Examples for TTML 2 WebVTT Mapping

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Examples for TTML 2 WebVTT Mapping

Andreas Tai
Dear all,

IRT has been working on four examples to test and demonstrate the
TTML-to-WebVTT mapping specified in "Mapping Between TTML and WebVTT",
W3C Editor's Draft 16 February 2016 [1].

The result can be found in the branch "webvtt-mapping" of the "IRT
EBU-TT-D Application Samples" repository on github:

https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping

The "IRT EBU-TT-D Application Samples" are published to support EBU-TT-D
implementation. As EBU-TT-D is a subset of TTML 1 and IMSC 1 the samples
can also be used for TTML and IMSC 1 implementation.

The "Application Samples" include TTML documents and example renderings
as images and videos.

Because a mapping from TTML to WebVTT based on the W3C mapping guideline
should deliver an equivalent result, the EBU-TT-D rendering videos are
also used to test the WebVTT rendering. It can be accessed by hosting
the webvtt [2] folder on a webserver and accessing the documents in the
html directory [3].

A description of the implementation experience will be provided soon.

Best regards,

Andreas

[1] http://w3c.github.io/ttml-webvtt-mapping/
[2]
https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping
[3]
https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping/html



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Re: Examples for TTML 2 WebVTT Mapping

Andreas Tai
Dear all,

I added now an explanation of the folder structure
(https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping).
To make the access of the html pages easier they are now also hosted on
github pages [1][2][3][4][5].

In the following I want to share some implementation experiences.

-----------------------------------------------------
Complexity of mapping for developers
-----------------------------------------------------
The developer, who had implemented the mapping samples, had good
knowledge of TTML and basic knowledge WebVTT. He noticed first that the
mapping document alone was not sufficient to do the mapping. He needed
to go back to the WebVTT spec to implement some of the features. One
problem he had was that the WebVTT spec has advanced since the mapping
document has been written. In case of a bad result it was therefore
difficult to decide if this was an error of the mapping document or if
the WebVTT spec has changed since publication of the mapping document
(see for example [6][7]). Although he was quite open to the WebVTT spec
at the beginning he found some of the principles in WebVTT hard to
understand and also fundamentally different to TTML. At the end it
needed around three iterations and consultation with a mapping document
editor to come to a conforming mapping.


-------------------------------------
Preservation of information
-------------------------------------
Especially in respect to the use of regions there seems no lossless
mapping to VTT regions. One difficulty was to test WebVTT regions
because lack of implementation (they seem one provider that implemented
this feature). It is not clear if styling of the region by using the
::cue-region pseudo-element fully works. For the parts we tested
(background color of region) we could not find an implementation to
prove it. For the region feature and other features the preservation of
presentation intention depends largely on conforming implementation (see
below).


-------------------------------------------------------------------
Testing against existing WebVTT implementation
-------------------------------------------------------------------
The testing of WebVTT implementation against expected rendering gave a
very mixed result. By using the mapping strategies defined in the
document none of the implementations we looked at provided the expected
result. Also the feature where implementations deliver unexpected
renderings varied across the implementations. In some cases the
deviations have been so serious that the rendering was not acceptable
given the original intent.

------------------------------------------
Spec Version
------------------------------------------
The mapping exercise confirmed what was suspected before: the mapping
document faces one main problem in respect to the version of the WebVTT
spec. It needs to take into account three versions:

a) the version that was the reference at time of writing (of the mapping
doc)

b) the latest version of the spec

c) the version of the spec that is implemented in different clients

This task is impossible to solve at the moment. For fixing errors in the
documents it seems best to stick with the spec version at time of
writing because that version was seen as stable at that time and seems
still a reference for some implementations. Changes in the WebVTT spec
can be incorporated once the spec gets finally a stable status. It may
also be helpful to signal which of the WebVTT features are at risk and
which of the new edits have been implemented already.

------------------------------------------
Summary
------------------------------------------
At the moment it cannot be recommended to use the mapping document in
operation. Apart from the fact that it is still an editor’s draft, the
results could lead to unacceptable presentations in some environments.
Although in theory the results could be much better this takes more
conforming and interoperable WebVTT implementations to become an asset.

I would like to encourage members of the TTML and VTT communities to
review the few sample mappings. If there is an issue or error please
check first the mapping document. If it is an error in the mapping
document please file an issue there[7]. If there is an error how the
mapping is applied in the samples, please file an issue in the
IRT-EBU-TT-D Application Sample repository.

Best regards,

Andreas


[1]
http://irt-open-source.github.io/irt-ebu-tt-d-application-samples/webvtt-mapping/html/backgroundColor-region-p-span-002-html.html
[2]
http://irt-open-source.github.io/irt-ebu-tt-d-application-samples/webvtt-mapping/html/backgroundColor-region-p-span-002_region-html.html
[3]
http://irt-open-source.github.io/irt-ebu-tt-d-application-samples/webvtt-mapping/html/displayalign-before-001-html.html
[4]
http://irt-open-source.github.io/irt-ebu-tt-d-application-samples/webvtt-mapping/html/four-active-regions-001-html.html
[5]
https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/blob/webvtt-mapping/webvtt-mapping/html/writing-mode-tbrl-001-html.html
[6] https://github.com/w3c/ttml-webvtt-mapping/issues/24
[7] https://github.com/w3c/ttml-webvtt-mapping/issues/21
[8] https://github.com/w3c/ttml-webvtt-mapping/issues
[9]
https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/issues



Am 05.05.2016 um 16:32 schrieb Andreas Tai:

> Dear all,
>
> IRT has been working on four examples to test and demonstrate the
> TTML-to-WebVTT mapping specified in "Mapping Between TTML and WebVTT",
> W3C Editor's Draft 16 February 2016 [1].
>
> The result can be found in the branch "webvtt-mapping" of the "IRT
> EBU-TT-D Application Samples" repository on github:
>
> https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping 
>
>
> The "IRT EBU-TT-D Application Samples" are published to support
> EBU-TT-D implementation. As EBU-TT-D is a subset of TTML 1 and IMSC 1
> the samples can also be used for TTML and IMSC 1 implementation.
>
> The "Application Samples" include TTML documents and example
> renderings as images and videos.
>
> Because a mapping from TTML to WebVTT based on the W3C mapping
> guideline should deliver an equivalent result, the EBU-TT-D rendering
> videos are also used to test the WebVTT rendering. It can be accessed
> by hosting the webvtt [2] folder on a webserver and accessing the
> documents in the html directory [3].
>
> A description of the implementation experience will be provided soon.
>
> Best regards,
>
> Andreas
>
> [1] http://w3c.github.io/ttml-webvtt-mapping/
> [2]
> https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping
> [3]
> https://github.com/IRT-Open-Source/irt-ebu-tt-d-application-samples/tree/webvtt-mapping/webvtt-mapping/html
>
>
>


--
------------------------------------------------
Andreas Tai
Production Systems Television IRT - Institut fuer Rundfunktechnik GmbH
R&D Institute of ARD, ZDF, DRadio, ORF and SRG/SSR
Floriansmuehlstrasse 60, D-80939 Munich, Germany

Phone: +49 89 32399-389 | Fax: +49 89 32399-200
http: www.irt.de | Email: [hidden email]
------------------------------------------------

registration court&  managing director:
Munich Commercial, RegNo. B 5191
Dr. Klaus Illgner-Fehns
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