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Re: the *precise* definition of 1em

From: David Perrell <davidp@earthlink.net>
Date: Sat, 19 Jul 1997 18:21:53 -0700
Message-Id: <199707200122.SAA20419@sweden.it.earthlink.net>
To: "Joel N. Weber II" <devnull@gnu.ai.mit.edu>
Cc: <www-style@w3.org>
Joel N. Weber II wrote:
> But the above example may not be worth considering, since the 50
point
> font might be too far away to ever be considered.

IMHO, 50pts is not an approximation for 11.

This discussion is really not about em, it's about the meaning of
"approximated". Once you determine that, you've got your em. Em is
nothing but a variable, local to each element, that is initialized with
the em value of the parent element. If the font-size for the element is
declared, the em value is altered accordingly. The value of em must be
resolved before any dependent properties are computed. In your
examples, font-size is being declared but can't be resolved without
some computation, given that CSS1 calls for an 'approximation'.

> one is availible as 9 points and 12 points.  Thus we use 12 points.
> 
> two is available as 10 and 13 points.  The same is true for three,
four
> five, and six.  So for these 5 fonts, we use 10 points.
> 
> The average will be 62/6 = 10 points.
> 
> Thus, the 1em will cause the font named one to appear smaller, since
> 10 (the average) is closer to 9 points than 12 points.

I don't understand what you're saying. How can the 12pt font "appear
smaller" than the 10pt font? If we're talking scalable fonts, there is
no issue. For a non-scalable font, such as a bitmap screen font, the
glyphs will the glyphs of a 12pt font, but all relative measurements
will assume a 10pt font. Therefore, if the element's line-height were
1em, the 12pt glyphs might very well overlap on subsequent lines.

> These cases are obscure, but if we need to make sure that 1em
> will give us the same size as he parent element, the average rule
> won't always work reliably.

If the element consists of a variety of font sizes, what is the
font-size of the element? Whatever you decide that is, that is the em
value for the element.

You could simply take the metrics of the over/undersized font(s), scale
them, use the over/undersized glyphs but space/overlap them in
accordance with the scaled metrics. And forget about approximation, use
the value specified by the author.

But, isn't this so obscure as to be moot? Doesn't the opsys/GDI do most
of the work of displaying text strings, including the scaling of bitmap
screen fonts?

David Perrell
Received on Saturday, 19 July 1997 21:22:26 GMT

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