[NTG-context] open type feature "frac"

Hans Hagen j.hagen at xs4all.nl
Sat Sep 29 00:06:51 CEST 2018


On 9/28/2018 10:56 PM, Ulrike Fischer wrote:
> Due to a question on tex.sx
> (https://tex.stackexchange.com/questions/452969/luatex-fontfeaturefractions-on-turns-first-digits-into-superscripts)
> I looked again (see
> https://www.mail-archive.com/ntg-context@ntg.nl/msg84822.html) at
> the frac feature.
> 
> The output is still not really satisfactory. With mode=node numbers
> with slash looks okay, but single numbers are rather odd: only the
> last digit has the correct size:
> 
> \starttext
> 
> \font\test={file:linlibertine_r.otf:mode=node;script=latn;language=DFLT;+frac}
> \test
> 787347 125

it all depends on the font and lookups i guess

the logic can be a combination of:

1 - replace digit when followed by digit or slash
2 - replace digit when preceded by slash
3 - replace digit when preceded by fract digit

so a last digit not preceded will not

in fact one can then as well replace all digits non contextual

> 1/7 1/2 3/4 5/6 7/8 9/10 11/12 31415/27182 1000/1000000
> 
> \font\test={file:linlibertine_r.otf:mode=base;script=latn;language=DFLT;+frac}
> \test
> 787347 125
> 
> 1/7 1/2 3/4 5/6 7/8 9/10 11/12 31415/27182 1000/1000000

forget about base mode ... dependent on how the font does frac single 
substitutions will be applied (maybe ligs)

> \stoptext

> (It could be a font bug, the output with xetex isn't good either).
fonts probably have a limited set of lookups

(the 5 and 7 suggests some inconsistency in the lookups)

i must admit that i've seen so many bad frac implementations that i 
never use it (and it's definitely something that should be applied very 
selectively, not on a whole text stream)

Hans

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