[Ffmpeg-devel] 4XM audio codec_tag

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> Hi
>
> On Sun, Nov 05, 2006 at 09:36:57PM +0000, M?ns Rullg?rd wrote:
>> Reimar D?ffinger <Reimar.Doeffinger at stud.uni-karlsruhe.de> writes:
>> 
>> > Hello,
>> > On Sun, Nov 05, 2006 at 09:00:14PM +0000, M?ns Rullg?rd wrote:
>> >> Does any of the above make sense to you?
>> >
>> > It's mostly about getting something more specific than whether it makes
>> > sense.
>> 
>> I just don't understand how you guys can find it so unimportant to be
>> accurate.  Ever heard of interoperability?
>
> yes, and we loose alot of interoperability if we dissaow every
> demuxer to try list of common codec_tag -> codec_ids (=riff.c)

Why should the RIFF tags be given this special status?  Why not have
all demuxers search the codec lists for *all* formats while we're at
it?

>> > I on the other hand was mostly thinking of completely new formats, like
>> > all of those game formats, that do not usually appear in AVI.
>> > E.g. if the line
>> > vst->codec->codec_tag = MKTAG('R', 'J', 'P', 'G');
>> > was removed from nuv.c (which it probably should), then should an entry
>> > for it be added to riff.c?
>> 
>> I'd say no.  We have no choice but to include some nonstandard tags if
>> we want to support all the files out there.  That does not mean we
>> should be contributing to the mess.
>
> we contribute by not having a tag there,

How do we contribute to the mess by refusing to write non-standard files?

> if a user wants to have 4xm in avi he will mux it in avi

If his apps refuse to do it he won't.

> theres -vtag and -atag to set codec tags no need for a table,

Those flags encourage users to create non-standard files.  Personally
I'd be happy to see them go.

> the user can override it,

About time that changed.

> and if theres no entry then every user will invent one, that mess is
> several magnitudes bigger

That still relies on the apps allowing non-standard tags.  Most users
are too lazy to write their own apps just to let them use a
non-standard codec tag.

>> > MPlayer has long supported that format in avi I think,
>> 
>> So what?
>
> iam against droping support for decoding existing files

So support decoding of nonstandard files.  Encoding them should be
very strongly discouraged, if not totally refused.

>> > I would tend towards: better clutter the table a bit and support more
>> > formats however rare than having a cleaner table.
>> 
>> Again you've drifted from the question of sharing the codec ID table
>> between unrelated formats, and treating the RIFF table as some kind of
>> absolute reference.
>> 
>> Just face the facts: formats use different tags to identify codecs, no
>> matter how much you pretend that all of them use the RIFF values.  The
>> only way to properly handle this is by using one codec tag table per
>> format.  End of story.
>
> i have no problem with one table per format, what i have a problem with
> is that libav* would fail decoding a file if no match is found and that
> it would leave the codec tag decission to the end user and not suggest
> a default one if theres none in the one table for the target format
>
> if OTOH there is one table per format and a default fallback table then
> thats something different with which iam fine

A "fallback table" doesn't make any sense at all.  None.  Zero.  Nil.
Either a format supports some particular codec, in which case its ID
table will have an entry for that codec, or the codec is not
supported, in which case failure is the only sensible option.
"Suggesting" to use a tag from some other random format instead is
utterly senseless.  Such behavior is what created the AVI mess in the
first place.

-- 
M?ns Rullg?rd
mru at inprovide.com