1####################################################### 2designspaceLib: Read, write, and edit designspace files 3####################################################### 4 5Implements support for reading and manipulating ``designspace`` files. 6Allows the users to define axes, rules, sources, variable fonts and instances, 7and their STAT information. 8 9.. toctree:: 10 :maxdepth: 1 11 12 python 13 xml 14 scripting 15 16 17Notes 18===== 19 20Paths and filenames 21------------------- 22 23A designspace file needs to store many references to UFO files. 24 25- designspace files can be part of versioning systems and appear on 26 different computers. This means it is not possible to store absolute 27 paths. 28- So, all paths are relative to the designspace document path. 29- Using relative paths allows designspace files and UFO files to be 30 **near** each other, and that they can be **found** without enforcing 31 one particular structure. 32- The **filename** attribute in the ``SourceDescriptor`` and 33 ``InstanceDescriptor`` classes stores the preferred relative path. 34- The **path** attribute in these objects stores the absolute path. It 35 is calculated from the document path and the relative path in the 36 filename attribute when the object is created. 37- Only the **filename** attribute is written to file. 38- Both **filename** and **path** must use forward slashes (``/``) as 39 path separators, even on Windows. 40 41Right before we save we need to identify and respond to the following 42situations: 43 44In each descriptor, we have to do the right thing for the filename 45attribute. Before writing to file, the ``documentObject.updatePaths()`` 46method prepares the paths as follows: 47 48**Case 1** 49 50:: 51 52 descriptor.filename == None 53 descriptor.path == None 54 55**Action** 56 57- write as is, descriptors will not have a filename attr. Useless, but 58 no reason to interfere. 59 60**Case 2** 61 62:: 63 64 descriptor.filename == "../something" 65 descriptor.path == None 66 67**Action** 68 69- write as is. The filename attr should not be touched. 70 71**Case 3** 72 73:: 74 75 descriptor.filename == None 76 descriptor.path == "~/absolute/path/there" 77 78**Action** 79 80- calculate the relative path for filename. We're not overwriting some 81 other value for filename, it should be fine. 82 83**Case 4** 84 85:: 86 87 descriptor.filename == '../somewhere' 88 descriptor.path == "~/absolute/path/there" 89 90**Action** 91 92- There is a conflict between the given filename, and the path. The 93 difference could have happened for any number of reasons. Assuming 94 the values were not in conflict when the object was created, either 95 could have changed. We can't guess. 96- Assume the path attribute is more up to date. Calculate a new value 97 for filename based on the path and the document path. 98 99Recommendation for editors 100-------------------------- 101 102- If you want to explicitly set the **filename** attribute, leave the 103 path attribute empty. 104- If you want to explicitly set the **path** attribute, leave the 105 filename attribute empty. It will be recalculated. 106- Use ``documentObject.updateFilenameFromPath()`` to explicitly set the 107 **filename** attributes for all instance and source descriptors. 108 109 110Common Lib Key Registry 111======================= 112 113public.skipExportGlyphs 114----------------------- 115 116This lib key works the same as the UFO lib key with the same name. The 117difference is that applications using a Designspace as the corner stone of the 118font compilation process should use the lib key in that Designspace instead of 119any of the UFOs. If the lib key is empty or not present in the Designspace, all 120glyphs should be exported, regardless of what the same lib key in any of the 121UFOs says. 122 123 124Implementation and differences 125============================== 126 127The designspace format has gone through considerable development. 128 129 - the format was originally written for MutatorMath. 130 - the format is now also used in fontTools.varlib. 131 - not all values are be required by all implementations. 132 133Varlib vs. MutatorMath 134---------------------- 135 136There are some differences between the way MutatorMath and fontTools.varlib handle designspaces. 137 138 - Varlib does not support anisotropic interpolations. 139 - MutatorMath will extrapolate over the boundaries of 140 the axes. Varlib can not (at the moment). 141 - Varlib requires much less data to define an instance than 142 MutatorMath. 143 - The goals of Varlib and MutatorMath are different, so not all 144 attributes are always needed. 145 146 147Rules and generating static UFO instances 148----------------------------------------- 149 150When making instances as UFOs from a designspace with rules, it can 151be useful to evaluate the rules so that the characterset of the UFO 152reflects, as much as possible, the state of a variable font when seen 153at the same location. This can be done by some swapping and renaming of 154glyphs. 155 156While useful for proofing or development work, it should be noted that 157swapping and renaming leaves the UFOs with glyphnames that are no longer 158descriptive. For instance, after a swap ``dollar.bar`` could contain a shape 159without a bar. Also, when the swapped glyphs are part of other GSUB variations 160it can become complex very quickly. So proceed with caution. 161 162 - Assuming ``rulesProcessingLast = True``: 163 - We need to swap the glyphs so that the original shape is still available. 164 For instance, if a rule swaps ``a`` for ``a.alt``, a glyph 165 that references ``a`` in a component would then show the new ``a.alt``. 166 - But that can lead to unexpected results, the two glyphs may have different 167 widths or height. So, glyphs that are not specifically referenced in a rule 168 **should not change appearance**. That means that the implementation that swaps 169 ``a`` and ``a.alt`` also swap all components that reference these 170 glyphs in order to preserve their appearance. 171 - The swap function also needs to take care of swapping the names in 172 kerning data and any GPOS code. 173 174Version history 175=============== 176 177Version 5.0 178----------- 179 180The format was extended to describe the entire design space of a reasonably 181regular font family in one file, with global data about the family to reduce 182repetition in sub-sections. "Reasonably regular" means that the sources and 183instances across the previously multiple Designspace files are positioned on a 184grid and derive their metadata (like style name) in a way that's compatible with 185the STAT model, based on their axis positions. Axis mappings must be the same 186across the entire space. 187 1881. Each axis can have labels attached to stops within the axis range, analogous to the 189 `OpenType STAT <https://docs.microsoft.com/en-us/typography/opentype/spec/stat>`_ 190 table. Free-standing labels for locations are also allowed. The data is intended 191 to be compiled into a ``STAT`` table. 1922. The axes can be discrete, to say that they do not interpolate, like a distinctly 193 constructed upright and italic variant of a family. 1943. The data can be used to derive style and PostScript names for instances. 1954. A new ``variable-fonts`` element can subdivide the Designspace into multiple subsets that 196 mix and match the globally available axes. It is possible for these sub-spaces to have 197 a different default location from the global default location. It is required if the 198 Designspace contains a discrete axis and you want to produce a variable font. 199 200What is currently not supported is e.g. 201 2021. A setup where different sources sit at the same logical location in the design space, 203 think "MyFont Regular" and "MyFont SmallCaps Regular". (this situation could be 204 encoded by adding a "SmallCaps" discrete axis, if that makes sense). 2052. Anisotropic locations for axis labels. 206 207Older versions 208-------------- 209 210- In some implementations that preceed Variable Fonts, the `copyInfo` 211 flag in a source indicated the source was to be treated as the default. 212 This is no longer compatible with the assumption that the default font 213 is located on the default value of each axis. 214- Older implementations did not require axis records to be present in 215 the designspace file. The axis extremes for instance were generated 216 from the locations used in the sources. This is no longer possible. 217 218