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

[Anson]

On 5/26/05, Werner LEMBERG <address@hidden> wrote:
> 
> > I have compiled a file documenting different metrics I have come
> > across (see attached rtf.)
> 
> Nice!  Note that FreeType tries to hide (resp. unify) the various
> metrics definitions as much as possible.

Any particular reasons for this?  As far as I could tell the metrics
definitions I could find are slightly different from each other.

> > 1. What is the best way to get the ascent and descent values for Latin
> > characters?  (effectively 'A' to 'Z' and 'a' to 'z'.)
> 
> This is only 52 glyphs.  Simply get the metrics of them and do the
> calculation by yourself!  BTW, there is another complication: Many
> glyphs have some vertical `overshoot' so that they are slightly larger
> than the (global) ascent value of the font.  This is an optical
> correction normally done for glyphs with round shapes at the top or
> bottom.  Examples are glyph `o' or `m'.

I have done this.  Attached are the two metrics I got.  I have 2 more
questions regarding this:

1. There is discrepancy between the calculated metrics (under DIY) and
those retrieved from the OpenType OS2 Table.  i.e.  If I compare
winAscender/winDescender with Latin-1 in the Times New Roman, or the
ascender under OS2 table and DIY in the New Courier.  Do you know why?

2. This _really_ puzzles me.  I call FT_Set_Pixel_Sizes to set the
character dimensions of the desired face object (in the way shown in
the attached source code.)  But if I sum up any pairs of
ascender/descender none of them are close to the set size.  Do you
know why?

> > 2. What is "AFM file" as mentioned in sTypoAscender and sTypoDescender
> > documentation?
> 
> Adobe Font Metrics files are text files which accompany Type 1
> PostScript fonts (which don't contain metrics information).  On
> Windows box you'll sometimes fine PFM files which are something
> similar.
> 
> 
>    Werner

Thanks very much again for your help Werner.  Your responses have been
very helpful.

Anson

NewCourier30.txt
Description: Text document

TimesNewRoman34.txt
Description: Text document

metrics.cpp
Description: Binary data