Re: [FFmpeg-devel] [FFmpeg-cvslog] fftools/ffmpeg: add loopback decoding

Re: [FFmpeg-devel] [FFmpeg-cvslog] fftools/ffmpeg: add loopback decoding

[Gyan Doshi]

On 2024-03-13 12:57 pm, Anton Khirnov wrote:
> ffmpeg | branch: master | Anton Khirnov<anton@khirnov.net>  | Mon Feb 19 10:27:44 2024 +0100| [a9193f7b7d65aafa326e25571c6672636a8ee3d2] | committer: Anton Khirnov
>
> fftools/ffmpeg: add loopback decoding
...
> +E.g. the following example:
> +
> +@example
> +ffmpeg -i INPUT                                        \
> +  -map 0:v:0 -c:v libx264 -crf 45 -f null -            \
> +  -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' \
> +  -map '[stack]' -c:v ffv1 OUTPUT

Can you add an example showing the assignment of a specific decoder 
along with a lavc generic + private option to a loopbacked stream?

Are there any limitations to loopback decoding e.g. would a vpx w/alpha 
encode be decoded back to a alpha pix fmt?

Regards,
Gyan
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Re: [FFmpeg-devel] [FFmpeg-cvslog] fftools/ffmpeg: add loopback decoding

[Anton Khirnov]

Quoting Gyan Doshi (2024-03-13 08:49:52)
> 
> 
> On 2024-03-13 12:57 pm, Anton Khirnov wrote:
> > ffmpeg | branch: master | Anton Khirnov<anton@khirnov.net>  | Mon Feb 19 10:27:44 2024 +0100| [a9193f7b7d65aafa326e25571c6672636a8ee3d2] | committer: Anton Khirnov
> >
> > fftools/ffmpeg: add loopback decoding
> ...
> > +E.g. the following example:
> > +
> > +@example
> > +ffmpeg -i INPUT                                        \
> > +  -map 0:v:0 -c:v libx264 -crf 45 -f null -            \
> > +  -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' \
> > +  -map '[stack]' -c:v ffv1 OUTPUT
> 
> Can you add an example showing the assignment of a specific decoder 
> along with a lavc generic + private option to a loopbacked stream?

Sent a patch with some notes on this.

> Are there any limitations to loopback decoding e.g. would a vpx w/alpha 
> encode be decoded back to a alpha pix fmt?

I don't know how vpx alpha works, was it a separate stream in the
demuxer or something like that?

Loopback decoders work exactly like normal decoders, except their
packets arrive from an encoder instead of a demuxer.

-- 
Anton Khirnov
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Re: [FFmpeg-devel] [FFmpeg-cvslog] fftools/ffmpeg: add loopback decoding

[Gyan Doshi]

On 2024-03-14 03:46 pm, Anton Khirnov wrote:
> Quoting Gyan Doshi (2024-03-13 08:49:52)
>
>> Are there any limitations to loopback decoding e.g. would a vpx w/alpha
>> encode be decoded back to a alpha pix fmt?
> I don't know how vpx alpha works, was it a separate stream in the
> demuxer or something like that?

The encoder attaches the coded alpha as side data to the packet. Only a 
couple of containers support this SD.
The demuxer, in turn, attaches the alpha as SD upon export.

> Loopback decoders work exactly like normal decoders, except their
> packets arrive from an encoder instead of a demuxer.
In terms of syntax, do all LB streams have to be specified in the first 
output? Does that output muxer have to be null, or can those packets be 
routed to both output and LB? Regards, Gyan
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Re: [FFmpeg-devel] [FFmpeg-cvslog] fftools/ffmpeg: add loopback decoding

[Anton Khirnov]

Quoting Gyan Doshi (2024-03-15 05:12:41)
> 
> 
> On 2024-03-14 03:46 pm, Anton Khirnov wrote:
> > Quoting Gyan Doshi (2024-03-13 08:49:52)
> >
> >> Are there any limitations to loopback decoding e.g. would a vpx w/alpha
> >> encode be decoded back to a alpha pix fmt?
> > I don't know how vpx alpha works, was it a separate stream in the
> > demuxer or something like that?
> 
> The encoder attaches the coded alpha as side data to the packet. Only a 
> couple of containers support this SD.
> The demuxer, in turn, attaches the alpha as SD upon export.

The packets arrive at the decoder exactly as the encoder produced them,
so if that's enough then it should work.

> > Loopback decoders work exactly like normal decoders, except their
> > packets arrive from an encoder instead of a demuxer.
> In terms of syntax, do all LB streams have to be specified in the first 
> output?

Actually they are not a part of the output, they form their own parsing
unit equivalent to -i/output. Probably this could be documented better.

> Does that output muxer have to be null, or can those packets be 
> routed to both output and LB?

The muxer can be anything, it does not know anything about loopback
decoders. The only limitation in this regard is that it is not
(currently) possible to avoid muxing entirely.

-- 
Anton Khirnov
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