Re: kerning/ligatures in opentype fonts

[Werner LEMBERG]

> If for a font "ff" leads to collisions, and the ligature is not
> correct at that point, is it recommended to insert space or to
> switch the font?

Yes, at least for German.

> Or, stated differently: Should I consider it a sign of low quality
> of a font if consecutive letters collide or touch unpleasantly, even
> if a ligature exists?

It depends.  Basically, it's a locale thing.  For example, in Turkish
you must not use an `fi' ligature at all to avoid misinterpretation
with `fı'.  I can imagine that an OpenType font's German language
support recognizes the sequence <f> <ZWNJ> <f> (in the GPOS table),
moving the second `f' glyph slightly to the right to avoid collision.

Cf.

  https://code.google.com/p/chromium/issues/detail?id=22240#c37

The whole, very long Chromium bug report explains issues with German
Fraktur, BTW.

>> As mentioned by Urs, there is even a (semi)automatic solution
>> called `selnolig' which uses a dictionary of German words to find
>> and handle such non-ligatures properly (this is, you don't have to
>> insert "| any more).
>
> Amazing.  IIUC, that requires the /text rendering engine/ to bring
> it's own dictionary?

Well, the TeX engine comes with hyphenation patterns, so why not
having something special for `fi' and `fl' handling, which you can
only resolve with dictionaries?

> (How) does it work if the word is hyphenated at a different
> position?

The massaging of the data happens in multiple passes, if necessary.

> I thought the rendering engine is called with small units of text,
> such as the part before and after the hyphen separately; so the
> context is sent additionally?

TeX normally handles paragraphs as units.


    Werner