[featureVars] expose method to addGSUBFeatureVariations to existing VF

Fixes https://github.com/fonttools/fonttools/issues/3357

Lib/fontTools/varLib/__init__.py

@@ -863,7 +863,7 @@ def _add_COLR(font, model, master_fonts, axisTags, colr_layer_reuse=True):
         colr.VarIndexMap = builder.buildDeltaSetIndexMap(varIdxes)
 
 
-def load_designspace(designspace):
+def load_designspace(designspace, log_enabled=True):
     # TODO: remove this and always assume 'designspace' is a DesignSpaceDocument,
     # never a file path, as that's already handled by caller
     if hasattr(designspace, "sources"):  # Assume a DesignspaceDocument
@@ -911,10 +911,11 @@ def load_designspace(designspace):
                 axis.labelNames["en"] = tostr(axis_name)
 
         axes[axis_name] = axis
-    log.info("Axes:\n%s", pformat([axis.asdict() for axis in axes.values()]))
+    if log_enabled:
+        log.info("Axes:\n%s", pformat([axis.asdict() for axis in axes.values()]))
 
     axisMappings = ds.axisMappings
-    if axisMappings:
+    if axisMappings and log_enabled:
         log.info("Mappings:\n%s", pformat(axisMappings))
 
     # Check all master and instance locations are valid and fill in defaults
@@ -944,20 +945,23 @@ def load_designspace(designspace):
     # Normalize master locations
 
     internal_master_locs = [o.getFullDesignLocation(ds) for o in masters]
-    log.info("Internal master locations:\n%s", pformat(internal_master_locs))
+    if log_enabled:
+        log.info("Internal master locations:\n%s", pformat(internal_master_locs))
 
     # TODO This mapping should ideally be moved closer to logic in _add_fvar/avar
     internal_axis_supports = {}
     for axis in axes.values():
         triple = (axis.minimum, axis.default, axis.maximum)
         internal_axis_supports[axis.name] = [axis.map_forward(v) for v in triple]
-    log.info("Internal axis supports:\n%s", pformat(internal_axis_supports))
+    if log_enabled:
+        log.info("Internal axis supports:\n%s", pformat(internal_axis_supports))
 
     normalized_master_locs = [
         models.normalizeLocation(m, internal_axis_supports)
         for m in internal_master_locs
     ]
-    log.info("Normalized master locations:\n%s", pformat(normalized_master_locs))
+    if log_enabled:
+        log.info("Normalized master locations:\n%s", pformat(normalized_master_locs))
 
     # Find base master
     base_idx = None
@@ -972,7 +976,8 @@ def load_designspace(designspace):
         raise VarLibValidationError(
             "Base master not found; no master at default location?"
         )
-    log.info("Index of base master: %s", base_idx)
+    if log_enabled:
+        log.info("Index of base master: %s", base_idx)
 
     return _DesignSpaceData(
         axes,
@@ -1308,6 +1313,30 @@ def _feature_variations_tags(ds):
     return sorted({t.strip() for t in raw_tags.split(",")})
 
 
+def addGSUBFeatureVariations(vf, designspace, featureTags=(), *, log_enabled=False):
+    """Add GSUB FeatureVariations table to variable font, based on DesignSpace rules.
+
+    Args:
+        vf: A TTFont object representing the variable font.
+        designspace: A DesignSpaceDocument object.
+        featureTags: Optional feature tag(s) to use for the FeatureVariations records.
+            If unset, the key 'com.github.fonttools.varLib.featureVarsFeatureTag' is
+            looked up in the DS <lib> and used; otherwise the default is 'rclt' if
+            the <rules processing="last"> attribute is set, else 'rvrn'.
+            See <https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#rules-element>
+        log_enabled: If True, log info about DS axes and sources. Default is False, as
+            the same info may have already been logged as part of varLib.build.
+    """
+    ds = load_designspace(designspace, log_enabled=log_enabled)
+    if not ds.rules:
+        return
+    if not featureTags:
+        featureTags = _feature_variations_tags(ds)
+    _add_GSUB_feature_variations(
+        vf, ds.axes, ds.internal_axis_supports, ds.rules, featureTags
+    )
+
+
 def main(args=None):
     """Build variable fonts from a designspace file and masters"""
     from argparse import ArgumentParser

Tests/varLib/varLib_test.py

@@ -1,7 +1,13 @@
 from fontTools.colorLib.builder import buildCOLR
 from fontTools.ttLib import TTFont, newTable
 from fontTools.ttLib.tables import otTables as ot
-from fontTools.varLib import build, build_many, load_designspace, _add_COLR
+from fontTools.varLib import (
+    build,
+    build_many,
+    load_designspace,
+    _add_COLR,
+    addGSUBFeatureVariations,
+)
 from fontTools.varLib.errors import VarLibValidationError
 import fontTools.varLib.errors as varLibErrors
 from fontTools.varLib.models import VariationModel
@@ -1009,6 +1015,32 @@ Expected to see .ScriptCount==1, instead saw 0""",
             save_before_dump=True,
         )
 
+    def test_varlib_addGSUBFeatureVariations(self):
+        ttx_dir = self.get_test_input("master_ttx_interpolatable_ttf")
+
+        ds = DesignSpaceDocument.fromfile(
+            self.get_test_input("FeatureVars.designspace")
+        )
+        for source in ds.sources:
+            ttx_dump = TTFont()
+            ttx_dump.importXML(
+                os.path.join(
+                    ttx_dir, os.path.basename(source.filename).replace(".ufo", ".ttx")
+                )
+            )
+            source.font = ttx_dump
+
+        varfont, _, _ = build(ds, exclude=["GSUB"])
+        assert "GSUB" not in varfont
+
+        addGSUBFeatureVariations(varfont, ds)
+        assert "GSUB" in varfont
+
+        tables = ["fvar", "GSUB"]
+        expected_ttx_path = self.get_test_output("FeatureVars.ttx")
+        self.expect_ttx(varfont, expected_ttx_path, tables)
+        self.check_ttx_dump(varfont, expected_ttx_path, tables, ".ttf")
+
 
 def test_load_masters_layerName_without_required_font():
     ds = DesignSpaceDocument()