[FFmpeg-devel] [PATCH 001/279] Add a new channel layout API

[Lynne]

9 Dec 2021, 14:12 by anton at khirnov.net:

> Quoting Lynne (2021-12-08 13:57:45)
>
>> 8 Dec 2021, 13:16 by anton at khirnov.net:
>>
>> > Quoting Lynne (2021-12-08 10:02:34)
>> >
>> >> 8 Dec 2021, 02:06 by jamrial at gmail.com:
>> >>
>> >> > From: Anton Khirnov <anton at khirnov.net>
>> >> >
>> >> > The new API is more extensible and allows for custom layouts.
>> >> > More accurate information is exported, eg for decoders that do not
>> >> > set a channel layout, lavc will not make one up for them.
>> >> >
>> >> > Deprecate the old API working with just uint64_t bitmasks.
>> >> >
>> >> > Expanded and completed by Vittorio Giovara <vittorio.giovara at gmail.com>
>> >> > and James Almer <jamrial at gmail.com>.
>> >> > Signed-off-by: Vittorio Giovara <vittorio.giovara at gmail.com>
>> >> > Signed-off-by: James Almer <jamrial at gmail.com>
>> >> > ---
>> >> >  libavutil/channel_layout.c | 498 ++++++++++++++++++++++++++++++------
>> >> >  libavutil/channel_layout.h | 510 ++++++++++++++++++++++++++++++++++---
>> >> >  libavutil/version.h        |   1 +
>> >> >  3 files changed, 906 insertions(+), 103 deletions(-)
>> >> > 
>> >> >  /**
>> >> >  * @}
>> >> > @@ -128,6 +199,157 @@ enum AVMatrixEncoding {
>> >> >  AV_MATRIX_ENCODING_NB
>> >> >  };
>> >> > 
>> >> > +/**
>> >> > + * @}
>> >> > + */
>> >> > +
>> >> > +/**
>> >> > + * An AVChannelCustom defines a single channel within a custom order layout
>> >> > + *
>> >> > + * Unlike most structures in FFmpeg, sizeof(AVChannelCustom) is a part of the
>> >> > + * public ABI.
>> >> > + *
>> >> > + * No new fields may be added to it without a major version bump.
>> >> > + */
>> >> > +typedef struct AVChannelCustom {
>> >> > +    enum AVChannel id;
>> >> > +} AVChannelCustom;
>> >> >
>> >>
>> >> Consider adding a 25-byte or so of a description field string here that
>> >> would also be copied if the frame/layout is copied?
>> >> That way, users can assign a description label to each channel,
>> >> for example labeling each channel in a 255 channel custom layout
>> >> Opus stream used in production at a venue somewhere.
>> >> Also, consider additionally reserving 16-bytes here, just in case.
>> >>
>> >
>> > That would enlarge the layout size by a significant amount. Keep in mind
>> > that this is all per frame. And for something that very few people would
>> > want to use.
>> >
>> > These descriptors, if anyone really needs them, can live in side data.
>> >
>>
>> That's fine with me.
>> Can we at least have an opaque uint64_t? I'd accept a uintptr_t,
>> or an int too, or even a single-byte uint8_t.
>>
>
> like this?
>
> diff --git a/libavutil/channel_layout.h b/libavutil/channel_layout.h
> index 7b77a74b61..7b2ba12532 100644
> --- a/libavutil/channel_layout.h
> +++ b/libavutil/channel_layout.h
> @@ -252,7 +252,8 @@ enum AVMatrixEncoding {
>  * No new fields may be added to it without a major version bump.
>  */
>  typedef struct AVChannelCustom {
> -    enum AVChannel id;
> +    enum AVChannel id:16;
> +    int      reserved:16;
>  } AVChannelCustom; 
>

"reserved" rather than "opaque"?
16 bits for both? Could you keep the id as a plain (32-bit, on
a sane compiler) enum, since we may need to add flags, one
never knows, and make the opaque field a plain uint32_t?
64-bits per channel overall isn't too bad.