[FFmpeg-trac] #11551(avcodec:new): [HEVC] Incorrect PCM coding block length derivation for YUV400 (was: [HEVC] Incorrect PCM Coding Block Length Derivation For YUV400)

[FFmpeg]

#11551: [HEVC] Incorrect PCM coding block length derivation for YUV400
------------------------------------+-----------------------------------
             Reporter:  ksthey      |                    Owner:  (none)
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  avcodec
              Version:  git-master  |               Resolution:
             Keywords:  hevc        |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  1           |
------------------------------------+-----------------------------------
Changes (by MasterQuestionable):

 * analyzed:  0 => 1
 * cc: MasterQuestionable (added)
 * component:  ffmpeg => avcodec
 * keywords:  Hevc => hevc
 * summary:  [HEVC] Incorrect PCM Coding Block Length Derivation For YUV400
     => [HEVC] Incorrect PCM coding block length derivation for YUV400

Comment:

 ͏    In "libavcodec/hevc/hevcdec.c", function "hls_pcm_sample":
 ͏    The derivation of length below doesn't consider the case of YUV400
 (!`chroma_format_idc == 0`).
 ͏    Which results in incorrect byte skipped, and parsing errors for later
 blocks: when YUV400 PCM blocks appear.

 ͏
 https://github.com/FFmpeg/FFmpeg/blob/0ce8868878c90153e45648b180cef59a9bcb0109/libavcodec/hevc/hevcdec.c#L1668-L1671
 ͏    Modify the code as below does solve the problem:
 [[
 {{{#!diff
 --- libavcodec/hevc/hevcdec.c   2025-04-02T07:25:38Z
 +++ libavcodec/hevc/hevcdec.c   2025-04-22T03:17:36Z
 @@ -1668,4 +1668,6 @@
      int length = cb_size * cb_size * sps->pcm.bit_depth + (
 +    sps->chroma_format_idc != 0 ? (
      ( cb_size >> sps->hshift[1] ) * ( cb_size >> sps->vshift[1] ) +
      ( cb_size >> sps->hshift[2] ) * ( cb_size >> sps->vshift[2] )
 -    ) * sps->pcm.bit_depth_chroma;
 +    ) * sps->pcm.bit_depth_chroma
 +    : 0 );
 }}}
 ]]
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11551#comment:1>
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