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Re: [ft] How to determine font ascent

From: Werner LEMBERG
Subject: Re: [ft] How to determine font ascent
Date: Wed, 12 Oct 2005 08:59:19 +0200 (CEST)

> In order to correctly display glyphs I need to know somehow the
> baseline and the "top baseline" (don't know the correct term, the
> term "ascent" seems used for many things that aren't what I mean) of
> the font.  For example, if you don't understand what I mean,
> fontforge displays these two horizontal lines with thin gray lines,
> and even shows the "top baseline" Y coord in the Font Info -> Common
> -> Ascent entry.

This information is not available for all fonts, thus FreeType doesn't
expose it.

> The FT_Face structure contains the max bounding box (bbox) of all
> the character in the font, and the ascender/descender values;
> however these are not the values I'm looking for - the 'ascender'
> value given by freetype is the highest point even hit by a single
> character, and not the "top value" intended for normal characters,
> but which can be exceeded by e.g. diacritics.

I think you are looking for something you don't need at all.

> I need this to properly vertically center a character, otherwise I get
> what you can see on this screenshot:
> The only way I can think of to achieve vertical centering is to know
> how much of a character may go below the baseline (here the
> "descender" value works fine, provided baseline is always at 0 (is
> this true?)).  And if I also will know how much a character may go
> *above* the *top baseline*, I can take the biggest of these two
> values and add such an amount of space below the bottom baseline,
> and above the top baseline, thus glyphs will look vertically
> centered.

The concept of `vertically centering' is wrong IMHO.  Instead, I
suggest you arrange the menu items as if they were normal text, this
is, you put the glyphs of a line onto its baseline, and the glyphs one
line above with a distance of FT_Face->height from the previous
baseline.  This assures that

  . all lines of the menu have the same vertical distance

  . you use the typographically correct distance the font has been
    designed with

  . it works even for glyphs like `(' and `)' which often extend the
    font's default ascent or descent value

  . it works with all font formats


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