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Posted by Alan J. Flavell on 11/23/08 11:43
On Sun, 26 Mar 2006, Carolyn Marenger wrote:
> Points are a unit of measure commonly used in the print industry.
> If I remember correctly, 72 points equals one inch.
CSS size units are set out in detail in the CSS specification[1], it
isn't really necessary to have a good memory.[2]
> Pixels have to do with screen resolution, and their size varies
> monitor to monitor.
Indeed. However, the CSS px units as defined in the CSS specification
aren't (necessarily) physical pixels - they are *supposed* to be
adjusted to the physical resolution, and other parameters, of the
viewing situation.
The important points (no pun intended) to keep in mind are:
1) neither pt units nor px units are, in fact, calibrated on typical
user displays, so you can ask for them till you're blue in the face,
but you'll only get what the user's display was set up to show.
2) Even if pt or px units *were* calibrated, they wouldn't be what the
user wants, since they make no allowances for their viewing
preferences, eyesight etc.
And a secondary point is that MSIE makes it rather difficult for users
to scale text to their preferences, if the author has specified the
size in px or pt units. That alone would be a good reason to avoid
them, even if the other considerations didn't go against them already.
So, the bottom line is that the right choice of units for general WWW
work is em or percent, based on the user's choice of size, whatever
that might be. It meets all of the right criteria, except for those
annoying people who refuse to make their choice of font size and keep
complaining (typically) that it's too large - see
http://css.nu/articles/font-analogy.html
pt units might be useful specifically for print media; whereas
(properly-scaled) px units might be good for a collective viewing
situation (e.g projection), where individual users don't get to adjust
the display to their preferences.
Coming back to the original question, which you were aiming to answer:
the whole purpose of having different sizing systems is so that they
can behave differently when the viewing situation is varied.
Consequently, there can be no straightforward lookup table to convert
from one to another.
regards
[1] Although in theory CSS 2.1 is still a "working draft", it's the
closest thing we have to the current state of affairs, so I'll cite it
here instead of being too pedantic:
http://www.w3.org/TR/CSS21/syndata.html#length-units
The CSS px unit - like it or not - isn't simply a pixel, but is based
on an angle subtended at the typical viewing distance, as is depicted
there in the spec. One might argue it would have been better to call
it something else than a px, to avoid confusion; but what's done is
done, as far as specifying it is concerned. *Implementing* it is a
different matter, though.
[2] Historically, there have been several different "point" units, but
as you rightly say, the one used in this context is 1/72nd of an inch.
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