[varLib.interpolatable] Support discrete axes in .designspace

Fixes https://github.com/fonttools/fonttools/issues/3597

Lib/fontTools/varLib/interpolatable.py

@@ -135,6 +135,7 @@ def test_gen(
     kinkiness=DEFAULT_KINKINESS,
     upem=DEFAULT_UPEM,
     show_all=False,
+    discrete_axes=[],
 ):
     if tolerance >= 10:
         tolerance *= 0.01
@@ -150,7 +151,9 @@ def test_gen(
         # ... risks the sparse master being the first one, and only processing a subset of the glyphs
         glyphs = {g for glyphset in glyphsets for g in glyphset.keys()}
 
-    parents, order = find_parents_and_order(glyphsets, locations)
+    parents, order = find_parents_and_order(
+        glyphsets, locations, discrete_axes=discrete_axes
+    )
 
     def grand_parent(i, glyphname):
         if i is None:
@@ -701,6 +704,7 @@ def main(args=None):
     fonts = []
     names = []
     locations = []
+    discrete_axes = set()
     upem = DEFAULT_UPEM
 
     original_args_inputs = tuple(args.inputs)
@@ -713,8 +717,13 @@ def main(args=None):
             designspace = DesignSpaceDocument.fromfile(args.inputs[0])
             args.inputs = [master.path for master in designspace.sources]
             locations = [master.location for master in designspace.sources]
+            discrete_axes = {
+                a.name for a in designspace.axes if not hasattr(a, "minimum")
+            }
             axis_triples = {
-                a.name: (a.minimum, a.default, a.maximum) for a in designspace.axes
+                a.name: (a.minimum, a.default, a.maximum)
+                for a in designspace.axes
+                if a.name not in discrete_axes
             }
             axis_mappings = {a.name: a.map for a in designspace.axes}
             axis_triples = {
@@ -879,7 +888,13 @@ def main(args=None):
                 glyphset[gn] = None
 
     # Normalize locations
-    locations = [normalizeLocation(loc, axis_triples) for loc in locations]
+    locations = [
+        {
+            **normalizeLocation(loc, axis_triples),
+            **{k: v for k, v in loc.items() if k in discrete_axes},
+        }
+        for loc in locations
+    ]
     tolerance = args.tolerance or DEFAULT_TOLERANCE
     kinkiness = args.kinkiness if args.kinkiness is not None else DEFAULT_KINKINESS
 
@@ -896,6 +911,7 @@ def main(args=None):
             tolerance=tolerance,
             kinkiness=kinkiness,
             show_all=args.show_all,
+            discrete_axes=discrete_axes,
         )
         problems = defaultdict(list)
 

Lib/fontTools/varLib/interpolatableHelpers.py

@@ -293,17 +293,19 @@ def add_isomorphisms(points, isomorphisms, reverse):
             )
 
 
-def find_parents_and_order(glyphsets, locations):
+def find_parents_and_order(glyphsets, locations, *, discrete_axes=set()):
     parents = [None] + list(range(len(glyphsets) - 1))
     order = list(range(len(glyphsets)))
     if locations:
         # Order base master first
-        bases = (i for i, l in enumerate(locations) if all(v == 0 for v in l.values()))
+        bases = [
+            i
+            for i, l in enumerate(locations)
+            if all(v == 0 for k, v in l.items() if k not in discrete_axes)
+        ]
         if bases:
-            base = next(bases)
-            logging.info("Base master index %s, location %s", base, locations[base])
+            logging.info("Found %s base masters: %s", len(bases), bases)
         else:
-            base = 0
             logging.warning("No base master location found")
 
         # Form a minimum spanning tree of the locations
@@ -317,9 +319,17 @@ def find_parents_and_order(glyphsets, locations):
             axes = sorted(axes)
             vectors = [tuple(l.get(k, 0) for k in axes) for l in locations]
             for i, j in itertools.combinations(range(len(locations)), 2):
+                i_discrete_location = {
+                    k: v for k, v in zip(axes, vectors[i]) if k in discrete_axes
+                }
+                j_discrete_location = {
+                    k: v for k, v in zip(axes, vectors[j]) if k in discrete_axes
+                }
+                if i_discrete_location != j_discrete_location:
+                    continue
                 graph[i][j] = vdiff_hypot2(vectors[i], vectors[j])
 
-            tree = minimum_spanning_tree(graph)
+            tree = minimum_spanning_tree(graph, overwrite=True)
             rows, cols = tree.nonzero()
             graph = defaultdict(set)
             for row, col in zip(rows, cols):
@@ -330,7 +340,7 @@ def find_parents_and_order(glyphsets, locations):
             parents = [None] * len(locations)
             order = []
             visited = set()
-            queue = deque([base])
+            queue = deque(bases)
             while queue:
                 i = queue.popleft()
                 visited.add(i)
@@ -339,6 +349,9 @@ def find_parents_and_order(glyphsets, locations):
                     if j not in visited:
                         parents[j] = i
                         queue.append(j)
+            assert len(order) == len(
+                parents
+            ), "Not all masters are reachable; report an issue"
 
         except ImportError:
             pass