Kitty shrinks glyphs with their ink box wider than the advance width.

[be5invis]

cc. @jpotier

@jpotier built an Iosevka build with ligations, however Kitty zooms out the glyphs that having larger ink box. In Iosevka, there are glyphs that extends out the x=0 and x=500 boundary, to connect to other glyphs to form a ligature.

[jpotier]

Oh, I just opened the same bug >< I'll remove it

[be5invis]

Per @jpotier's font file, (download), the calt GSUB is defined like:

The hyphen glyph is replaced by a bar that overflows the left boundary to connect with the < glyph.

The other ligations are defined in the similar manner:

[kovidgoyal]

That's not going to work in kitty. There are two types of ligatures that will work, either substitute a single glyph for '<-' that is two chars wide or a dummy glyph + single glyph that is two chars wide with a negative left bearing. For example, from Fira Code for '<-'

hb-shape --cluster-level=1 --show-extents  ~/work/env/conf/fonts/FiraCode-Regular.otf '<-'     
[LIG=0+600<0,0,0,0>|less_hyphen.liga=1+600<-477,618,952,-610>]

Notice the first glyph is a dummy glyph and the second has a negative left bearing and is two cells wide. This pattern needs to be followed for most ligatures anyway, since they cannot typically be created by leaving some glyphs unchanged and changing only other glyphs. So stick to that pattern for all ligatures and it will work fine.

[be5invis]

@kovidgoyal
Well, in Iosevka, actually, the ligations could be arbitrary length, like this:

So, I cannot stick to that pattern.

[jpotier]

@kovidgoyal can the single glyph be of more that 2 times a base glyph width? In iosevka there's the WwW glyph, how does this one work?

[kovidgoyal]

Well, kitty is never going to support arbitrary length ligatures, as it works by storing cell level rendered bitmaps (technically alpha masks) on the GPU for performance, and that scheme has to have a finite max ligature length, otherwise glyph lookup times become too large.

@jpotier substitute N for 2 in my previous post and it remains the same. The only limitation is that in kitty the max value for N is 9. Basically N character string must be replaced by a ligature that is N glyphs wide.

[be5invis]

@kovidgoyal Hmmm, you will experience a lot of problems when dealing with complex scripts (like Indian scripts, Arabic, ...)

[jpotier]

Ok. So, only way to get it to work for me, would be to change how ligatures are done in the font. I have no experience in this field, though.

@be5invis well kitty is a terminal emulator. Maybe that's not going to be a problem :D

[kovidgoyal]

Yes, I know. I dont intend to ever support complex scripts in kitty. It is fundamentally a character grid, scripts that dont work in character grids are not going to be renderable.

[jpotier]

@be5invis How hard would it be for me to implement ligatures by glyph substitution for Iosevka? Any tools you know on Linux for that task?

[kovidgoyal]

@be5invis I'm not sure I understand why you want to support arbitrary length ligature in any case? If one wants to draw solid lines, the way to do it is to use the unicode box drawing characters, not replacing ------- with a ligature. Maybe there's some use case I am overlooking.

[be5invis]

@kovidgoyal You know, in functional languages like Haskell there are very long operators (look at them in Lens!), so, hmmm, I made the arbitrary-length operators.

[kovidgoyal]

I'm not familiar with Haskell -- but surely they are not arbitrarily sized operators! I imagine sticking to a max of five characters would be ample -- kitty itself supports ligatures of up to 9 characters, this is because of Source Code Pro with has ligatures for [WARNING]