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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: Timed Text Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Thu, 22 May 2014 11:13:20 +0000
Message-Id: <E1WnQw4-0003oM-ST@stuart.w3.org>
To: public-tt@w3.org
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 11:13:22 UTC

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