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Re: Making pt a non-physical unit

From: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Date: Thu, 7 Jan 2010 17:35:56 +0100
Message-ID: <cb7bb73a1001070835h27562cddqa71bcdb44df5fa42@mail.gmail.com>
To: Boris Zbarsky <bzbarsky@mit.edu>
Cc: Jonathan Kew <jonathan@jfkew.plus.com>, www-style mailing list <www-style@w3.org>, "Robert O'Callahan" <robert@ocallahan.org>
On Thu, Jan 7, 2010 at 3:16 PM, Boris Zbarsky <bzbarsky@mit.edu> wrote:
> On 1/7/10 4:16 AM, Giuseppe Bilotta wrote:
>> Additionally, aside from layout liquidity (how many web pages
>> actually still make use of that, though?), the screen rendering would
>> be just a scaled version of the print rendering
> This is false, since font metrics for a 16px font are not just 4/3 those for
> a 12px font.

The fact that bitmap and high-quality vector fonts may have different
metrics at different design sizes is precisely one of the reasons why
fixing the px/pt ratio actually makes sense. What design size and
rendering size should be selected when a given font-size is specified
in a CSS?

Should a 16px font-size for a a vector font (whose design sizes are
usually in terms of pt) select a 12pt design size (at 96dpi), or a 6pt
design size (at 192dpi)? Conversely for a 12pt font-size for a bitmap

Fixing the px/pt ratio helps preventing that the same size as
specified in pt come out as different design sizes depending on where
the thing is rendered: so a 12pt design size would be equivalent to a
16px design size regardless of whether the rendering is done on a
96dpi device, on a 72dpi device or on a 600dpi device, so you would
actually be selecting the same design size for both screen and print.

>> My vote goes to moving px from being a relative to being an absolute
>> unit of measure, equal to 3/4pt.
> Then there would be no unit at all to express "visible size" in CSS. That's
> what px do right now, and imo is the one thing that's most important for
> units used in CSS....

I'm not sure what you mean by "visible size". px as a single pixel
makes sense for screen media, but it makes much less sense for print
media, unless you plan to make 1px = 1 printer dot. (Which is why the
CSS spec has that discussion that basically implies that the physical
size of a px should be more or less equivalent to the pixel size of a
96dpi monitor held at arms length).

The proposal I'm putting my vote for is to make px=3/4pt=1/96in and
then consider px the 'fundamental unit' in contexts where it makes
sense (monitor, maybe projectors too) while taking pt (or in or cm or
any other physical unit) as fundamental in contexts such as print or
tablet devices where the actual physical dimensions have a higher

This means that where px makes little sense (print media) you get
sensible results for the physical units, and where the physical units
make little sense (screen monitor) you get sensible results for the px
unit. Additionally, you're actually getting consistent font rendering
because the same design size is being used.

Giuseppe "Oblomov" Bilotta
Received on Thursday, 7 January 2010 16:36:51 UTC

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