[Mp4-tech] Re: the range of level_prefix in CAVLC

[Nancy Johnson]

Winger and Gary,
  Thanks a lot. Will this limit on level_prefix for High Profile be added to the SPEC or it is derived from the request (ex., section 8.5.10 on Cij) that the AC/DC levels should be within -2^(7+BitDepth) and 2^(7+BitDepth)-1 (for 8-bit video, BitDepth=8)?
  If it is the latter, I analyzed the mechanism for CAVLC and found that when level_prefix=19, it is still possible for level[i] out of range of [-2^15,2^15-1]. See the following for detail:
  According to section 7.3.5.3.1, the max possible value for abs(level[i]) when level_prefix=19 is:
  (((min(15,level_prefix) << suffixLength) + level_suffix + ((1<<(level_prefix-3))-4096) + 4)/2
  Since the max possible value of level_suffix is 2^16-1 for level_prefix=19 and suffixLength <= 6, so the max possible value for abs(leve[i]) is (15<<6 + 2^16-1 + 2^16 - 4096 + 4)/2 = 63969 = 0xF9E1, which is out of range of [-2^15,2^15-1]. If level_prefix=18, the abs(level[i]) will <= (15<<6 + 2^15-1 + 2^15-4096+4)/2=0x79E1, which is in range of [-2^15,2^15-1].
  Could you please point where I missed for the above analysis? Thanks.
  Rgs,
  Nancy
"Winger, Lowell" <Lowell.Winger lsi.com> wrote:
Nancy, for High Profile the maximum level_prefix is 19 for 8-bit video.
Best Regards,
Lowell Winger, LSI Logic
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