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Re: ISSUE-318 (HRM glyph copy assumes no sub-pixel positioning): Hypothetical Render Model glyph copy assumes no sub-pixel positioning [TTML IMSC 1.0]

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Fri, 23 May 2014 08:22:25 -0700
Message-ID: <CAF_7JxBTyn0rXpFXaRANfQw_pu7JMuCsEuG_ootBgtzeesHABA@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
Hi Nigel,

>  This does not take into account sub-pixel positioning of anti-aliased text,
> which would result in different per-pixel buffer values for a glyph that
> would otherwise be considered identical using the current criteria.

Not all implementations will support "sub-pixel positioning of
anti-aliased text".

Implementations that choose to support that feature, and thus will not
be able to use a glyph buffer, will need to implement a
correspondingly higher glyph rendering rate to accommodate documents
compliant with the IMSC Text Profile.

Best,

-- Pierre

On Thu, May 22, 2014 at 4:13 AM, Timed Text Working Group Issue
Tracker <sysbot+tracker@w3.org> wrote:
> ISSUE-318 (HRM glyph copy assumes no sub-pixel positioning): Hypothetical Render Model glyph copy assumes no sub-pixel positioning [TTML IMSC 1.0]
>
> http://www.w3.org/AudioVideo/TT/tracker/issues/318
>
> Raised by: Nigel Megitt
> On product: TTML IMSC 1.0
>
> IMSC 1 FPWD includes in the Hypothetical Render Model a test for how two glyphs can be considered identical for buffer copying purposes. This does not take into account sub-pixel positioning of anti-aliased text, which would result in different per-pixel buffer values for a glyph that would otherwise be considered identical using the current criteria.
>
> For presentation devices that layout text using sub-pixel accuracy and render glyphs with anti-aliasing this test of identity will fail resulting in wrongly painted glyphs.
>
> I propose that an extra criterion is added to the glyph identity test that the post-layout sub-pixel offset relative to the pixel grid, horizontally and vertically, is identical.
>
> [1] https://dvcs.w3.org/hg/ttml/raw-file/ea1a92310a27/ttml-ww-profiles/ttml-ww-profiles.html#paint-text
>
>
>
Received on Friday, 23 May 2014 15:23:15 UTC

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