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Re: Document Coordinate System intepretation

From: Glenn Adams <glenn@skynav.com>
Date: Tue, 23 May 2017 23:09:19 -0600
Message-ID: <CACQ=j+dTKB08jgV4fCHwak9+hC4OviX8JyQNmSCrtyWirmXDEA@mail.gmail.com>
To: Pierre-Anthony Lemieux <pal@sandflow.com>
Cc: "public-tt@w3.org" <public-tt@w3.org>
On Tue, May 23, 2017 at 10:50 PM, Pierre-Anthony Lemieux <pal@sandflow.com>
wrote:

> Hi Glenn et al.,
>
> Before I continue filing issues on PR #321, I thought I would confirm
> (or not) my summary understanding of the document coordinate system as
> currently proposed.
>
> - the coordinate system is in logical pixels
>

yes, effectively in the storage (sample) coordinate space (as governed by
SAR)


> - the aspect ratio of each of the logical pixels is set by the value
> of Pixel Aspect Ratio (PAR) of the root container
>

hmm, it is the aspect ratio of the physical pixels that PAR applies to, or,
to be more precise, pixels in the display coordinate space (as governed by
DAR); logical pixels in the storage coordinate space (as governed by SAR)
have no aspect ratio per se;


> - the origin of the coordinate system is the top left corner of the
> root container
>

yes


> - the positive x-axis points to the right edge of the root container,
>

yes


> while the positive y-axis points to the bottom edge of the root
>

yes


> contain
> - the extent of the root container in this coordinate system is set
> either by (a) tts:extent if specified in "px" or (b) arbitrarily by
> the document processing context such that the ratio of the width
> divided by the height is equal to the computed Storage Aspect Ratio
> (SAR) of the root container
>

yes for (a), for (b), this is not exactly what I wrote, which is

otherwise, the resolution of the root container region is determined
(arbitrarily) by the document processing context
<#terms-document-processing-context> in a manner that respects the resolved
values of SAR and PAR as determined by *H.1 Aspect Ratios*
<#root-container-region-semantics-aspect-ratios> above

in saying that the resolved value of SAR be respected, what I mean is that
width divided by height selected by the document processing context must
equal SAR, and that SAR * PAR must still equal DAR;


>
> Thanks,
>
> -- Pierre
>
>
Received on Wednesday, 24 May 2017 05:10:13 UTC

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