Re: [Freetype] Line space calculation in FT 1

[Rob Kramer]

Hi Antoine,

On Thursday 22 April 2004 05:48 pm, Antoine Leca wrote:
> > I hope this mailing list is still alive.. :)
>
> Yes, because it is used for FT2 as well ;-)

Wow, thanks for the detailed help!

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

I'm stuck with FT1 for historical reasons on this one..

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

"easy" isn't the word I would use, seeing what follows. :)

Does FT2 take care of this sort of rather horrible calculations?

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

Thanks!

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

> 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>

So, I should always use the latter formula?

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

If that's the case, how come the values are the same as in os2?

(gdb) p *fp.os2
$3 = {version = 0, xAvgCharWidth = 476, usWeightClass = 700, usWidthClass = 5,
<snip>
  usFirstCharIndex = 32, usLastCharIndex = 64258, sTypoAscender = 935,
  sTypoDescender = -200, sTypoLineGap = 85, usWinAscent = 1191,
  usWinDescent = 506, ulCodePageRange1 = 0, ulCodePageRange2 = 0}
(gdb) p *fp.horizontal
$4 = {Version = 65536, Ascender = 935, Descender = -200, Line_Gap = 85,
<snip>

Does the horizontal.Version == 0xffff mean it's not valid perhaps?

For the sake of a little experiment, let's use these values to calculate the 
BTBD: 

external leading = max (0, 85 - ((1191 + 506) - (935 + 200))) = 0. 

BTBD = 1191 + 506 + 0 = 1697

Is that correct? To me it feels dodgy that the external leading is zero.. How 
do I convert the 1697 to a number of pixels?

> 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,

How would one know if it's an original OS/2 flavoured table?

Cheers!

    Rob


-- 
Gods don't kill people, people with gods kill people.