[FFmpeg-user] Verifying lossless rewrapping/transcoding in one step?

[Paul B Mahol]

On Mon, Aug 5, 2019 at 11:13 PM Peter B. <pb at das-werkstatt.com> wrote:

> Hi Paul!
>
> On 05/08/2019 16:25, Paul B Mahol wrote:
> > On Mon, Aug 5, 2019 at 4:16 PM Peter B. <pb at das-werkstatt.com> wrote:
> >
> >> [...]
> >> For example, instead of "CRC32=...." let's say:
> >> "v:0:CRC32=...
> >> v:1:CRC32=...
> >> a:0:CRC32...
> >> a:1:CRC32...
> >> a:2...
> >> "
> >>
> > Hash muxer does not work like that.
>
> Oh :(
>
>
> > Why would you need hash muxer for multiple streams?
>
> Because I want to verify that every stream was transferred losslessly
> from source to target.
> For both: audio and video.
> (and actually also non-AV tracks, any time-based metadata, etc)
>
>
> > You can use several maps to produce hash of more stream at once, each
> output directed to separate file.
> >
> > ffmpeg.exe -i fate-suite/mxf/C0023S01.mxf -vn -map 0:1 -f framecrc
> > first_audio_stream.framecrc -map 0:2 -f framecrc
> > second_audio_stream.framecrc
>
> Thanks!
>
> However, that's what I'm already doing (-f hash), but I'm writing for
> fully-automated transcoding scenarios, so I need to parse the source
> file to know how many streams (of which type), and then have another
> part that puts together the right commandline recipe, and then write
> code that generates names for each stream-hash output file, and then,
> and then I need to keep track of all these files, and then ...
>
> And I believe I'm neither the first nor the last person to write code
> for automating this.
> So I thought: wouldn't it be great to find a nicer way of validating
> "output-fidelity"? :)
>
> Any ideas?
>
>
> What I don't understand:
> If the hash muxer output can generate a single hash for the sums of all
> streams, I presume it has access to all streams. Since it can also
> output individual hashes to individual files, why isn't it possible to
> do the same, but store the hashcodes in variables (associated with their
> source stream), and then write that information in one single file?
>
>
> I like puzzles :)
> Thank you very much in advance!
>

If I understand correctly, basically need hash of each stream separately
(for framehash this already works).

That is quite possible to be implemented. Feel free to open feature request
on FFmpeg bug tracker.