Re: ANN: Emacs on GNUstep / OS X 8.0-rc1

[Adrian Robert]

On Mar 9, 2005, at 2:36 PM, Fred Kiefer wrote:

> Adrian Robert wrote:
> > On Mar 8, 2005, at 10:15 AM, M. Uli Kusterer wrote:
> > > At 16:42 Uhr -0500 07.03.2005, Adrian Robert wrote:
> > >
> > > > http://emacs-on-aqua.sf.net/
> > > >
> > > > Status respecting backends on GNUstep is unchanged: works on Xlib, 
> > > > has
> > > > rendering anomalies under Art.  Specifically, menus are rendered 
> > > > blank
> > > > when they change, and the letter 'f' is rendered in proportional 
> > > > spacing
> > > > (from NSString -drawAtPoint:withAttributes:) in all fonts tried (in 
> > > > some
> > > > cases the character after the 'f' must be 'i' or another 'f').
> > > > "defaults write Emacs GSBackend libgnustep-xlib" is necessary.
> > >
> > >
> > >  Not sure whether you already know the source of the "f" problem and 
> > > simplified the explanation, or whether you don't know where this 
> > > comes from, but this looks to me like ligatures are turned on where 
> > > they shouldn't. Ligatures are a typographical fanciness for 
> > > representing certain character pairs with a single character that 
> > > looks like an amalgamation of the two. Often done for fi, fl and a 
> > > few others.
> > >
> > >  I think there should be a way to turn of ligatures, but you may 
> > > have to use the Cocoa text system directly instead of using 
> > > NSString's drawAtPoint:.
> > Yes, the rendering issue did have to do with ligature, but the fault 
> > is not in Art, but in GUI.  Art just reveals the issue because it 
> > implements ligature whereas Xlib does not.
> > The problem is in [GSLayoutManager -_run_cache_attributes], where it 
> > currently sets a glyph run's 'ligature' attribute by default to '1'.  
> > Either it should set this to '0' if the font is monospaced, or some 
> > lower level should ignore ligature=1 for fixed fonts.
>
> Not sure if this really is the problem. Having the ligature attribute 
> at 1 is some sort of default value, meaning: use ligatures as much as 
> suitable for the font. The back art code tries to only use ligatures 
> if they are available. I would rather expect that the code there 
> somehow fails (_generateGlyphsForRun:at:). Or are you saying that you 
> are using a monospaced font which has ligatures available as well? But 
> even then, where would be the problem?
> Looks like somebody needs to dig deeper into the art font handling to 
> find out.

Somebody more experienced than myself may have more to say here, but 
back-art first does this:

ligature_fi = FTC_CMapCache_Lookup(ftc_cmapcache, &cmap, 0xfb01);

then later on uses ligature_fi if it is non-null and the ligature 
attribute is 1.

I guess FTC_CMapCache_Lookup() is some freetype thing?  If so, then it 
appears monospaced fonts CAN have ligatures set for them.  In any case, 
a monospaced font is often used precisely because of its property that 
each character consumes the same amount of space, so you can organize a 
text area like a grid, line up columns, etc..  Accordingly, on OS X, 
ligatures are not displayed for monospaced fonts, even those that say 
isFixedPitch=NO and don't show up in the font panel under "Fixed 
Width".  This is true even if I force ligatures on using string 
attributes.

So Art should probably explicitly ignore the ligatures even if it finds 
them for monospaced fonts.