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Re: [css3-values] Unit normalization and types

From: David Woolley <forums@david-woolley.me.uk>
Date: Sat, 28 Mar 2009 11:11:17 +0000
Message-ID: <49CE05D5.1000303@david-woolley.me.uk>
To: "www-style@w3.org" <www-style@w3.org>
Giovanni Campagna wrote:

> According to the current text, the only absolute units are cm, mm, pt,
> in and pc.
> Does this means that px units are converted to cm (or in if the
> developer is British) in the Computed Values, then back to px in the
> Actual Values?
> What absolute unit should be chosen?
> What is the meaning of converting px to cm, considering that the UA
> has probably no knowlegde about the diagonal of my monitor?

Pixels have been a confusing area for a long time.  Officially they 
actually represent a proportion of the screen (window?) and I believe 
that some browsers have even attempted to do that.  More rationally, 
though they are asymptotic to that for small physical pixels, but always 
a integral number of physical pixels, or integral fraction of one.

For accessibility purposes, they should be treated as being absolute 
units, and therefore their use should be avoided.

It should always be possible for a user agent with real display to 
convert to cm, as the user agent cannot display cm correctly without 
knowing the physical pixel size.  What it knows may be wrong, and often 
is wrong on common GUI systems, although I believe both Windows and X 
have provision for proper calibration.  Obviously, any conversion to cm, 
on a browser, that maintains physical pixel integrity, needs to involve 
rounding to physical pixels before converting.

I've ignored the issue of rectangular pixels that sometimes comes up 
when discussing the px unit.

PS.  It might bave been better to address each sub-topic in a different 
article, as there is as risk that only one will get discussed.

-- 
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
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Received on Saturday, 28 March 2009 11:12:25 GMT

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