[varLib.models] Add getMasterScalars

Lib/fontTools/varLib/models.py

@@ -472,6 +472,22 @@ class VariationModel(object):
             for support in self.supports
         ]
 
+    def _getMasterScalarsRecurse(self, out, j, weights, scalar=1):
+        for i, weight in weights.items():
+            influence = -weight * scalar
+            out[i] += influence
+            self._getMasterScalarsRecurse(out, i, self.deltaWeights[i], influence)
+
+    def getMasterScalars(self, targetLocation):
+        out = []
+        for i, (weights, support) in enumerate(zip(self.deltaWeights, self.supports)):
+            scalar = supportScalar(targetLocation, support)
+            out.append(scalar)
+            self._getMasterScalarsRecurse(out, i, weights, scalar)
+
+        out = [out[self.mapping[i]] for i in range(len(out))]
+        return out
+
     @staticmethod
     def interpolateFromDeltasAndScalars(deltas, scalars):
         v = None

Tests/varLib/models_test.py

@@ -397,7 +397,7 @@ class VariationModelTest(object):
             )
 
     @pytest.mark.parametrize(
-        "locations, axisOrder, masterValues, instanceLocation, expectedValue",
+        "locations, axisOrder, masterValues, instanceLocation, expectedValue, masterScalars",
         [
             (
                 [
@@ -422,6 +422,7 @@ class VariationModelTest(object):
                     "axis_B": 0.5,
                 },
                 37.5,
+                [0.25, 0.0, 0.0, -0.25, 0.5, 0.5],
             ),
         ],
     )
@@ -432,8 +433,11 @@ class VariationModelTest(object):
         masterValues,
         instanceLocation,
         expectedValue,
+        masterScalars,
     ):
         model = VariationModel(locations, axisOrder=axisOrder)
-        interpolatedValue = model.interpolateFromMasters(instanceLocation, masterValues)
 
+        interpolatedValue = model.interpolateFromMasters(instanceLocation, masterValues)
         assert interpolatedValue == expectedValue
+
+        assert masterScalars == model.getMasterScalars(instanceLocation)