[Gimp-gui] opentype feature support

[Liam R E Quin <liam holoweb net>]

Inkscape and LibreOffice both have some openType font feature support.
It's a major part of OpenType, and it's big in graphic design. Examples
include
* alternate glyphs, that might be accessed with a "gyph palette"
* enabling small caps, different styles of numerals
* control over ligatures (fi, ffi, ct, st) and historical forms (these
vary by language, but tall-s is an example, e.g. used in Fraktur as
well as older Roman types, and moderately widely supported by fonts
today)
* swash forms, very popular in graphic design - e.g. a long
calligraphic tail on a Q or R

LibreOffice has a font name scheme that looks a bit like URI/URL
parameters, and it'd be good if those names also worked in Inksape,
Krita, GIMP etc., which suggests maybe fontconfig support and an XDG
spec.

The GIMP text tool should support a glyphs palette, in which users can
pick characters & alternate characters. This could also be an external
service shared with other programs butright now  i'mthinking there'd be
a glyph doc.

GIMP could also support setting/unsetting font features over ranges
(span) of text, just as it supports bold & italics today. The hard part
here is the user interface.

I'm right now thinking it'd be like other things in gimp -
* create new "Font" based on existing one
* a dialogue (dock) to edit opentype  font features, e.g.  kerning
on/off, and to give the "font" a  name
* then you'd be able to use this "font" in the existing text tools.

Users should also be able to find fonts with specific opentype
features, and specific language support. We could use the existing
tagging mechanism for this, and add some default tags. I've tried this
and suspect there may be some performance issues, but they are
resolvable.

The gtk4-demo program emphasizes internationalization rather than
graphic design in the way it shows font features; libreoffice has a
moderately compact widget for enabling/disabling font features. But, as
with Inkscape, it's hidden under a button to make it "advanced".

I think this is because it's hard to see how to make a generic UI for
it. There's some 30 or so (i forget) different font features.

But they are not all equal. For example, turning kerning on or off is
one of them. This is much more basic than controlling historical
ligatures & a UI could treat them separately.

We also have a split text UI today, with on-canvas editing and also
tool options editing e.g. for choosing a default font. And a text
editing box.

An alternate design would be to present font feature settings as
properties of the text (like bold and underline) rather than of the
font, so you'd select a span of text and click "discretionary ligatures
n, please" or "small caps here"

A compromise might be if the drop-down list of fonts on-canvase for the
text tool kept the most recently used few fonts copied at the top of
the list, so if you'd defined your own variants you could switch to
them easily.

What  do you think?

(i can't promise i'll  ever actually get to much of this but if we 
have a discussion maybe it can happen bit by bit)

Liam / slave ankh / demiboy / whatever

-- 
Liam Quin, https://www.delightfulcomputing.com/
Available for XML/Document/Information Architecture/XSLT/
XSL/XQuery/Web/Text Processing/A11Y training, work & consulting.
Barefoot Web-slave, antique illustrations:  http://www.fromoldbooks.org