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Re: [SVGMobile12] SVGT12-207: Why are UAs allowed to have varying definitions of "px"?

From: Chris Lilley <chris@w3.org>
Date: Thu, 6 Apr 2006 15:15:53 +0200
Message-ID: <972066893.20060406151553@w3.org>
To: www-svg@w3.org
Cc: Ian Hickson <ian@hixie.ch>

Hello www-svg,

Ian Hickson <ian@hixie.ch> wrote:
>> Operating systems vary in their ability to say what the display density
>> is. Some do not provide such a call at all; others always return
>> hard-coded or inaccurate values (72, 75, 96, 100 and 120 are popular
>> values encountered).
>> Therefore, a "should" seems appropriate here.
> I was not debating whether it was appropriate or not. My request was
> to have the specification list these points explicitly if there were
> any, or to have the requirement strengthened if there were not.

Having the requirement strengthened from should to must does sound as if
you are debating it. Sticking for the moment with 'should', which means
that the implementation is normally to follow the requirement unless it
has good reason not to, one possible reason not to was given in the
quoted response above:some platforms do not make the requisite display
density information available to perform the calculation.

We were tempted to  explicitly list those reasons in the spec - must do
this unless the display density is not known. However, that would be a
mistake. There could be other valid reasons. Some that spring to mind:

- a hovering 'magnifying glass' for those with diminished visual acuity
- editing application with 'preview' rendering simulating other devices
- print preview
- thumbnailing application

We cannot claim to have thought of all the possible good reasons that
might call for a different display density to be used for some reason.

> Thus, your response does not satisfy my request.

On further consideration, a should still seems to be the correct term;
an explicit list of allowed exceptions does not seem beneficial and is
contrary to accepted practice for a 'should'; strengthening to a must
would prevent a number of useful real-world applications.

In the interests of clarity, we added the material about lack of APIs
for display density to the spec, but as an example.

 Chris Lilley                    mailto:chris@w3.org
 Chair, W3C SVG Working Group
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Thursday, 6 April 2006 13:16:01 UTC

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