Re: [FFmpeg-devel] [PATCH 3/3] avformat/electronicarts: add option to return alpha channel in the main video stream in VP6A codec

From: Marton Balint <cus@passwd.hu>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 3/3] avformat/electronicarts: add option to return alpha channel in the main video stream in VP6A codec
Date: Mon, 14 Nov 2022 23:05:44 +0100 (CET)
Message-ID: <362f6c69-b72d-447-7ab5-4a24695c58c8@passwd.hu> (raw)
In-Reply-To: <166844365819.20155.3362908166990565436@lain.khirnov.net>

On Mon, 14 Nov 2022, Anton Khirnov wrote:

> Quoting Marton Balint (2022-11-13 19:44:41)
>> Signed-off-by: Marton Balint <cus@passwd.hu>
>> ---
>>  doc/demuxers.texi            | 18 ++++++++++++++++
>>  libavformat/electronicarts.c | 42 +++++++++++++++++++++++++++++++-----
>>  libavformat/version.h        |  2 +-
>>  3 files changed, 56 insertions(+), 6 deletions(-)
>>
>> diff --git a/doc/demuxers.texi b/doc/demuxers.texi
>> index 2b6dd86c2a..f07f3f5318 100644
>> --- a/doc/demuxers.texi
>> +++ b/doc/demuxers.texi
>> @@ -285,6 +285,24 @@ This demuxer accepts the following option:
>>
>>  @end table
>>
>> +@section ea
>> +
>> +Electronic Arts Multimedia format demuxer.
>> +
>> +This format is used by various Electronic Arts games.
>> +
>> +@subsection Options
>> +
>> +@table @option
>> +
>> +@item merge_alpha @var{bool}
>> +
>> +Normally the VP6 alpha channel (if exists) is returned as a secondary video
>> +stream,
>
> Why? And why keep it as the default?

VP6 alpha in EA format is a second VP6 encoded video stream where only the 
Y component is used and is interpreted as the alpha channel of the first 
VP6 stream. The alpha VP6 stream is muxed separately from the main VP6 
stream, has its own stream headers and packet headers. In theory the two 
streams might not even have the same resolution (although most likely 
that is not something that is seen or supported in the wild), but the 
format is capable of doing it.

Merged VP6 alpha (also known as the VP6A codec) means that a packet of the 
video stream contains the corresponding packet of both VP6 substreams like 
this:
{OffsetOfAlpha, DataPacket, AlphaDataPacet}
So data and alpha data of a frame is merged to a single packet, this is 
how VP6 video with alpha is muxed in FLV and SWF.

So the first approach is more like how the demuxer sees data in the EA 
format, unfortunately it is different to what the FLV or SWF format 
expects, so - having no better place for it in the framework - I decided 
to do an optional format conversion in the EA demuxer.

I did not want to change the default, but certainly doable if people 
prefer it.

Regards,
Marton
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