     if(bitCount < 2)
       put_bits(PrefixVLCTable[valPrefixVLC][bitCount], 1)
     else
       put_bits(0, 1)
     if(bitCount >= 2)
       k++
     bitCount++
   }

   if(bitCount < 2)
     put_bits(PrefixVLCTable[valPrefixVLC][bitCount], 1)
   else
     put_bits(1, 1)

   if(k > 0)
     put_bits(symbolValue, k)

                Figure 27: Generating bits from symbolValue

   where a codeword generated from put_bits(v, n) is interpreted as a
   binary representation of an n-bit unsigned integer value v with the
   most significant bit written first.