W3C home > Mailing lists > Public > public-tt@w3.org > February 2003

RE: TT and subtitling

From: <Johnb@screen.subtitling.com>
Date: Mon, 3 Feb 2003 09:41:53 -0000
Message-ID: <11E58A66B922D511AFB600A0244A722E6C5FE8@NTMAIL>
To: glenn@xfsi.com
Cc: public-tt@w3.org

> > GAA> One potential issue with the SVG font format is its apparent
> > GAA> lack of support for bitmap as opposed to outline glyph 
> > representations.
> > 
	CL> That is a strength, not a feature. How are you going to cope
with a
	CL> range of display sizes and resolutions with a bitmap font?

	GAA>Obviously, if some author used embedded bitmaps, then it is
clearly
	GAA>sub-optimal for device interoperability. On the other hand in
some
	GAA>contexts, it might be that the author or perhaps the delivery
system
	GAA>knows precisely what the required display sizes and resolutions
that
	GAA>are required.

	This is essentially the situation in TV subtitling - the display
resolution is known and fixed. All subtitling is performed using bitmaps,
(open is a bitmap burnt over video, Teletext use a pre-rasterised font in
the decoder chip of the TV, DVB uses bitmaps that are RLE encoded).

	GAA>On the other hand, clearly it would be a mistake, and I would
	GAA>never advocate sole reliance on use of bitmap representations; I
just
	GAA>want to be certain that we meet the needs of common authoring
and
	GAA>delivery systems, where there are many more uses made of bitmap
glyph
	GAA>downloads than outlines at this time.

	I totally agree.... in the emission context there is a fair
probablility that conversion to a bitmap form will occur prior to emission
for TT (simply because of prior art). It is one of my companies current aims
to add the ability for bitmaps to be propogated through the chain from
authoring to display, for channel identification, logos etc. Whilst these
usages may not fall into the TT charter, the ability of TT to carry bitmap
data would IMHO considerably enhance its utility in the contexts in which it
is likely to be used. Unicode does not cover every conceivable character -
and there are always notations that are private or not in common use that
may require transmission by TT. Some of these may not be efficiently carried
by SVG.

	GAA>One possible way this might be used is as follows:

	GAA>1. authoring and distribution system specify outlines inline;

	GAA>2. emission system that knows device capabilities
pre-rasterizes,
	GAA>changing outlines to bitmaps;

	GAA>If we fail to define a way to embed bitmaps, then in this
scenario,
	GAA>the emission system would be forced to use a different content
format
	GAA>or to extend it in a potentially non-interoperable way.

	Personally I feel that in most cases the cause is lost for existing
**emission systems** (e.g. TV, DAB, DVD) adopting TT. However, for new
applications there is an opportunity provided that the TT standard is
flexible enough. For me, the major role for TT will be in the authoring,
storage and data exchange contexts.

	Certainly if TT were to be adopted for subtitling for our purposes
it would need a parallel or extension mechanism to carry timed graphic data.
Current multimedia standards (eg SMIL) are generally not appropriate for
subtitling.

	GAA>I would like to see us be able to support this scenario without
	GAA>requiring a different content format or a non-standard
extension.

> G.
> 
Received on Monday, 3 February 2003 04:39:38 GMT

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