Re: What's an em

Erik van der Poel wrote at 02/02/00 06:15

>My understanding is that the "perceived" size is so dependent on the
>x-height, that the heading wouldn't *look* smaller than continuous text
>on the same page.

Let me give an example, using the "oh so realistic" comparisons 
previously cited in the CSS2 spec. Compare the font size adjust at the 
extremes: Verdana and Flemish Script.

     If I set a heading in 24pt Flemish Script, but the
     user decides that they really want everything in Verdana
     then the font size adjust mechanism will scale the "real"
     size by 0.5 to 12pt to match the "perceived" font size.

Correct?

Isn't it reasonable to assume that continuous text on the same page might 
be set at 12pt? Thus the heading would be indistinguishable in size from 
the text. Perhaps more realistic is that a heading might be 18pt, thus 
with font size adjust it would scale to 9pt - smaller than the text.

Perhaps other more realistic outcomes are that a large x-height sans like 
Verdana is being used in continuous text, or as sub heads, to match the 
"perceived" size of the text, thus in the designers intended style the 
sans is set perhaps 1pt smaller than the text (this is quite common).

But the user says, "I want Times", thus the sans sub heads and emphasis 
in text get scaled by 1.26 the size of the text, ie perhaps (10pt - 1pt) 
* 1.26 = 11.34 - bigger than the continuous text, which wasn't the 
designers intention either.

Perceived size isn't just a factor of x-height, it's a factor of design, 
ie relationships between x, cap, ascender and descender.

>
>> Do you want to give an example of where you think knowing the x-height
>> might be something useful?
>
>Suppose the document author chose Verdana at a certain size. Now suppose
>the reader doesn't have Verdana...

I think then the document creator should have used alternatives, some of 
which were guaranteed to be on the system. In continuous text I think 
document creators generally specify the default sytem fonts as they can 
be assumed to be available. They try to represent something that all 
users will see, rather than what their real preference would be.


-- Clive

Received on Thursday, 3 February 2000 10:26:11 UTC