[Matroska-general] Subtitles in MP4 .. a mess !!

Christian HJ Wiesner chris at matroska.org
Sun Dec 14 22:31:03 CET 2003

Read here : 

*[quote]ND Subtitles & Chapters issue ; from bobolobo, member of the 
Nero Digital dev team :*

I'd like to bring some clarifications about ND mp4 files, Subtitles and 
Chapters support as it seems to generate much discussions, lack of 
understanding, etc. I hope this could enlight you on the way it is done 
and why.

Basically, the mpeg-4 standard doesn't specify a way to handle subtitles 
and chapters. That is to say the notion of movie subtitles and chapters 
doesn't exist in the standard. From that assumption, there is no reason 
to say subtiltes or chapters are ISO compliant or not since there is no 
spec for this.

However mpeg-4 is a very rich standard and provide several tools one can 
use to implement the support of subtitles and chapters. For the 
subtitles, we can have :

- Streaming Text Format (14496-17)
- private data

The first approach (2D BIFS) consists in the use of the complex mpeg-4 
system layer to describe a presentation composed of multiple objects 
(audio stream, video stream, 2D bitmap, etc.). In the absolute this 
should be the best way to implement subtitles. However, due to its 
complexity it would involve a lot of work for us which simply means more 
delay before the release ! Beside this constraint, we were also faced to 
the fact that such complex system management would become a real 
headache on embedded platforms (ie standalones) were resource are 
extrexemly limited. Therefore a fast adoption of MP4 format in this 
area. Just imagine us telling standalone manufacturers : to support MP4 
it's quite easy, just read the 1000 pages of mpeg-4 BIFS system 
standard, then make it fit in your 16 kB of memory and you'll get it ! 
You'll understand that solution was not reasonable for us and that's why 
we prefer in our first stage to not use this implementation. But in 
futur evolutions, the addition of new features like menus, 
interactivity, and with the spreading of more powerful chips, we may 
seriously consider this solution.

The second solution "Streaming Text Format" is a new part of the 
standard not approved yet. It uses text data rather than bitmap for the 
subtitles and relies on the previous system layer. Considering subtitles 
in DVD are bitmap it would require some OCR process. In conclusion this 
solution is even harder to implement (imagine bitmap to unicode 
conversion for chinese or arabic language !). So we coulnd't choose this 

And finaly the last solution uses mp4 file user data to store our info. 
Since the mp4 file provides private data space, we simply put our 
compressed data here. These user data must be discarded by decoders that 
don't know how to handle them so that shouldn't hurt.
This was the best compromise we found and basically this doesn't differ 
much from other system like vobsub which stores data in a file beside.

For the chapter list, we applied the same recipe since there was no 
really other mean to achieve this.

I hope this clarify a little bit things and help you to understand why 
these choice were done.

And last but not least, for those who are concerned about ND MP4 files 
support on their standalone, be sure we're making much efforts in 
promoting ISO MP4 support in standalones. Discussions are on the way and 
hopefuly mp4 support will come quite soon.[/quote]

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