designspaceLib: Read, write, and edit designspace files
Implements support for reading and manipulating
Allows the users to define axes, rules, sources, variable fonts and instances,
and their STAT information.
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
InstanceDescriptorclasses 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
method prepares the paths as follows:
descriptor.filename == None descriptor.path == None
write as is, descriptors will not have a filename attr. Useless, but no reason to interfere.
descriptor.filename == "../something" descriptor.path == None
write as is. The filename attr should not be touched.
descriptor.filename == None descriptor.path == "~/absolute/path/there"
calculate the relative path for filename. We’re not overwriting some other value for filename, it should be fine.
descriptor.filename == '../somewhere' descriptor.path == "~/absolute/path/there"
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.
documentObject.updateFilenameFromPath()to explicitly set the filename attributes for all instance and source descriptors.
Common Lib Key Registry
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.
rulesProcessingLast = True:
We need to swap the glyphs so that the original shape is still available. For instance, if a rule swaps
a.alt, a glyph that references
ain a component would then show the new
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.altalso 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.
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.
Each axis can have labels attached to stops within the axis range, analogous to the OpenType STAT table. Free-standing labels for locations are also allowed. The data is intended to be compiled into a
The axes can be discrete, to say that they do not interpolate, like a distinctly constructed upright and italic variant of a family.
The data can be used to derive style and PostScript names for instances.
variable-fontselement 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.
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).
Anisotropic locations for axis labels.
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.