Re: [ft] scalable font metrics, ascent/descent/height, metrics struct

[Werner LEMBERG]

> By far I have found 3 sets of ascent/descent/height metrics.  We
> have ascender/descender/height from FT_Face.  We have
> ascender/descender/height from FT_Size_Metrics.  We also have
> sTypoAscender/sTypoDescender from TT_Face.  How do they differ from
> each other?

You haven't yet read the documentation files I've pointed to, right?

>From http://www.microsoft.com/OpenType/OTSpec/recom.htm:

  sTypoAscender is used to determine the optimum offset from the top
  of a text frame to the first baseline.  sTypoDescender is used to
  determine the optimum offset from the last baseline to the bottom of
  the text frame.  The value of (sTypoAscender - sTypoDescender) is
  recommended to equal one em.  [...]

  For Western fonts, the Ascender and Descender fields in Type 1
  fonts' AFM files are a good source of sTypoAscender and
  sTypoDescender, respectively.  The Minion Pro font family (designed
  on a 1000-unit em), for example, sets sTypoAscender = 727 and
  sTypoDescender = -273.

  sTypoAscender, sTypoDescender and sTypoLineGap specify the
  recommended line spacing for single-spaced horizontal text.  The
  baseline-to-baseline value is expressed by:

    OS/2.sTypoAscender - OS/2.sTypoDescender + OS/2.sTypoLineGap

  sTypoLineGap will usually be set by the font developer such that the
  value of the above expression is approximately 120% of the em.  The
  application can use this value as the default horizontal line
  spacing.  The Minion Pro font family (designed on a 1000-unit em),
  for example, sets sTypoLineGap = 200.

On this page you find more details how the various metrics data
*within an OpenType file* differ.

A complete description of FreeType's glyph metrics can be found at
http://freetype.freedesktop.org/freetype2/docs/glyphs/glyphs-3.html.
In case this file doesn't explain everything, it is a bug in the
documentation, and I ask you to send questions (again) to the
freetype-devel list so that it is going to be fixed.


    Werner