[PD] Gem: mpeg vs mpeg-3

IOhannes m zmoelnig zmoelnig at iem.at
Mon Apr 20 11:06:01 CEST 2015

On 2015-04-19 22:08, Peter P. wrote:
> * IOhannes zmölnig <zmoelnig at iem.at> [2015-04-19 02:52]:
>> Am 18. April 2015 21:30:20 MESZ, schrieb "Peter P." <peterparker at fastmail.com>:
>>> mpeg3video_seek: frame accurate seeking without a table of contents 
>>> is no longer supported.  Use mpeg3toc <mpeg file> <table of contents>
>>> to generate a table of contents and load the table of contents instead.
>>> what to do?
>> How about "Use mpeg3toc <mpeg file> <table of contents>" ?
> Yes, of course everyone could do that. But why is Gem no longer able to
> play alea.mpg by default,

because Gem does not do a low-level decoding of the mpg files. instead
it delegates the job to one of multiple backends.

now Gem has two backends that are specialised on decoding mpeg files
(the other backends are more generic regarding formats):
- libMPEG: this is *very* old and afaik not available in any current
distribution; so of very limited use.
- libMPEG3: this is what you are trying to use. libMPEG3 used to be
directly support flame accurate seeking (which is what Gem uses), but no
longer does so. instead you are now asked to pregenerate a TOC-file.

btw, there is no "default" backend: Gem uses what is available.
in practice i would recommend the GMERLIN-backend.

> and wouldn't it make more sense to mpeg3toc
> the file alea.mpg in git already? I assume more users with limited
> knowledge would come up with similar questions eventually.

hmm well.
the problem is: afaict, mpeg3toc produces a proprietary (non-standard,
application-specific) file, currently only used by libMPEG3.
any other backend will likely not be able to do anything with that file
(e.g. play it back) and libMPEG3 might decide switch to another format
whenever they please - after all the TOC file is just a helper file to
offline some of the work.
users who want to be able to play back other mpg-files (besides the
alea.mpg) would need to create the TOC-file for their media anyhow.

so to answer your question: i'd rather not.

btw, the problem seems to be not very pressing, as the toc-file is
required since ages (though i cannot remember the specific date) and few
people have complaint so far. (probably few people want to playback MPEG
files; and even fewer use libMPEG3 for that).

> I am still wondering which library to install on Debian in order to have
> "mpeg" support in Gem, and if that would have any effect (or if it is
> deprecated).

the "gem" debian package recommends "gem-plugin-gmerlin" as the
*default* media decoding backend, and it seems that this one is well
capable of playing back alea.mpg.


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