Re: [Freefont-bugs] ligatures

[Steve White]

Karl

On Feb 17, 2008 1:36 AM, Karl Berry <address@hidden> wrote:
>     But...these, I think, have their own internal idea of which
>     replacements to do.
>
> In the case of TeX, the ligatures are completely controlled by the TFM
> file.  That information comes from the metrics that come with the
> original font, so whatever you specify would, ultimately, be used.
>
I see.  That's good.

>     It looks like many apps that will do obligatory replacements, simply
>     ignore the discretionary ones.
>
> Not surprising.  I wouldn't want an application to treat `st' and `ffi'
> the same, which is where we came in ...
>
Part of the reason for my concern was that I had only seen 'liga' and
'dlig'.  This isn't enough to distinguish necessary, everyday, and
nice ligatures.

Now I have seen 'rlig' ("required") which is the proper table for
ligatures in scripts that are incorrect without them.

>     Could it be there is no standard policy as to how apps should handle
>     these tables?
>
> Even if there is a "standard" policy written down somewhere (I don't
> know of any myself), I would venture to say that there are plenty of
> applications that do whatever the heck they feel like.
>
Well yeah.  But that's why we distinguish between "standards" and
"natural laws".

There is a standard.  All of this is in OpenType.
A listing of tables from OpenType is here
    http://en.wikipedia.org/wiki/OpenType
The MS docs go into much more detail, e.g.
    http://www.microsoft.com/typography/otfntdev/arabicot/features.aspx

The Wiki article remarks that the MacOS lags a bit on conformance to
this standard.

> It sounds like what would work best in practice is to specify the
> ligatures we actually want to be used, like ffi, as obligatory, even if
> they may not be obligatory in a natural language sense.  Whereas marking
> ones we don't want to be used "by default", like st, to be
> discretionary.
>
Not as I now understand it.

The ffi etc should be in 'liga', which is used by many programs by default.
The ligatures required by some language scripts should be in 'rlig'.

I'm not sure about dlig.  I wouldn't put the st there.  It's too odd.
There is also an "hlig" for historical.  But these are all very
special-purpose, and I have my doubts about going into so much detail
that could never capture all the special purposes:  For such things,
maybe it is better to leave them to the document's author to choose.

Cheers!