[luatex] Picking defaults for axes on variable fonts

[luigi scarso]

On Sun, Apr 25, 2021 at 4:03 PM Sergio Callegari <sergio.callegari at gmail.com>
wrote:

> On Fri, 23 Apr 2021 09:57:13 +0200 luigi.scarso at gmail.com (luigi
> scarso) wrote:
>
> > On Fri, Apr 23, 2021 at 9:48 AM Sergio Callegari <sergio.callegari at
> > unibo.it>
> > wrote:
> >
> > > Hi,
> > >
> > > I have noticed that luatex now supports variable fonts and I've started
> > > experimenting. I am asking a question here but I am unsure if it is
> > > actually tied to luatex or luaotfload or maybe fontspec. In case,
> please
> > > be so kind to address me to the correct forum.
> > >
> > >
> > hm, luatex and luahbtex don't support variable fonts directly,
> > it's a matter of the format (latex,  context, optex... ) .
> Thanks for providing this info. Can you be so kind to expand a little,
> as there are still a couple of things that are not 100% clear to me?
>
> - Common widom found on the net is that variable fonts work only with
> the LuaTeX engine, no others and that this become possible only with
> LuaTex 1.13. As a matter of fact, xeTeX works without an hiccup on the
> file, but after it it is impossible to make a PDF because dvipdfmx
> reports "invalid font" . This suggests that the engine and specific the
> part of it responsible for the PDF generation plays a role. The only
> item that comes to my mind given the situation is lua. If so, would it
> be luaotfload the upstream to ask the question to?
>
>
yes


> - The errors reported when attempting to use variable fonts appear to
> happen /after/ the format and the tex machinery has completed its work,
> that is after the:
>
>   406 words of node memory still in use:
>     3 hlist, 1 vlist, 1 rule, 2 glue, 3 kern, 1 glyph, 4 attribute, 48
> glue_spec
> , 4 attribute_list, 1 write nodes
>     avail lists: 2:17,3:4,4:1,5:28,6:2,7:116,9:14
>
> What appears to fail is the step following this one, that is the PDF
> generation. This looks consistent with what happens in xetex. But if I
> am correct, in both pdftex and luatex the pdf generation is unseparable
> from the engine, apart from the role of luaotfload...
>
> Any suggestion?
>
>
LuaTeX has  a pdf and dvi backend (as pdfTeX) and a native management of
"traditional"
postscript / truetype /opentype fonts -- but not the font variations.
"Variable fonts" are managed by the format which, basically create an
instance of a "traditional"  font
from the values of the axes --- this is possible because LuaTeX has the
concept of "virtual" font, see
https://www.tug.org/svn/texlive/trunk/Master/texmf-dist/doc/luatex/base/luatex.pdf?view=log

The first implementation (in ConTeXt) was in 2017:
http://www.gust.org.pl/bachotex/2017-pl/presentations/hhagen-2-2017.pdf
https://www.tug.org/TUGboat/tb38-2/tb119hagen-variable.pdf

-- 
luigi
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