# Re: Making pt a non-physical unit

From: Christoph Päper <christoph.paeper@crissov.de>
Date: Fri, 15 Jan 2010 22:40:27 +0100
Message-Id: <4D629E0E-B66C-45B5-9FCB-52C2645B7513@crissov.de>
To: www-style <www-style@w3.org>
```Robert O'Callahan:
>
> Then we can define the CSS absolute length units:
> 1in == 96px
> 1pt == 4/3px
> 1pc == 16px
> 1cm == 96/2.54px
> 1mm = 96/25.4px

1mm ~= 3.7795px <=> 127mm == 480px  =>  1in = 25.4mm     1mm ~
2.8346pt

When CSS drops the true meaning of physical units, it can just as
well go on and change their definitions to make them more practical.
That means ‘mm’ and ‘cm’ (which no longer resembled millimetres and
centimetres anyway) would be integer or small fraction multiples of
‘px’. This would of course depart the relation of CSS inches to CSS
centimetres from the real world 2.54 cm/in.

1mm == 3.75px   <=>   4mm == 15px   =>  1in = 25.6mm     1mm =
2.8125pt
or 1mm == 3.8px    <=>   5mm == 19px   =>  1in ~ 25.263mm   1mm =
2.85pt
or 1mm == 3.84px   <=>  25mm == 96px   =>  1in = 25mm       1mm =
2.88pt
or 1mm == 4px                          =>  1in = 24mm       1mm = 3pt
1cm == 10mm

Actually I am kidding, mostly. If I read the discussion correctly,
the current closest to consensus proposal is something like this:

@media screen, projection, handheld {
1px := 1dot; /* maybe default zoom for smaller pixels */
}
@media screen, projection {
1pt := 4/3px; 1in := 72pt;
1mm := 5/127in;
}
@media print, handheld {
1mm := 1 mm; /* SI unit, probably rounded to 1dot */
1pt := 1/72in; 1in := 25.4mm;
}
@media print {
1px := 0.75pt = 15twip;
}
@media all {
1pc := 12pt;
1cm := 10mm;
}
```
Received on Friday, 15 January 2010 21:41:01 UTC

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