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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: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Fri, 23 May 2014 15:39:10 +0000
To: Pierre-Anthony Lemieux <pal@sandflow.com>
CC: Timed Text Working Group <public-tt@w3.org>
Message-ID: <CFA52959.1E439%nigel.megitt@bbc.co.uk>
Hi Pierre,

I guess that's reasonable - it would be worth appending to the
introductory blurb in 7 paragraph 2 a sentence such as: "Nor does it
indicate all of the processing requirements for an implementation; for
example it does not model sub-pixel character positioning and anti-aliased
glyph rendering which an implementation may use to produce easily readable
output." to make it absolutely clear to implementers that they shouldn't
be looking here for hints but if they are, then they need to think bigger.

kind regards,

Nigel


On 23/05/2014 16:22, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:

>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-w
>>w-profiles.html#paint-text
>>
>>
>>
Received on Friday, 23 May 2014 15:39:43 UTC

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