[HEVC] Incorrect PCM coding block length derivation for YUV400

[ksthey]

In hevcdec.c, 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.
=============================================================================

int length = cb_size * cb_size * sps->pcm.bit_depth +

(((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;

=============================================================================

Modify the code as below does solve the problem:
=============================================================================

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) : 0);

=============================================================================

[MasterQuestionable]

͏    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:
[[

libavcodec/hevc/hevcdec.c

@@ -1668 +1668 @@
     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 );

]]

[Balling]

Can you send a patch then?

[MasterQuestionable]

͏    Unfamiliar with the committing procedure.
͏    [ As mentioned: ​https://trac.ffmpeg.org/ticket/11190#comment:6 ]

͏    Please eligible reviewers review and submit?

[Balling]

4eca335d94ca0126669a8241834159df356f9389