Re: [Freetype] Line space calculation in FT 1

[Antoine Leca]

Hi Rob,

On Thursday, April 22, 2004 8:06 AM
Rob Kramer <address@hidden> va escriure:

> I hope this mailing list is still alive.. :)

Yes, because it is used for FT2 as well ;-)

However, you should really think about switching to FT2: FT1 is not really
maintained, only the obvious bugs are corrected.


> I'm looking for a way to calculate a good starting value for the
> distance between two baselines. In FT2, i've seen some people use
> some factor of face->size->metrics->height for that. Or perhaps
> FT_FaceRec.height, although I'm not sure how that is different from
> the other height field.
<snip>
> How can I calculate this height in FT1?


FT1 is Truetype only. Hence baseline distance is much easier to sort out,
there is less possible variation.

Basically, you should follow Microsoft's rules here:
<URL:http://www.microsoft.com/typography/OTSPEC/recom.htm>

It is expressed for font designers, so you should read it backwards, but it
should give you the result;

If there is an OS/2 table, Windows-flavour (prop->os2->version != 0xFFFF),
you can use it:

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

prop->os2->sTypoAscender - prop->os2->sTypoDescender +
prop->os2->sTypoLineGap

</CITE>

However, for reasons known only to themselves, Windows designers did use a
formula a bit more complex:
<CITE>
prop->os2->usWinAscent + prop->os2->usWinDescent
    + MAX(0, prop->os2->sTypoLineGap
        - ((prop->os2->usWinAscent + prop->os2->usWinDescent)
            - (prop->horizontal->Ascender - prop->horizontal->Descender)))
</CITE>

Beware on the units: os2 fields are integers, horizontal ones are
fixed-point!


When there is no OS/2 table (prop->os2->version == 0xFFFF), or when only the
original OS/2-flavour is available (68-byte format; they are not handled
correctly by FreeType; since nobody asks, this is not a problem in
practice), the only remaining way is the traditional Apple one,

prop->horizontal->Ascender + prop->horizontal->Descender +
prop->horizontal->Line_Gap


Please double check the signs: I did copy documentations. If it happens to
be wrong, please feel free to comment here, so this will get recorded.


Hope it helps,


Antoine