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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: Michael Dolan <mdolan@newtbt.com>
Date: Thu, 22 May 2014 07:44:10 -0700
To: "'Timed Text Working Group'" <public-tt@w3.org>
Message-ID: <00a801cf75cc$4c6aa770$e53ff650$@newtbt.com>
This and issue 317 call to light the difference between alignment with the related media (video) object versus display rendering.  All that can really be contemplated by an author is alignment with the related coded video, not how the resulting  decoded video+text is mapped to an (entirely unknown to the author) display resolution, framerate, etc.

The HRD is ONLY about document conformance.

	Mike

-----Original Message-----
From: Timed Text Working Group Issue Tracker [mailto:sysbot+tracker@w3.org] 
Sent: Thursday, May 22, 2014 4:13 AM
To: public-tt@w3.org
Subject: ISSUE-318 (HRM glyph copy assumes no sub-pixel positioning): Hypothetical Render Model glyph copy assumes no sub-pixel positioning [TTML IMSC 1.0]

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 Thursday, 22 May 2014 14:44:45 UTC

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