Re: what defines the font size?

[Hans Hagen <j.hagen@xs4all.nl>]

On 10/18/2018 10:37 AM, Taco Hoekwater wrote:
> Hi Pablo,
> 
> Let me go back an try to answer the original question:
> 
>> On 15 Oct 2018, at 21:21, Pablo Rodriguez <oinos@gmx.es> wrote:
>>
>> Dear list,
>>
>> sorry for this basic question, but what defines the size in a font?
> 
> The font designer decides on the ‘natural’ size of the font.
> There are two parts to this.
> 
> First, what the ‘natural’ size indicates is the designer’s
> _intended use size_ for the font, such that when you plan
> to use the font “Times-Roman” without any special rescaling,
> it should in fact be equivalent to “Times-Roman at ’natural size’”.
> 
> For most fonts, this ‘natural size’ is 10pt, but special display
> or footnote fonts may have a different intended use size, and the
> font designer may have made special glyph adjustments for that
> purpose. For example, the computer modern family has special fonts
> with a ‘natural’ size anywhere between 5pt and 17pt. The glyphs in
> the specific fonts with a smaller ‘natural’ size (like 8pt)
> are in fact a little bit bolder and wider than the same glyphs in
> the font designed to be used at 10pt. This makes sense when you
> consider that the 8pt font is likely be used along with the 10pt
> font for e.g. footnotes. The 10pt font used at 8pt size would look
> thinner and weaker than the actual font designed for 8pt.
> 
> Second, a design size in points like ‘10pt’ is somewhat misleading,
> because what it actually is, is just a different way of saying “at
> the expected size for traditional main text”. The “10pt" is not
> necessarily a measure of _anything_ in the font. In fact, font designers
> sometimes do not use a “XXpt” design size at all.The Minion font family
> has fonts with names like "Minion Pro Caption" and "Minion Pro Display”,
> which is actually a better indication of the information the font
> designer wants to convey.
> 
> 
> That leaves the question of what the actual size is of a font used
> at “10pt”. As explained above, there are no hard rules. But usually
> for a modern font the “10pt" is the _vertical_ space needed to enclose
> all of the ascenders and descenders in the font when all the glyphs
> are overlaid on top of each other. Traditionally, this was also the
> with of an ‘em’, going back to the Roman era, where inscribed text fitted
> characters into a square. But these days that is no longer always the
> case, since some font families have condensed or extended members
> (and it really only applied to ‘upright’ fonts anyways).
> 
> In short:
> 
>> If two fonts have the same size, I think there may be a dimension which
>> has the same length in both. Which one is this?
> 
> No, there is no such thing. "TeX Gyre Bonum and TeX Gyre Adventor at twelve
> point” really only means this:
> 
>    "TeX Gyre Bonum at a somewhat larger size than the TeX Gyre Bonum designer
>    intended and TeX Gyre Adventor at a somewhat larger size than the TeX Gyre
>    Adventor designer intended"
> 
> Nothing more, nothing less.
A nice explanation.

In context one can actually scale in two more ways than standard tex but 
it's more a gimmick than useful:

\starttext

% at : fraction of design size
% sa : fraction of current bodyfont size
% ht : fraction of font ascender
% cp : fraction of height of glyph X

\starttexdefinition ShowThem #1#2#3
     \startoverlay
         {\color[trace:r]{\definedfont[#1*default #3]This is a just a 
simple line.}}
         {\color[trace:g]{\definedfont[#2*default #3]This is a just a 
simple line.}}
     \stopoverlay
\stoptexdefinition

\ShowThem {dejavu-sans} {ibmplexsans-regular} {at 12pt}
\ShowThem {dejavu-sans} {ibmplexsans-regular} {sa 1}
\ShowThem {dejavu-sans} {ibmplexsans-regular} {ht 12pt}
\ShowThem {dejavu-sans} {ibmplexsans-regular} {cp 12pt}

\blank

\ShowThem {dejavu-serif} {texgyrepagella-regular} {at 12pt}
\ShowThem {dejavu-serif} {texgyrepagella-regular} {sa 1}
\ShowThem {dejavu-serif} {texgyrepagella-regular} {ht 12pt}
\ShowThem {dejavu-serif} {texgyrepagella-regular} {cp 12pt}

\blank

\ShowThem {lmroman12-regular} {lucidabrightot} {at 12pt}
\ShowThem {lmroman12-regular} {lucidabrightot} {sa 1}
\ShowThem {lmroman12-regular} {lucidabrightot} {ht 12pt}
\ShowThem {lmroman12-regular} {lucidabrightot} {cp 12pt}

\stoptext





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