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##################################
varLib: OpenType Variation Support
##################################
The ``fontTools.varLib`` package contains a number of classes and routines
for handling, building and interpolating variable font data. These routines
rely on a common set of concepts, many of which are equivalent to concepts
in the OpenType Specification, but some of which are unique to ``varLib``.
Terminology
-----------
axis
"A designer-determined variable in a font face design that can be used to
derive multiple, variant designs within a family." (OpenType Specification)
An axis has a minimum value, a maximum value and a default value.
designspace
The n-dimensional space formed by the font's axes. (OpenType Specification
calls this the "design-variation space")
scalar
A value which is able to be varied at different points in the designspace:
for example, the horizontal advance width of the glyph "a" is a scalar.
However, see also *support scalar* below.
default location
A point in the designspace whose coordinates are the default value of
all axes.
location
A point in the designspace, specified as a set of coordinates on one or
more axes. In the context of ``varLib``, a location is a dictionary with
the keys being the axis tags and the values being the coordinates on the
respective axis. A ``varLib`` location dictionary may be "sparse", in the
sense that axes defined in the font may be omitted from the location's
coordinates, in which case the default value of the axis is assumed.
For example, given a font having a ``wght`` axis ranging from 200-1000
with default 400, and a ``wdth`` axis ranging 100-300 with default 150,
the location ``{"wdth": 200}`` represents the point ``wght=400,wdth=200``.
master
The value of a scalar at a given location. **Note that this is a
considerably more general concept than the usual type design sense of
the term "master".**
normalized location
While the range of an axis is determined by its minimum and maximum values
as set by the designer, locations are specified internally to the font binary
in the range -1 to 1, with 0 being the default, -1 being the minimum and
1 being the maximum. A normalized location is one which is scaled to the
range (-1,1) on all of its axes. Note that as the range from minimum to
default and from default to maximum on a given axis may differ (for
example, given ``wght min=200 default=500 max=1000``, the difference
between a normalized location -1 of a normalized location of 0 represents a
difference of 300 units while the difference between a normalized location
of 0 and a normalized location of 1 represents a difference of 700 units),
a location is scaled by a different factor depending on whether it is above
or below the axis' default value.
support
While designers tend to think in terms of masters - that is, a precise
location having a particular value - OpenType Variations specifies the
variation of scalars in terms of deltas which are themselves composed of
the combined contributions of a set of triangular regions, each having
a contribution value of 0 at its minimum value, rising linearly to its
full contribution at the *peak* and falling linearly to zero from the
peak to the maximum value. The OpenType Specification calls these "regions",
while ``varLib`` calls them "supports" (a mathematical term used in real
analysis) and expresses them as a dictionary mapping each axis tag to a
tuple ``(min, peak, max)``.
box
``varLib`` uses the term "box" to denote the minimum and maximum "corners" of
a support, ignoring its peak value.
delta
The term "delta" is used in OpenType Variations in two senses. In the
more general sense, a delta is the difference between a scalar at a
given location and its value at the default location. Additionally, inside
the font, variation data is stored as a mapping between supports and deltas.
The delta (in the first sense) is computed by summing the product of the
delta of each support by a factor representing the support's contribution
at this location (see "support scalar" below).
support scalar
When interpolating a set of variation data, the support scalar represents
the scalar multiplier of the support's contribution at this location. For
example, the support scalar will be 1 at the support's peak location, and
0 below its minimum or above its maximum.
.. toctree::
:maxdepth: 2
builder
cff
errors
featureVars
instancer
interpolatable
interpolate_layout
iup
merger
models
mutator
mvar
plot
varStore
.. automodule:: fontTools.varLib
:inherited-members:
:members:
:undoc-members:
|
