[instancer.solver] Handle when peak falls on new axis default

Lib/fontTools/varLib/instancer/__init__.py

@@ -255,9 +255,6 @@ def changeTupleVariationAxisLimit(var, axisTag, axisLimit):
         if tent is None:
             newVar.axes.pop(axisTag)
         else:
-            # Sometimes happens when can't be solved. Eg. if a dirac function
-            # (tent with upper==peak==lower) is placed at axisLimit's default
-            # location.
             assert tent[1] != 0, tent
             newVar.axes[axisTag] = tent
         newVar.scaleDeltas(scalar)

Lib/fontTools/varLib/instancer/solver.py

@@ -252,7 +252,27 @@ def _solve(tent, axisLimit, negative=False):
         out.append((scalar1 - gain, loc1))
         out.append((scalar2 - gain, loc2))
 
-    return out
+
+    # If peak ended up being zero, nudge it to the next value
+    EPSILON = 1 / (1 << 14)
+    new_out = []
+    for scalar,triple in out:
+        if scalar == 0:
+            continue
+
+        if triple is None:
+            new_out.append((scalar, triple))
+            continue
+
+        lower,peak,upper = triple
+        if peak == axisDef:
+            assert not (lower == upper == axisDef)
+
+            peak += EPSILON if lower == axisDef else -EPSILON
+
+        new_out.append((scalar, (lower,peak,upper)))
+
+    return new_out
 
 
 @lru_cache(128)
@@ -279,6 +299,6 @@ def rebaseTent(tent, axisLimit):
     sols = _solve(tent, axisLimit)
 
     n = lambda v: normalizeValue(v, axisLimit, extrapolate=True)
-    sols = [(scalar, (n(v[0]), n(v[1]), n(v[2])) if v is not None else None) for scalar,v in sols if scalar != 0]
+    sols = [(scalar, (n(v[0]), n(v[1]), n(v[2])) if v is not None else None) for scalar,v in sols]
 
     return sols

Tests/varLib/instancer/solver_test.py

@@ -213,6 +213,17 @@ class RebaseTentTest(object):
                     (1, (0, .4, 1.99994)),
                 ]
             ),
+            # Dirac delta at new default. Fancy!
+            pytest.param(
+                (.5, .5, .5), (0, .5, 1),
+                [
+                    (1, None),
+                    (-1, (0, 0.0001220703, 1)),
+                    (-1, (0, 1, 1)),
+                    (-1, (-1, -0.0001220703, 0)),
+                    (-1, (-1, -1, 0)),
+                ]
+            ),
         ],
     )
     def test_rebaseTent(self, tent, axisRange, expected):