If you are working with OpenType Variable Fonts, there are a few additional functions you should use to specify the variation-axis settings of your font object. Without doing so, your variable font's font object can still be used, but only at the default setting for every axis (which, of course, is sometimes what you want, but does not cover general usage).
HarfBuzz manages variation settings in the
hb_variation_t data type, which holds a tag for the
variation-axis identifier tag and a value for its
setting. You can retrieve the list of variation axes in a font
binary from the face object (not from a font object, notably) by
calling hb_ot_var_get_axis_count(face)
to
find the number of axes, then using
hb_ot_var_get_axis_infos()
to collect the
axis structures:
axes = hb_ot_var_get_axis_count(face); ... hb_ot_var_get_axis_infos(face, 0, axes, axes_array);
For each axis returned in the array, you can can access the
identifier in its tag. HarfBuzz also has
tag definitions predefined for the five standard axes specified
in OpenType (ital
for italic,
opsz
for optical size,
slnt
for slant, wdth
for
width, and wght
for weight). Each axis also
has a min_value, a
default_value, and a max_value.
To set your font object's variation settings, you call the
hb_font_set_variations()
function with an
array of hb_variation_t variation settings. Let's
say our font has weight and width axes. We need to specify each
of the axes by tag and assign a value on the axis:
unsigned int variation_count = 2; hb_variation_t variation_data[variation_count]; variation_data[0].tag = HB_OT_TAG_VAR_AXIS_WIDTH; variation_data[1].tag = HB_OT_TAG_VAR_AXIS_WEIGHT; variation_data[0].value = 80; variation_data[1].value = 750; ... hb_font_set_variations(font, variation_data, variation_count);
That should give us a slightly condensed font ("normal" on the
wdth
axis is 100) at a noticeably bolder
weight ("regular" is 400 on the wght
axis).
In practice, though, you should always check that the value you
want to set on the axis is within the
[min_value,max_value]
range actually implemented in the font's variation axis. After
all, a font might only provide lighter-than-regular weights, and
setting a heavier value on the wght
axis will
not change that.
Once your variation settings are specified on your font object, however, shaping with a variable font is just like shaping a static font.