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Re: Making pt a non-physical unit

From: Ambrose LI <ambrose.li@gmail.com>
Date: Tue, 12 Jan 2010 19:11:37 -0500
Message-ID: <af2cae771001121611u4b7ff5e1y8354efa390948a47@mail.gmail.com>
To: Christoph Päper <christoph.paeper@crissov.de>
Cc: www-style W3C Group <www-style@w3.org>
2010/1/12 Christoph Päper <christoph.paeper@crissov.de>:
> If we redefined only ‘pt’ (with 3pt = 4px) that reason basically vanishes.
> The ‘pt’ replacement (e.g. Tex’s ‘bp’ or just ‘p’) and the other absolute
> units would then still be rendered physically correct when the physical
> viewport dimensions are known (e.g. @media print including PDF export,
> probably also handhelds). If they are unknown (@media screen) or unknowable
> (@media projection), which authors must avoid, I can think of several
> fallback solutions:
> 1. make 1bp = 1pt = 4/3px (i.e. “96dpi” resolution),
> 2a. assume a certain standard viewport, e.g. about 30cm = 12in high (i.e.
> portrait A4, landscape A3; 19"@5:4, 20"@4:3, 22"@8:5, 24"@16:9 screen
> diameters), and know its logical dimensions of course,
> 2b. like 2a, but also apply media specific magnification (e.g. 1 @media
> screen, 2 @media [hd]tv, 5 @media projection),
> 3. any better algorithm someone comes up with.
> That is basically what Robert O'Callahan wrote earlier, only with an
> algorithm defined in the specification:
[stuff deleted]
>>> I do see a use case for a device pixel unit,
> Which would be called ‘pel’ or ‘dot’, I guess.

I don't know what I want to say about these new units, nor do I have a
better suggestion, but I think this is going to make things even more

TeX users who know what bp is will see bp and assume that pt means
1/72.27" (since bp = 1/72", which is bigger than 1/72.27", hence "big"
point), which was not, is not, and will never be the case.

And pel, dot, and px basically mean the same thing.

FWIW, in Postscript, a width of a minimum device pixel (no "minimum
dot size" or any consideration whether you can see the resulting line)
can be specified, and it is represented as 0 (zero points, or 0bp
since Postscript points are TeX bp).

Received on Wednesday, 13 January 2010 00:12:10 UTC

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