RE: Padding on tt:p and tt:span elements

At the risk of confusing this further….

The ‘boxing’ or ‘padding’ displayed in a typical broadcast application is generally applied by determining the size of the rendered text, and then extending that determined bounding box by a known amount (of pixels or percentage). Typically the ‘box’ extends a greater extent in the horizontal direction than in the vertical direction. Note: boxes can also fuse together, or take the horizontal dimension from the largest of a set of boxes (when in rows).

In TTML as I understand it(as a result of derivation from xsl:fo?), there is no possible mechanism that can set the region size as a result of a calculation of the rendered text size on the display. In contrast to broadcast practises, in TTML the text is fitted inside a predefined region (or overflows / clips), rather than the region (growing) fitting the text.

Note: the above is really only relevant to subtitling / captioning applications that do not assume fixed fonts… i.e. this does not apply to Teletext or CEA 608/708.
Further, this is perhaps only relevant if you wish to use TTML to recreate an exact replica of a broadcast presentation… which may be relevant to SMPTE-TT for example, but may not be relevant to EBU-TT.
In EBU-TT  an alternative markup (that identifies a boxing strategy) might be acceptable to indicate what a display processor (or transformation engine) might be expected to do.

Regards,

John Birch

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From: Andreas Tai [mailto:tai@irt.de]
Sent: 25 April 2012 10:35
To: Glenn Adams
Cc: public-tt
Subject: Re: Padding on tt:p and tt:span elements


Am 24.04.2012 16:23, schrieb Glenn Adams:

On Tue, Apr 24, 2012 at 1:41 AM, Andreas Tai <tai@irt.de<mailto:tai@irt.de>> wrote:
As in TTML a region has always a predefined size, tt:p- and tt:span elements are the better choices to apply a non-transparent background color on a subtitle block that dynamically grows and shrinks with the corresponding text.

I'm not sure I understand what you mean by "has always a predefined size". If by this you mean the "same" size, then I would not agree. A region's size (extent) can be changed at any time by using <tt:set>.

With "predefined" I wanted to express that the width and the height of the region are set before the text is rendered. The values are defined in a TTML document by a tts:extent attribute that applies to a region or (if no tts:extent attribute applies) by the default value "auto" which means that the region has the same width and height as the root container.

Maybe the timing semantic of "predefined" and "before" can be a bit misleading here. It is maybe more that the width and height of the region are independent of the size of the content that is flowed into the region.

Hope this makes it clearer.

Unfortunately padding cannot be applied to tt:p and tt:span elements and the very common use case to have empty space between text and the border of a centered "text box" with dynamic width is hard to implement (for an illustration of two use cases see the attached images).
Therefore the EBU-Working group that specifies the TTML subset EBU-TT sees the requirements to add padding as an applicable style property for the tt:p and tt:span elements.
Best regards,
Andreas




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