[designspaceLib] Update documentation and add version 5

Doc/source/designspaceLib/index.rst

@@ -1,18 +1,217 @@
-##############
+#######################################################
-designspaceLib
+designspaceLib: Read, write, and edit designspace files
-##############
+#######################################################
 
-MutatorMath started out with its own reader and writer for designspaces.
+Implements support for reading and manipulating ``designspace`` files.
-Since then the use of designspace has broadened and it would be useful
+Allows the users to define axes, rules, sources, variable fonts and instances,
-to have a reader and writer that are independent of a specific system.
+and their STAT information.
 
 .. toctree::
    :maxdepth: 1
 
-   readme
+   python
+   xml
    scripting
 
-.. automodule:: fontTools.designspaceLib
+
-   :inherited-members:
+Notes
-   :members:
+=====
-   :undoc-members:
+
+Paths and filenames
+-------------------
+
+A designspace file needs to store many references to UFO files.
+
+-  designspace files can be part of versioning systems and appear on
+   different computers. This means it is not possible to store absolute
+   paths.
+-  So, all paths are relative to the designspace document path.
+-  Using relative paths allows designspace files and UFO files to be
+   **near** each other, and that they can be **found** without enforcing
+   one particular structure.
+-  The **filename** attribute in the ``SourceDescriptor`` and
+   ``InstanceDescriptor`` classes stores the preferred relative path.
+-  The **path** attribute in these objects stores the absolute path. It
+   is calculated from the document path and the relative path in the
+   filename attribute when the object is created.
+-  Only the **filename** attribute is written to file.
+-  Both **filename** and **path** must use forward slashes (``/``) as
+   path separators, even on Windows.
+
+Right before we save we need to identify and respond to the following
+situations:
+
+In each descriptor, we have to do the right thing for the filename
+attribute. Before writing to file, the ``documentObject.updatePaths()``
+method prepares the paths as follows:
+
+**Case 1**
+
+::
+
+    descriptor.filename == None
+    descriptor.path == None
+
+**Action**
+
+-  write as is, descriptors will not have a filename attr. Useless, but
+   no reason to interfere.
+
+**Case 2**
+
+::
+
+    descriptor.filename == "../something"
+    descriptor.path == None
+
+**Action**
+
+-  write as is. The filename attr should not be touched.
+
+**Case 3**
+
+::
+
+    descriptor.filename == None
+    descriptor.path == "~/absolute/path/there"
+
+**Action**
+
+-  calculate the relative path for filename. We're not overwriting some
+   other value for filename, it should be fine.
+
+**Case 4**
+
+::
+
+    descriptor.filename == '../somewhere'
+    descriptor.path == "~/absolute/path/there"
+
+**Action**
+
+-  There is a conflict between the given filename, and the path. The
+   difference could have happened for any number of reasons. Assuming
+   the values were not in conflict when the object was created, either
+   could have changed. We can't guess.
+-  Assume the path attribute is more up to date. Calculate a new value
+   for filename based on the path and the document path.
+
+Recommendation for editors
+--------------------------
+
+-  If you want to explicitly set the **filename** attribute, leave the
+   path attribute empty.
+-  If you want to explicitly set the **path** attribute, leave the
+   filename attribute empty. It will be recalculated.
+-  Use ``documentObject.updateFilenameFromPath()`` to explicitly set the
+   **filename** attributes for all instance and source descriptors.
+
+
+Common Lib Key Registry
+=======================
+
+public.skipExportGlyphs
+-----------------------
+
+This lib key works the same as the UFO lib key with the same name. The
+difference is that applications using a Designspace as the corner stone of the
+font compilation process should use the lib key in that Designspace instead of
+any of the UFOs. If the lib key is empty or not present in the Designspace, all
+glyphs should be exported, regardless of what the same lib key in any of the
+UFOs says.
+
+
+Implementation and differences
+==============================
+
+The designspace format has gone through considerable development.
+
+ -  the format was originally written for MutatorMath.
+ -  the format is now also used in fontTools.varlib.
+ -  not all values are be required by all implementations.
+
+Varlib vs. MutatorMath
+----------------------
+
+There are some differences between the way MutatorMath and fontTools.varlib handle designspaces.
+
+ -  Varlib does not support anisotropic interpolations.
+ -  MutatorMath will extrapolate over the boundaries of
+    the axes. Varlib can not (at the moment).
+ -  Varlib requires much less data to define an instance than
+    MutatorMath.
+ -  The goals of Varlib and MutatorMath are different, so not all
+    attributes are always needed.
+
+
+Rules and generating static UFO instances
+-----------------------------------------
+
+When making instances as UFOs from a designspace with rules, it can
+be useful to evaluate the rules so that the characterset of the UFO
+reflects, as much as possible, the state of a variable font when seen
+at the same location. This can be done by some swapping and renaming of
+glyphs.
+
+While useful for proofing or development work, it should be noted that
+swapping and renaming leaves the UFOs with glyphnames that are no longer
+descriptive. For instance, after a swap ``dollar.bar`` could contain a shape
+without a bar. Also, when the swapped glyphs are part of other GSUB variations
+it can become complex very quickly. So proceed with caution.
+
+ -  Assuming ``rulesProcessingLast = True``:
+ -  We need to swap the glyphs so that the original shape is still available.
+    For instance, if a rule swaps ``a`` for ``a.alt``, a glyph
+    that references ``a`` in a component would then show the new ``a.alt``.
+ -  But that can lead to unexpected results, the two glyphs may have different
+    widths or height. So, glyphs that are not specifically referenced in a rule
+    **should not change appearance**. That means that the implementation that swaps
+    ``a`` and ``a.alt`` also swap all components that reference these
+    glyphs in order to preserve their appearance.
+ -  The swap function also needs to take care of swapping the names in
+    kerning data and any GPOS code.
+
+Version history
+===============
+
+Version 5.0
+-----------
+
+The format was extended to describe the entire design space of a reasonably
+regular font family in one file, with global data about the family to reduce
+repetition in sub-sections. "Reasonably regular" means that the sources and
+instances across the previously multiple Designspace files are positioned on a
+grid and derive their metadata (like style name) in a way that's compatible with
+the STAT model, based on their axis positions. Axis mappings must be the same
+across the entire space.
+
+1. Each axis can have labels attached to stops within the axis range, analogous to the
+   `OpenType STAT <https://docs.microsoft.com/en-us/typography/opentype/spec/stat>`_
+   table. Free-standing labels for locations are also allowed. The data is intended
+   to be compiled into a ``STAT`` table.
+2. The axes can be discrete, to say that they do not interpolate, like a distinctly
+   constructed upright and italic variant of a family.
+3. The data can be used to derive style and PostScript names for instances.
+4. A new ``variable-fonts`` element can subdivide the Designspace into multiple subsets that
+   mix and match the globally available axes. It is possible for these sub-spaces to have
+   a different default location from the global default location. It is required if the
+   Designspace contains a discrete axis and you want to produce a variable font.
+
+What is currently not supported is e.g.
+
+1. A setup where different sources sit at the same logical location in the design space,
+   think "MyFont Regular" and "MyFont SmallCaps Regular". (this situation could be
+   encoded by adding a "SmallCaps" discrete axis, if that makes sense).
+2. Anisotropic locations for axis labels.
+
+Older versions
+--------------
+
+-  In some implementations that preceed Variable Fonts, the `copyInfo`
+   flag in a source indicated the source was to be treated as the default.
+   This is no longer compatible with the assumption that the default font
+   is located on the default value of each axis.
+-  Older implementations did not require axis records to be present in
+   the designspace file. The axis extremes for instance were generated
+   from the locations used in the sources. This is no longer possible.
+

Doc/source/designspaceLib/python.rst

@@ -0,0 +1,199 @@
+################################
+1 DesignSpaceDocument Python API
+################################
+
+An object to read, write and edit interpolation systems for typefaces.
+Define sources, axes, rules, variable fonts and instances.
+
+Get an overview of the available classes in the Class Diagram below:
+
+.. figure:: v5_class_diagram.png
+   :width: 650px
+   :alt: UML class diagram of designspaceLib
+
+   UML class diagram of designspaceLib. Click to enlarge.
+
+.. contents:: Table of contents
+   :local:
+
+.. _designspacedocument-object:
+
+===================
+DesignSpaceDocument
+===================
+
+.. autoclass:: fontTools.designspaceLib::DesignSpaceDocument
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+.. _axis-descriptor-object:
+
+AxisDescriptor
+==============
+
+.. autoclass:: fontTools.designspaceLib::AxisDescriptor
+   :members:
+   :undoc-members:
+   :inherited-members: SimpleDescriptor
+   :member-order: bysource
+
+
+DiscreteAxisDescriptor
+======================
+
+.. autoclass:: fontTools.designspaceLib::DiscreteAxisDescriptor
+   :members:
+   :undoc-members:
+   :inherited-members: SimpleDescriptor
+   :member-order: bysource
+
+
+AxisLabelDescriptor
+-------------------
+
+.. autoclass:: fontTools.designspaceLib::AxisLabelDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+LocationLabelDescriptor
+=======================
+
+.. autoclass:: fontTools.designspaceLib::LocationLabelDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+RuleDescriptor
+==============
+
+.. autoclass:: fontTools.designspaceLib::RuleDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+Evaluating rules
+----------------
+
+.. autofunction:: fontTools.designspaceLib::evaluateRule
+.. autofunction:: fontTools.designspaceLib::evaluateConditions
+.. autofunction:: fontTools.designspaceLib::processRules
+
+
+.. _source-descriptor-object:
+
+SourceDescriptor
+================
+
+.. autoclass:: fontTools.designspaceLib::SourceDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+VariableFontDescriptor
+======================
+
+.. autoclass:: fontTools.designspaceLib::VariableFontDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+RangeAxisSubsetDescriptor
+-------------------------
+
+.. autoclass:: fontTools.designspaceLib::RangeAxisSubsetDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+ValueAxisSubsetDescriptor
+-------------------------
+
+.. autoclass:: fontTools.designspaceLib::ValueAxisSubsetDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+.. _instance-descriptor-object:
+
+InstanceDescriptor
+==================
+
+.. autoclass:: fontTools.designspaceLib::InstanceDescriptor
+   :members:
+   :undoc-members:
+   :member-order: bysource
+
+
+.. _subclassing-descriptors:
+
+=======================
+Subclassing descriptors
+=======================
+
+The DesignSpaceDocument can take subclassed Reader and Writer objects.
+This allows you to work with your own descriptors. You could subclass
+the descriptors. But as long as they have the basic attributes the
+descriptor does not need to be a subclass.
+
+.. code:: python
+
+    class MyDocReader(BaseDocReader):
+        axisDescriptorClass = MyAxisDescriptor
+        discreteAxisDescriptorClass = MyDiscreteAxisDescriptor
+        axisLabelDescriptorClass = MyAxisLabelDescriptor
+        locationLabelDescriptorClass = MyLocationLabelDescriptor
+        ruleDescriptorClass = MyRuleDescriptor
+        sourceDescriptorClass = MySourceDescriptor
+        variableFontsDescriptorClass = MyVariableFontDescriptor
+        valueAxisSubsetDescriptorClass = MyValueAxisSubsetDescriptor
+        rangeAxisSubsetDescriptorClass = MyRangeAxisSubsetDescriptor
+        instanceDescriptorClass = MyInstanceDescriptor
+
+    class MyDocWriter(BaseDocWriter):
+        axisDescriptorClass = MyAxisDescriptor
+        discreteAxisDescriptorClass = MyDiscreteAxisDescriptor
+        axisLabelDescriptorClass = MyAxisLabelDescriptor
+        locationLabelDescriptorClass = MyLocationLabelDescriptor
+        ruleDescriptorClass = MyRuleDescriptor
+        sourceDescriptorClass = MySourceDescriptor
+        variableFontsDescriptorClass = MyVariableFontDescriptor
+        valueAxisSubsetDescriptorClass = MyValueAxisSubsetDescriptor
+        rangeAxisSubsetDescriptorClass = MyRangeAxisSubsetDescriptor
+        instanceDescriptorClass = MyInstanceDescriptor
+
+    myDoc = DesignSpaceDocument(MyDocReader, MyDocWriter)
+
+
+==============
+Helper modules
+==============
+
+fontTools.designspaceLib.split
+==============================
+
+See :ref:`Scripting > Working with DesignSpace version 5 <working_with_v5>`
+for more information.
+
+.. automodule:: fontTools.designspaceLib.split
+
+
+fontTools.designspaceLib.stat
+=============================
+
+.. automodule:: fontTools.designspaceLib.stat
+
+
+fontTools.designspaceLib.statNames
+==================================
+
+.. automodule:: fontTools.designspaceLib.statNames

Doc/source/designspaceLib/scripting.rst

@@ -1,6 +1,6 @@
-#######################
+#########################
-Scripting a designspace
+3 Scripting a designspace
-#######################
+#########################
 
 It can be useful to build a designspace with a script rather than
 construct one with an interface like
@@ -88,6 +88,7 @@ OpenType <https://www.microsoft.com/typography/otspec/avar.htm>`__.
 
 .. code:: python
 
+    # (user space, design space), (user space, design space)...
     a1.map = [(0.0, 10.0), (401.0, 66.0), (1000.0, 990.0)]
 
 Make a source object
@@ -241,7 +242,7 @@ This is how you check the default font.
 Generating?
 ***********
 
-You can generate the UFO's with MutatorMath:
+You can generate the UFOs with MutatorMath:
 
 .. code:: python
 
@@ -251,3 +252,39 @@ You can generate the UFO's with MutatorMath:
 -  Assuming the outline data in the masters is compatible.
 
 Or you can use the file in making a **variable font** with varlib.
+
+
+.. _working_with_v5:
+
+**********************************
+Working with DesignSpace version 5
+**********************************
+
+The new version 5 allows "discrete" axes, which do not interpolate across their
+values. This is useful to store in one place family-wide data such as the STAT
+information, however it prevents the usual things done on designspaces that
+interpolate everywhere:
+
+- checking that all sources are compatible for interpolation
+- building variable fonts
+
+In order to allow the above in tools that want to handle designspace v5,
+the :mod:`fontTools.designspaceLib.split` module provides two methods to
+split a designspace into interpolable sub-spaces,
+:func:`splitInterpolable() <fontTools.designspaceLib.split.splitInterpolable>`
+and then
+:func:`splitVariableFonts() <fontTools.designspaceLib.split.splitVariableFonts>`.
+
+
+.. figure:: v5_split_downconvert.png
+   :width: 680px
+   :alt: Example process diagram to check and build DesignSpace 5
+
+   Example process process to check and build Designspace 5.
+
+
+Also, for older tools that don't know about the other version 5 additions such
+as the STAT data fields, the function
+:func:`convert5to4() <fontTools.designspaceLib.split.convert5to4>` allows to
+strip new information from a designspace version 5 to downgrade it to a
+collection of version 4 documents, one per variable font.

Doc/source/designspaceLib/v5_class_diagram.puml

@@ -0,0 +1,292 @@
+@startuml v5_class_diagram
+
+title
+  Designspace v5 Class Diagram
+
+  <size:12>Note: the ""Descriptor"" suffix is omitted from most classes
+end title
+
+' left to right direction
+
+skinparam class {
+BackgroundColor<<New>> PaleGreen
+}
+
+class DesignSpaceDocument {
++ formatVersion: str = None
++ <color:green><b><<New>> elidedFallbackName: str = None
++ rulesProcessingLast: bool = False
++ lib: Dict = {}
+}
+
+note left of DesignSpaceDocument::elidedFallbackName
+STAT Style Attributes Header field ""elidedFallbackNameID""
+end note
+
+abstract class AbstractAxis {
++ tag: str
++ name: str
++ labelNames: Dict[str, str]
++ hidden: bool
++ map: List[Tuple[float, float]]
++ <color:green><b><<New>> axisOrdering: int
+}
+DesignSpaceDocument *-- "*" AbstractAxis: axes >
+note right of AbstractAxis::axisOrdering
+STAT Axis Record field
+end note
+
+class Axis {
++ minimum: float
++ maximum: float
++ default: float
+}
+AbstractAxis <|--- Axis
+note bottom of Axis
+This is the usual
+Axis, with a range
+of values.
+end note
+
+class DiscreteAxis <<New>> {
++ values: List[float]
++ default: float
+}
+AbstractAxis <|--- DiscreteAxis
+note bottom of DiscreteAxis
+A discrete axis is not
+interpolable, e.g.
+Uprights vs Italics, and
+so has "discrete" stops
+instead of a continuous
+range of values.
+end note
+
+Axis .[hidden] DiscreteAxis
+
+class AxisLabel <<New>> {
++ userMinimum: Optional[float]
++ userValue: float
++ userMaximum: Optional[float]
++ name: str
++ elidable: bool
++ olderSibling: bool
++ linkedUserValue: Optional[float]
++ labelNames: Dict[str, str]
+
++ getFormat(): 1 | 2 | 3
+}
+note right of AxisLabel
+Label for a
+stop on an Axis
+(STAT format
+1,2,3)
+end note
+AbstractAxis *-- "*" AxisLabel: <<New>> \n axisLabels >
+
+class LocationLabel <<New>> {
++ name: str
++ location: Dict[str, float]
++ elidable: bool
++ olderSibling: bool
++ labelNames: Dict[str, str]
+}
+note right of LocationLabel
+Label for a
+freestanding
+location
+(STAT format 4)
+end note
+DesignSpaceDocument *--- "*" LocationLabel: <<New>> \n locationLabels >
+
+class Rule {
++ name: str
++ conditionSets: List[ConditionSet]
++ subs: Dict[str, str]
+}
+DesignSpaceDocument *- "*" Rule: rules >
+
+class Source {
++ name: Optional[str]
++ filename: str
++ path: str
++ layerName: Optional[str]
++ <color:brown><s><<Deprecated>> location: Location
++ <color:green><b><<New>> designLocation: AnisotropicLocation
+....
++ font: Optional[Font]
+....
++ familyName: Optional[str]
++ styleName: Optional[str]
++ <color:green><b><<New>> localisedFamilyName: Dict
+....
++ <color:brown><s><<Deprecated>> copyLib: bool
++ <color:brown><s><<Deprecated>> copyInfo: bool
++ <color:brown><s><<Deprecated>> copyGroups: bool
++ <color:brown><s><<Deprecated>> copyFeatures: bool
+....
++ muteKerning: bool
++ muteInfo: bool
++ mutedGlyphNames: List[str]
+----
++ <color:green><b><<New>> getFullDesignLocation(doc)
+}
+DesignSpaceDocument *-- "*" Source: sources >
+note right of Source::localisedFamilyName
+New field to allow generation
+of localised instance names using
+STAT information.
+end note
+note right of Source::copyGroups
+These fields are already not meaningful
+anymore in version 4 (the default source
+will be used as "neutral" for groups, info
+and features. ''copyLib'' can be emulated
+by putting content in the designspace's lib.
+end note
+
+note as NLocSource
+The location of
+sources can still only
+be defined in design
+coordinates, and now
+also by relying on
+axis defaults.
+
+To build the final,
+"full" location, a
+helper method is
+provided, that uses
+axis defaults and
+axis mappings to
+fill in the blanks.
+end note
+NLocSource . Source::designLocation
+NLocSource . Source::getFullDesignLocation
+
+class VariableFont <<New>> {
++ filename: str
++ lib: Dict
+}
+DesignSpaceDocument *--- "*" VariableFont: <<New>> \n variableFonts >
+note right of VariableFont
+A variable font is a
+subset of the designspace
+where everything interpolates
+(and so can be compiled into
+an OpenType variable font).
+end note
+
+abstract class AbstractAxisSubset <<New>> {
++ name: str
+}
+VariableFont *-- "*" AbstractAxisSubset: <<New>> \n axisSubsets >
+
+note right of AbstractAxisSubset
+An axis subset selects a range
+or a spot on each the available
+axes from the whole designspace.
+
+By default, only the default value
+of each axis is used to define the
+variable font.
+
+Continuous axes can be specified
+to include their full range instead;
+or a subset of the range.
+
+Discrete axes can be specified
+to include a different spot than the
+default.
+end note
+
+class RangeAxisSubset <<New>> {
++ userMinimum: float
++ userDefault: float
++ userMaximum: float
+}
+AbstractAxisSubset <|-- RangeAxisSubset
+
+class ValueAxisSubset <<New>> {
++ userValue: float
+}
+AbstractAxisSubset <|-- ValueAxisSubset
+
+class Instance {
++ filename: str
++ path: str
++ <color:brown><s><<Deprecated>> location: Location
++ <color:green><b><<New>> locationLabel: str
++ <color:green><b><<New>> designLocation: AnisotropicLocation
++ <color:green><b><<New>> userLocation: SimpleLocation
+....
++ font: Optional[Font]
+....
++ <color:orange><b><<Changed>> name: Optional[str]
++ <color:orange><b><<Changed>> familyName: Optional[str]
++ <color:orange><b><<Changed>> styleName: Optional[str]
++ <color:orange><b><<Changed>> postScriptFontName: Optional[str]
++ <color:orange><b><<Changed>> styleMapFamilyName: Optional[str]
++ <color:orange><b><<Changed>> styleMapStyleName: Optional[str]
++ localisedFamilyName: Dict
++ localisedStyleName: Dict
++ localisedStyleMapFamilyName: Dict
++ localisedStyleMapStyleName: Dict
+....
++ <color:brown><s><<Deprecated>> glyphs: Dict
++ <color:brown><s><<Deprecated>> kerning: bool
++ <color:brown><s><<Deprecated>> info: bool
+....
++ lib: Dict
+----
++ <color:green><b><<New>> clearLocation()
++ <color:green><b><<New>> getLocationLabelDescriptor(doc)
++ <color:green><b><<New>> getFullDesignLocation(doc)
++ <color:green><b><<New>> getFullUserLocation(doc)
+}
+DesignSpaceDocument *-- "*" Instance: instances >
+note right of Instance::locationLabel
+The location can now alternatively
+be a string = name of a LocationLabel
+(STAT format 4). The instance then
+adopts the location of that label.
+See the Decovar example file.
+end note
+note right of Instance::styleMapStyleName
+All the name field are now optional.
+Their default values will be computed
+using the provided STAT table data
+and the STAT rules from the spec.
+For styleMap{Family,Style}Name, they
+would be computed using the STAT
+""linkedUserValue"" fields.
+end note
+note right of Instance::glyphs
+This attribute has been replaced by rules
+end note
+note right of Instance::kerning
+All instances get kerning and info
+nowadays.
+end note
+
+note as NLocInstance
+The location of instances
+can now be defined using
+either:
+- a STAT LocationLabel
+- design coordinates
+- user coordinates.
+- relying on axis defaults
+
+To build the final, "full"
+location, a few helper
+methods are provided,
+that aggregate data from
+these sources and apply
+the axis mappings.
+end note
+NLocInstance . Instance::designLocation
+NLocInstance . Instance::getFullDesignLocation
+
+@enduml
+

Doc/source/designspaceLib/v5_split_downconvert.puml

@@ -0,0 +1,147 @@
+@startuml v5_split_downconvert
+
+title
+How to split and down-convert a DesignSpace version 5
+<size:14>so that existing tools can work with it
+end title
+
+
+start
+
+#lightgrey:DesignSpace 5.0
+- with STAT data
+- optional instance names
+- with discrete axes
+- with multiple VFs]
+
+note left
+  - STAT data means that compilers will need
+  to write that data to the ouput TTFs
+  - optional instance names means that
+  compilers will need to generate any missing
+  names using the STAT data
+  - discrete axes mean that not all sources
+  are compatible for interpolation
+  - multiple VFs means that compilers will
+  need to output several TTFs for one DS
+end note
+
+split
+split again
+
+:""splitInterpolating()"";
+
+note left
+  - Create one DS document per interpolating
+  sub-space, for example: with a discrete
+  axis for Upright vs Italics, create 2
+  DesignSpaces, one for the Uprights, and
+  one for the Italics.
+  - Expand all missing instance names using
+  the STAT data.
+  - Drop all the STAT data because as we start
+  taking out discrete axes, the sub-documents
+  lose STAT data anyway (only the full
+  document at the start of the process should
+  be used to generate the STAT table)
+end note
+
+split
+
+  #lightgrey:DesignSpace 5.0
+  - (no STAT data)
+  - (explicit instance names)
+  - (at discrete location #1)
+  - with multiple VFs]
+
+  note left
+    All sources in this sub-space are
+    (supposed to be) compatible for
+    interpolation
+  end note
+
+split again
+
+  #lightgrey:DesignSpace 5.0
+  - (no STAT data)
+  - (explicit instance names)
+  - (at discrete location #2)
+  - with multiple VFs]
+  detach
+
+split again
+
+  #lightgrey:etc
+  ...]
+  detach
+
+end split
+
+:check compatibility;
+:build compatible master TTFs with cu2qu;
+
+:""splitVariableFonts()"";
+
+note left
+  Create one DS document per variable font,
+  for example: the above document may
+  describe a weight and width space, out of
+  which we'll build 3 variable fonts:
+  full weight + width, weight only, width only.
+end note
+
+split
+
+  #lightgrey:DesignSpace 5.0
+  - (no STAT data)
+  - (explicit instance names)
+  - (at discrete location #1)
+  - (describing just VF #1)]
+
+  note left
+    This document looks very much
+    like version 4.1, you can just
+    change the version number at
+    the top and feed it to a tool
+    that doesn't know about v5;
+    see ""convert5to4()"".
+  end note
+
+split again
+
+  #lightgrey:DesignSpace 5.0
+  - (no STAT data)
+  - (explicit instance names)
+  - (at discrete location #1)
+  - (describing just VF #2)]
+  detach
+
+split again
+
+  #lightgrey:etc
+  ...]
+  detach
+
+end split
+
+:""varLib.build()"";
+
+#lightgrey:Variable font TTF
+No STAT data]
+
+end split
+
+:""buildVFStatTable()"";
+
+note left
+Apply STAT data using the full document
+from the start of the process + filtering
+entries based on this VF's location
+end note
+
+#lightgrey:Variable font TTF
+With STAT data]
+
+end
+
+@enduml