Re: ascender, descender, cap-height and x-height

Todd Fahrner wrote:
> 
> At 1:56 PM -0800 1/27/00, Erik van der Poel wrote:
> >Daniel Will-Harris wrote:
> >  >
> >  > It's hard to imagine that foundries and individual designers will go
> >  > back and change the tens of thousands of font designs they have to
> >  > fit some standard.
> >
> >I agree. But perhaps we could get the CSS implementations to apply some
> >sort of adjustment, as I mentioned earlier (font size: proposed plan).
> 
> Unless I'm reading incorrectly, your "proposed plan" is to withdraw
> all your previous suggestions regarding em, and do what everybody has
> always done.

That's the first stage of the proposed plan. The next stage is to see if
anybody actually wants to use any of those European fonts that Kent
mentioned. He claims that those fonts cram the accents into the em too,
whereas American fonts often have the accents outside (above) the em.

Perhaps my problem is that I believed him. Do you not believe that there
exist European fonts with accents *inside* the em?

> And that sounds reasonable to me; I'm still not sure
> which problem your suggestions were trying to solve, other than
> perhaps the untrustworthiness of type designers. :^)

If some fonts have the accents inside the em, and some fonts have them
outside, then basing font-size solely on TrueType's em will give you
divergent results, depending on the font.

> The trouble starts when Arial is not available, or Flemish Script or whatever.

Or when the user has selected one of those European fonts with accents
inside the em.

> >Yes, but that is not in the spirit of CSS. CSS allows the user to
> >override the author's choice of font. We need to carefully define what
> >happens to the size of that user's font.
> 
> And a proposed solution is font-size-adjust. The author must provide
> an aspect value (ex/em) for his or her first choice, so the UA has
> (at least half of) the information necessary to scale any
> substitutions appropriately. The results won't be entirely
> determinate, but that doesn't make them useless.

I agree that the results aren't useless. I was hoping to improve the
results by making the font-size part of it match up with reality (the
true height of the ascenders and the true depth of the descenders, as
opposed to the em, which Kent claims can be different for some fonts).

> (A bigger obstacle
> to determinacy is the lack of standard h&j rules.)

What are h&j rules?

> As for getting the available fonts' ex - looking at the bounding
> boxes of acemnorsuvwxz and z ought to do it.

Exactly.

Now, if looking at bounding boxes is a good solution for ex, then why
isn't looking at bounding boxes a good solution for em and font-size?
(Other than the fact that TrueType's square is also called "em".)

> For scripts lacking an obvious analog to ex, well. I don't know.
> Maybe look at extremely low-res display systems capable of
> representing such character sets. What is the  least number of pixels
> required (minimum ppem) to represent the entire set distinctly (plus
> any necessary diacritics)? Now subtract one pixel from this em. Which
> feature of the script can now no longer be represented adequately?
> This is analogous to the ex.

That is approaching it from the legibility angle. But ex and em are
really for scripts that use both upper and lower case letters, and have
certain heights associated with each. In other scripts, e.g. Japanese,
we don't have 2 separate groups like upper and lower case. It's just one
big group of characters that are basically square in nature. So the
height of that square seems like a good font-size to me. And we would
simply not apply font-size-adjust to such scripts. The answer may be
different for other scripts.

Erik

Received on Thursday, 27 January 2000 20:17:06 UTC