Comments on text of VP9 Bitstream & Decoding Process Specification v0.6

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Albert Liu

Nov 5, 2021, 7:38:37 PM11/5/21
to Codec Developers
Reading the VP9 spec went smoothly for the most part, however I had some comments:
  • 3 Symbols (and abbreviated terms): SEG_LVL_*, DCT/ADST options, and reset_frame_context options could be expressed as enums.
  • 3 Symbols (and abbreviated terms): Perhaps add symbols for the number of frame contexts, maximum number of subblocks per mode info, number of planes, and number of token types.
  • 6.1 Frame syntax: Perhaps header_size_in_bytes could be renamed to compressed_header_bytes to avoid confusion with headerBytes.
  • 6.2 Uncompressed header syntax: Many of the loop bounds in this call tree are expressed as integer literals rather than the corresponding symbols.
  • 6.4.24 Token syntax: The variable Tokens is really the coefs derived from the tokens rather than the tokens themselves.
  • 7.4.16 Token semantics: The names DCT_VAL_CAT1 etc. are used here, but in 9.3.1 Tree selection process, they are called DCT_VAL_CATEGORY1 etc.
  • 9.3.2 Probability selection process: Under "The probability depends on the value of FrameIsIntra" the two cases are reversed. I.e. If FrameIsIntra is equal to 0, the non-keyframe probabilities should be used.
  • B.2.1 Superframe index: frame_sizes[ i ] is specified as having type f(SzBytes). However, f() is supposed to take in a number of bits, not a number of bytes. Furthermore, it appears that the bytes are supposed to be stored in little-endian order rather than the big-endian order implied by f().
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