letterspacing, different types of ligatures and ZWNJ

[j_mach_wust shared-files de]

Hi

I am developing blackletter fonts. In these fonts, certain ligatures  
must not be broken up when letterspacing is increased (ch, ck, ſt,  
tz). I have encoded these with the OpenType feature "rlig" (required  
ligatures). The other ligatures must be broken up when letterspacing  
is increased. I have encoded these with the OpenType feature "liga".

An example: The character sequence "ch", encoded with "rlig", should  
always be rendered with the ch-ligature glyph, irrespective of  
letterspacing. On the other hand, the character sequence "fi", encoded  
with "liga", should be rendered with the fi-ligature glyph in normal  
text, but when the letterspacing is increased, it should be rendered  
with the f-glyph follwed by increased letterspace followed by the i- 
glyph. For more detailed explanations and screenshots, see:

http://unifraktur.sourceforge.net/letterspacing.html


On Ubuntu applications such as Firefox, this does not work correctly.  
I am not quite sure, but from what I understand, text rendering in  
Ubuntu Firefox is done by Pango, so I think this might be a bug in  
Pango.

The incorrect rendering in Ubuntu Firefox has two errors:

1. When the letterspacing is increased, the "liga" ligatures are still  
used, instead of the single characters' glyphs.

2. When letterspacing is increased, the ZWNJ (U+200C, required for  
German words such as "Zeit‌zone" 'time zone' that must not hava a tz- 
ligature) triggers double letterspacing as if it were a zero-width  
character. The ZWNJ should only inhibit ligatures, but it should not  
affect the letterspacing. Of course, fonts use a zero-width glyph for  
the ZWNJ. The text renderer, however, must be instructed to disregard  
the ZWNJ glyph when it comes to letterspacing.


Neither of these two errors occurs in Firefox on Mac OS X or in  
InDesign.

--
grüess
mach