*** Bug 786985 has been marked as a duplicate of this bug. ***

See: https://www.microsoft.com/typography/otspec/otvaroverview.htm

While fixing this, I like to also fix this old issue:
https://bugzilla.gnome.org/show_bug.cgi?id=95043

Suggested api is:

void
pango_font_description_set_variation_settings (PangoFontDescription *desc,
                                               const char           *settings);

With a syntax like

"wght=600,wdth=70".

For pango_font_description_to_string,
we could use @ to make the variation settings part:

"Noto Sans @wght=600,wdth=70"

We need a separate pango_font_description_to_display, which would convert
the variation settings to a human-readable string, using axis names, etc.

To get more useful names for variation axes, we need the font face to lookup
name strings.

I pushed some very preliminary work in wip/matthiasc/font-variations.
I just adds a string to PangoFontDescription, with some setters and getters.

Next step is to figure out how this gets passed down to actually loading a font. Is see

pango_fc_font_map_load_font
  pango_fc_font_map_load_fontset
    pango_fc_font_map_get_patterns
      pango_fc_fontset_new
        pango_fc_font_map_new_font

We store a shallow copy of the font description in the fontsetkey, and at some point we generate FcPatterns, and load an FcFont from it.

I think we can treat font variations like we treat the matrix here.

In the pangocairo case, we further have

pango_cairo_fc_font_map_create_font
  _pango_cairo_fc_font_new
    _pango_cairo_font_private_initialize

Which gets us close to the place where we create a cairo scaled font. We can pass the font variations down there and keep them in the _PangoCairoFontPrivateScaledFontData struct.

There is no cairo api to create a scaled font with variation settings. Do we need one ?

I guess the only place where we can really apply the font variation settings is in pango_cairo_fc_font_lock_face - where we can call FT_Set_Var_Design_Coordinates, right before handing out the FT_Face object.
Any place higher up risks that we run into issues with the several layers of font caching leading to unintentional sharing of FT_Face instances that have variation settings applied

I've put some more code into that branch, but it doesn't work

Created attachment 359792 [details] [review]
patch

This one works. It requires a cairo patch that has been sent to the list.

The patch adds new api, and i've marked it as PANGO_AVAILABLE_IN_1_42, which i guess means that we should branch off a stable 1.40 branch and finally open a new development cycle before landing this

Can you push your Pango stuff to a github or git.gnome branch as well?

its here https://git.gnome.org/browse/pango/log/?h=wip/matthiasc/font-variations