[FFmpeg-devel] [PATCH 03/11] mpeg2_metadata, cbs_mpeg2: Fix handling of colour_description

[Mark Thompson]

On 29/05/2019 05:12, Andreas Rheinhardt wrote:
> Mark Thompson:
>> On 22/05/2019 02:04, Andreas Rheinhardt wrote:
>>> If a sequence display extension is read with colour_description equal to
>>> zero, but a user wants to add one or more of the colour_description
>>> elements, then the colour_description elements the user did not explicitly
>>> request to be set are set to zero and not to the value equal to
>>> unknown/unspecified (namely 2). A value of zero is not only inappropriate,
>>> but explicitly forbidden. This is fixed by inferring the right default
>>> values during the reading process if the elements are absent; moreover,
>>> changing any of the colour_description elements to zero is now no longer
>>> permitted.
>>>
>>> Furthermore, if a sequence display extension has to be added, the
>>> earlier code set some fields to their default value twice. This has been
>>> changed, too.
>>>
>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>>> ---
>>>  libavcodec/cbs_mpeg2.c                 | 15 +++++++++++++++
>>>  libavcodec/cbs_mpeg2_syntax_template.c |  4 ++++
>>>  libavcodec/mpeg2_metadata_bsf.c        | 18 ++++++++++++------
>>>  3 files changed, 31 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c
>>> index 1d319e0947..437eac88a3 100644
>>> --- a/libavcodec/cbs_mpeg2.c
>>> +++ b/libavcodec/cbs_mpeg2.c
>>> @@ -71,6 +71,10 @@
>>>      (get_bits_left(rw) >= width && \
>>>       (var = show_bits(rw, width)) == (compare))
>>>  
>>> +#define infer(name, value) do { \
>>> +        current->name = value; \
>>> +    } while (0)
>>> +
>>>  #include "cbs_mpeg2_syntax_template.c"
>>>  
>>>  #undef READ
>>> @@ -79,6 +83,7 @@
>>>  #undef xui
>>>  #undef marker_bit
>>>  #undef nextbits
>>> +#undef infer
>>>  
>>>  
>>>  #define WRITE
>>> @@ -97,6 +102,15 @@
>>>  
>>>  #define nextbits(width, compare, var) (var)
>>>  
>>> +#define infer(name, value) do { \
>>> +        if (current->name != (value)) { \
>>> +            av_log(ctx->log_ctx, AV_LOG_WARNING, "Warning: " \
>>> +                   "%s does not match inferred value: " \
>>> +                   "%"PRId64", but should be %"PRId64".\n", \
>>> +                   #name, (int64_t)current->name, (int64_t)(value)); \
>>> +        } \
>>> +    } while (0)
>>> +
>>>  #include "cbs_mpeg2_syntax_template.c"
>>>  
>>>  #undef READ
>>> @@ -105,6 +119,7 @@
>>>  #undef xui
>>>  #undef marker_bit
>>>  #undef nextbits
>>> +#undef infer
>>>  
>>>  
>>>  static void cbs_mpeg2_free_user_data(void *unit, uint8_t *content)
>>> diff --git a/libavcodec/cbs_mpeg2_syntax_template.c b/libavcodec/cbs_mpeg2_syntax_template.c
>>> index b9d53682fe..87db0ad039 100644
>>> --- a/libavcodec/cbs_mpeg2_syntax_template.c
>>> +++ b/libavcodec/cbs_mpeg2_syntax_template.c
>>> @@ -144,6 +144,10 @@ static int FUNC(sequence_display_extension)(CodedBitstreamContext *ctx, RWContex
>>>          uir(8, transfer_characteristics);
>>>          uir(8, matrix_coefficients);
>>>  #endif
>>> +    } else {
>>> +        infer(colour_primaries,         2);
>>> +        infer(transfer_characteristics, 2);
>>> +        infer(matrix_coefficients,      2);
>>>      }
>>>  
>>>      ui(14, display_horizontal_size);
>>> diff --git a/libavcodec/mpeg2_metadata_bsf.c b/libavcodec/mpeg2_metadata_bsf.c
>>> index ba3a74afda..5aed41a008 100644
>>> --- a/libavcodec/mpeg2_metadata_bsf.c
>>> +++ b/libavcodec/mpeg2_metadata_bsf.c
>>> @@ -147,18 +147,12 @@ static int mpeg2_metadata_update_fragment(AVBSFContext *bsf,
>>>  
>>>              if (ctx->colour_primaries >= 0)
>>>                  sde->colour_primaries = ctx->colour_primaries;
>>> -            else if (add_sde)
>>> -                sde->colour_primaries = 2;
>>>  
>>>              if (ctx->transfer_characteristics >= 0)
>>>                  sde->transfer_characteristics = ctx->transfer_characteristics;
>>> -            else if (add_sde)
>>> -                sde->transfer_characteristics = 2;
>>>  
>>>              if (ctx->matrix_coefficients >= 0)
>>>                  sde->matrix_coefficients = ctx->matrix_coefficients;
>>> -            else if (add_sde)
>>> -                sde->matrix_coefficients = 2;
>>>          }
>>>      }
>>>  
>>> @@ -229,6 +223,18 @@ static int mpeg2_metadata_init(AVBSFContext *bsf)
>>>      CodedBitstreamFragment *frag = &ctx->fragment;
>>>      int err;
>>>  
>>> +#define VALIDITY_CHECK(name) do { \
>>> +        if (!ctx->name) { \
>>> +            av_log(bsf, AV_LOG_ERROR, "The value 0 for %s is " \
>>> +                                      "forbidden.\n", #name); \
>>> +            return AVERROR(EINVAL); \
>>> +        } \
>>> +    } while (0)
>>> +    VALIDITY_CHECK(colour_primaries);
>>> +    VALIDITY_CHECK(transfer_characteristics);
>>> +    VALIDITY_CHECK(matrix_coefficients);
>>> +#undef VALIDITY_CHECK
>>
>> Perhaps we could use the normal option checking to enforce this?  Suppose we change the range from [-1, 255] to [0, 255] and make the do-nothing default value 0.  Then the checks above become ctx->colour_primaries > 0 and this separate test is not needed.
>>
> There are two reasons why I didn't do this:
> 1. A user with only a background in (say) H.264 or H.265 might want to
> put a value of 0 in there, thinking it is a legel value. Said user is
> going to be surprised.
> 2. I have not given up hope on updating the AVCodecParameters (do you
> remember
> https://ffmpeg.org/pipermail/ffmpeg-devel/2018-December/236990.html ?)
> and if 0 means do nothing, then it is not in line with the rest of cbs
> and needs special handling there.
> But both reasons are very weak and so I will consider your suggestion.

Fair enough.  I have no actual objection to your approach; it was mostly a thought that the code could be slightly simpler.

Thanks,

- Mark