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RE: TTML and aspect ratio

From: John Birch <John.Birch@screensystems.tv>
Date: Wed, 30 Jan 2013 15:10:01 +0000
To: Glenn Adams <glenn@skynav.com>
CC: David Ronca <dronca@netflix.com>, "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8ABAAF0230F@SS-IP-EXMB-01.screensystems.tv>
Hi Glenn,

By "this" do you mean the fact that the origin of the root container region is determined by the presentation processor? We chose this method since, in the context of viewing captions/subtitles in HTML pages, we did not assume that captions/subtitles would overlay the related media, e.g., could be displayed at arbitrary other locations. That said, I suspect it is time we revisit this decision for TTML 1.1, and consider additional semantics or syntax to provide more authorial control or declarative ability.

Yes.. exactly – sorry to not be clear.
I also agree that it should be possible for captions, subtitles or other texts to be located outside of the video.
Additional semantics and syntax would be useful…

Comments about SDP-US understood ☺

Best regards,
John

John Birch | Screen | Strategic Partnerships Manager
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From: Glenn Adams [mailto:glenn@skynav.com]
Sent: 30 January 2013 15:06
To: John Birch
Cc: David Ronca; public-tt@w3.org
Subject: Re: TTML and aspect ratio


On Wed, Jan 30, 2013 at 7:51 AM, John Birch <John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv>> wrote:
Hi Glenn, all,

RE:
As presently defined by TTML, the origin of the root container region is determined by the external presentation context. [Note that the phrase "external authoring context" should be interpreted as external presentation context. In fact, the current text of TTML should probably be changed in this regard to always refer to external presentation context instead.] In other words, as presently defined, the root container origin is always determined by the presentation processor (or transformation processor when performing transformations).

One possible origin is the related media object's origin, another is some offset that corresponds to the typical safe area for the related media object. So, e.g., for a 4:3 480i NTSC related media we have:

SAR = 704:480
PAR = 10:11
DAR = 640:480

I regards this as dangerous since the arbitrary selection of an origin location relative to the underlying media by a presentation processor will lead to ambiguous presentations… i.e. the position (and size) of display of the timed text would be arbitrarily affected by the presentation processor used.

By "this" do you mean the fact that the origin of the root container region is determined by the presentation processor? We chose this method since, in the context of viewing captions/subtitles in HTML pages, we did not assume that captions/subtitles would overlay the related media, e.g., could be displayed at arbitrary other locations. That said, I suspect it is time we revisit this decision for TTML 1.1, and consider additional semantics or syntax to provide more authorial control or declarative ability.


Whilst in some scenarios this might be desirable (e.g. US captioning allows the viewer to select characteristics of text presentation) in other scenarios this would be undesirable (e.g. subtitling or tickers). I would welcome a resolution of this in TTML 1.1.
Some text to lock the ‘default origin’ to the origin of any associated media / presentation canvas might be useful, with a caveat that this may be specifically overridden (either in the file or by the external context?)

This may be better positioned within specific ‘profiles’ e.g. SDP-US rather than the base TTML?

No, because the current profile work (SDP-US) is strictly a subset profile. We aren't defining official profiles for TT (at least in this WG) that are superset profiles. [Note that others have done so.] The right place to define would be in TTML 1.1 if we want to add a new feature. We could also add clarifying text to TTML 1.0 SE to point out coming features or mechanisms to satisfy this.

First, however, I'd like to get the requirement well documented, and I don't believe we have done that yet.


Best regards,
John


John Birch | Screen | Strategic Partnerships Manager
Main Line : +44 1473 831700<tel:%2B44%201473%20831700> | Ext : 270 | Direct Dial : +44 1473 834532<tel:%2B44%201473%20834532>
Mobile : +44 7919 558380<tel:%2B44%207919%20558380> | Fax : +44 1473 830078<tel:%2B44%201473%20830078>
John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv> | www.screensystems.tv<http://www.screensystems.tv> | https://twitter.com/screensystems


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From: Glenn Adams [mailto:glenn@skynav.com<mailto:glenn@skynav.com>]
Sent: 30 January 2013 14:37
To: David Ronca
Cc: public-tt@w3.org<mailto:public-tt@w3.org>
Subject: Re: TTML and aspect ratio

On Tue, Jan 29, 2013 at 11:29 PM, David Ronca <dronca@netflix.com<mailto:dronca@netflix.com>> wrote:
CEA-608 mandates a 32x15 grid in a 4:3 display area.  CEA-708 allows either 32x15 (4:3) or 42:15 (16:9).  For 4:3, the position 10%,10% is a different location on the screen than it is for 16:9 content.. Further, displaying CEA-608 caption in a 16x9 display area will result in very poor presentation (as will presenting 16x9 CEA-708 caption in a 4:3 display area).  In reviewing the W3C-TTML, we can find no well-defined means to inform the client of the proper aspect ratio of the caption display area.  My question is how should a client determine the correct aspect ratio for the caption viewing area?

What you refer to as "caption display area" is called "root container region" by TTML 2.2 [1]:

Root Container Region

A logical region that establishes a coordinate system into which content regions are placed and optionally clipped.
The origin and extent of this container region is specified according to TTML 7.1.1 [2]:

If the tts:extent attribute is specified on the tt element, then it must adhere to 8.2.7 tts:extent<https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html#style-attribute-extent>, in which case it specifies the spatial extent of the root container region in which content regions are located and presented. If no tts:extent attribute is specified, then the spatial extent of the root container region is considered to be determined by the external authoring or presentation context. The root container origin is determined by the external authoring context.

The origin of non-root region areas generated by tt:region elements are located in relationship with the origin of the root container region, forming a single level hierarchy of regions.

As presently defined by TTML, the origin of the root container region is determined by the external presentation context. [Note that the phrase "external authoring context" should be interpreted as external presentation context. In fact, the current text of TTML should probably be changed in this regard to always refer to external presentation context instead.] In other words, as presently defined, the root container origin is always determined by the presentation processor (or transformation processor when performing transformations).

One possible origin is the related media object's origin, another is some offset that corresponds to the typical safe area for the related media object. So, e.g., for a 4:3 480i NTSC related media we have:

SAR = 704:480
PAR = 10:11
DAR = 640:480

For the TTML author, either SAR pixels or DAR pixels may be used for coordinates. If SAR is used, then in this case, the author would need to specify ttp:pixelAspectRatio='10 11' on the tt element. So, for example, the author might specify:

<tt ttp:pixelAspectRatio='10 11' tts:extent='704px 480px'>

or, if the coordinates are in DAR pixels instead of SAR pixels, then

<tt tts:extent='640px 480px'>

in this case the default ttp:pixelAspectRatio='1 1' applies.

It is up to the author to then specify the appropriate tts:origin and tts:extent values on each defined region element to position the region as desired within the root container region.

At present, TTML requires that tts:extent on the root tt element be specified in pixels rather than percentages. However, the tts:extent on region elements may be specified in pixels, ems, cells, or percentage. So, an author could specify the outer root container extent using pixels, then specify the non-root region extents either in percentage or in cells, which is another way of expressing percentages, since each cell is effectively:

cell width = ( 1 / cellResolution(columns) ) * 100%
cell height = ( 1 / cellResolution(rows) ) * 100%

[see TTML 6.2.1 [3] for more information on ttp:cellResolution]

Now, you raise the issue of presenting captions authored for 4:3 on a 16:9 screen (and vice-versa). At present, TTML doesn't specify anything explicit addressed to handle this situation. One option for a presentation processor would be to scale the coordinate space used to position regions, which would move regions to maintain relative positioning (but without font scaling). Another would be to scale both region coordinates and fonts. These operations may be modeled as performing TTML transformation processing as a step prior to TTML presentation processing. In an actual implementation, these steps could be merged.

If these mechanisms don't provide satisfactory results, then perhaps you can explain what you would like to see supported. Since we are defining TTML 1.1 at the current time, we have an opportunity to add new features.

[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html#terms

[2] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html#document-structure-vocabulary-tt

[3] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html#parameter-attribute-cellResolution


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