[FFmpeg-devel] [RFC] encoder profile validation

[Marton Balint]

On Wed, 20 May 2020, Anton Khirnov wrote:

> Quoting Marton Balint (2020-05-18 20:32:53)
>> 
>> 
>> On Mon, 18 May 2020, Anton Khirnov wrote:
>> 
>> > Quoting Marton Balint (2020-05-16 15:52:22)
>> >> Hi,
>> >> 
>> >> As you may know, a recent patchset enabled AVCodecContext->profile 
>> >> constants to reside in encoders.
>> >> 
>> >> In order to make a full transition to avctx->profile even in existing 
>> >> encoders which might use a private profile setting, we have to make sure 
>> >> only supported avctx->profile values are passed to encoders.
>> >> 
>> >> The fact that avctx->profile is not validated is already an issue, and 
>> >> assertions/segmentation faults can already happen in existing encoders 
>> >> (e.g.: aac, mpeg) if unsupported values are passed.
>> >> 
>> >> AVCodec have a .profiles attribute which supposed to contain the list of 
>> >> supported profiles. However this is problematic because
>> >> - AVCodecContext->profile is not validated against this list
>> >> - not all encoders define the list
>> >> - even if there is a list it might be defined as NULL_IF_CONFIG_SMALL
>> >> - some encoders support more or less than its currently defined list
>> >
>> > All of these sound like bugs that can and should be fixed.
>> >
>> >> - AVCodec->profiles contains AVProfiles which supposed to have a textual
>> >>    representation of each profile, which can cause profile name
>> >>    duplications/inconsistencies against libavcodec/profiles.c if the list
>> >>    is different to the one in codecs profile list.
>> >
>> > Why should it be different? Isn't that a bug that should be fixed?
>> 
>> The list can be different because possibly not all the profiles which are 
>> recognized by the decoder is supported by the encoder. Also the encoder 
>> can define pseudo profiles which it can support, for example the AAC 
>> encoders have support for FF_PROFILE_MPEG2_AAC_LOW which is LC profile 
>> with some features disabled.
>
> Don't understand - I would think AVCodec.profiles would list the
> profiles supported by that specific AVCodec instance. Is that not the
> case?

Yes, but you cannot share AVProfile lists among encoders or decoders of 
the same codec because they support different profiles.

Regards,
Marton