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• What is “interoperable TTML” ?

  1er janvier 2014, par silvia

  I’ve just tried to come to terms with the latest state of TTML, the Timed Text Markup Language.

  TTML has been specified by the W3C Timed Text Working Group and released as a RECommendation v1.0 in November 2010. Since then, several organisations have tried to adopt it as their caption file format. This includes the SMPTE, the EBU (European Broadcasting Union), and Microsoft.

  Both, Microsoft and the EBU actually looked at TTML in detail and decided that in order to make it usable for their use cases, a restriction of its functionalities is needed.

  EBU-TT

  The EBU released EBU-TT, which restricts the set of valid attributes and feature. “The EBU-TT format is intended to constrain the features provided by TTML, especially to make EBU-TT more suitable for the use with broadcast video and web video applications.” (see EBU-TT).

  In addition, EBU-specific namespaces were introduce to extend TTML with EBU-specific data types, e.g. ebuttdt:frameRateMultiplierType or ebuttdt:smpteTimingType. Similarly, a bunch of metadata elements were introduced, e.g. ebuttm:documentMetadata, ebuttm:documentEbuttVersion, or ebuttm:documentIdentifier.

  The use of namespaces as an extensibility mechanism will ascertain that EBU-TT files continue to be valid TTML files. However, any vanilla TTML parser will not know what to do with these custom extensions and will drop them on the floor.

  Simple Delivery Profile

  With the intention to make TTML ready for “internet delivery of Captions originated in the United States”, Microsoft proposed a “Simple Delivery Profile for Closed Captions (US)” (see Simple Profile). The Simple Profile is also a restriction of TTML.

  Unfortunately, the Microsoft profile is not the same as the EBU-TT profile : for example, it contains the “set” element, which is not conformant in EBU-TT. Similarly, the supported style features are different, e.g. Simple Profile supports “display-region”, while EBU-TT does not. On the other hand, EBU-TT supports monospace, sans-serif and serif fonts, while the Simple profile does not.

  Thus files created for the Simple Delivery Profile will not work on players that expect EBU-TT and the reverse.

  Fortunately, the Simple Delivery Profile does not introduce any new namespaces and new features, so at least it is an explicit subpart of TTML and not both a restriction and extension like EBU-TT.

  SMPTE-TT

  SMPTE also created a version of the TTML standard called SMPTE-TT. SMPTE did not decide on a subset of TTML for their purposes – it was simply adopted as a complete set. “This Standard provides a framework for timed text to be supported for content delivered via broadband means,…” (see SMPTE-TT).

  However, SMPTE extended TTML in SMPTE-TT with an ability to store a binary blob with captions in another format. This allows using SMPTE-TT as a transport format for any caption format and is deemed to help with “backwards compatibility”.

  Now, instead of specifying a profile, SMPTE decided to define how to convert CEA-608 captions to SMPTE-TT. Even if it’s not called a “profile”, that’s actually what it is. It even has its own namespace : “m608 :”.

  Conclusion

  With all these different versions of TTML, I ask myself what a video player that claims support for TTML will do to get something working. The only chance it has is to implement all the extensions defined in all the different profiles. I pity the player that has to deal with a SMPTE-TT file that has a binary blob in it and is expected to be able to decode this.

  Now, what is a caption author supposed to do when creating TTML ? They obviously cannot expect all players to be able to play back all TTML versions. Should they create different files depending on what platform they are targeting, i.e. a EBU-TT version, a SMPTE-TT version, a vanilla TTML version, and a Simple Delivery Profile version ? Should they by throwing all the features of all the versions into one TTML file and hope that the players will pick out the right things that they require and drop the rest on the floor ?

  Maybe the best way to progress would be to make a list of the “safe” features : those features that every TTML profile supports. That may be the best way to get an “interoperable TTML” file. Here’s me hoping that this minimal set of features doesn’t just end up being the usual (starttime, endtime, text) triple.

  UPDATE :

  I just found out that UltraViolet have their own profile of SMPTE-TT called CFF-TT (see UltraViolet FAQ and spec). They are making some SMPTE-TT fields optional, but introduce a new @forcedDisplayMode attribute under their own namespace “cff :”.

• Merge commit ’c6698dfe7cdbc7634f33245875488ed3fa4a8ced’

  4 juillet 2014, par Michael Niedermayer

  Merge commit ’c6698dfe7cdbc7634f33245875488ed3fa4a8ced’
  * commit ’c6698dfe7cdbc7634f33245875488ed3fa4a8ced’ :
  
    webpdec : Fix decoding of the huffman group indices.
  Merged-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/webp.c

• RTSP/RTMP Video Streaming Client iOS [closed]

  28 mai 2015, par nathansizemore

  I’m in need of a open source solution/library to stream RTSP/RTMP to an iOS Application. I need to build an app that connects to a media server, and opens the provided video stream. I believe there has to be libraries out there, but I have yet to find one that is open source, compiles, actually works, and runs on iOS 5+, iPhone 4+. I do not have a preference, RTMP or RTSP will suffice. Preferably the one with the least amount of work. I have RTSP working on the Android side, but nothing for iOS yet.

  This is what I already know from research today -

  RTSP

  • Seems possible using Live555/FFMPEG
  • MooncatVenture Group - Old FFMPEG, not compatible with ARMv7s (No updates/blogs/commits in over a year)
  • DFURTSPPlayer - This is a working example.

  RTMP

  • Seems possible using Live555/FFMPEG
  • A few libraries are out there for data messaging, but that is all
  • MidnightCoders Project - Does not seem video support is build yet, as Audio is not.

  I’ve never messed with anything video related before, so encoding, frame rate, key frame, chunks, etc... is pretty foreign to me. Right now, it seems building a static binary from Live555/FFMPEG is the only solution to my problem. If so, can anyone give me a simple quickstart guide or links to a blog/example someone has out there ? I’m not looking for anything crazy, just a simple

  1. Download This - LINK
  2. Compile it like this - LINK
  3. Place it into X Folder in Xcode
  4. Create X Object
  5. Read Stream API here - LINK

  If not, anyone want to point me to a working open source library ?

  Oh yeah, this happens to be my first iPhone app and first time in Objective-C. Awesome first project, yeah ?