Re: letterspacing, different types of ligatures and ZWNJ

[Behdad Esfahbod <behdad behdad org>]

Hi,

You are right.  Pango has both issues that you pointed out.  I filed the
following bugs so I don't forget about these:

  https://bugzilla.gnome.org/show_bug.cgi?id=628370

  https://bugzilla.gnome.org/show_bug.cgi?id=628371

behdad

On 08/30/10 12:30, j_mach_wust shared-files de wrote:
> 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.
>