Test var model error (#2217)

* [varLib.models] Add test for modeling rounding error

Tests https://github.com/fonttools/fonttools/pull/2214

If you flip demo to True, it does a slower test and demos the new error as well
as the error the old code was producing (ie. rounding deltas post-modeling).

Indeed, the new error is always capped by 0.5 as expected, whereas the old one
was unbounded. Here's the worst-case error of the bad code:

...
240     0.42    4.8
...

240 is just the line number. 0.42 is new error. 4.8 is old error.

* turn test_modeling_error into a parametrized pytest test

Like the other test methods in the same module, all those whose name starts with 'test_' are automatically discovered and run by pytest which is our default test runner. So there is no need to call the test method itself in the top-level module scope. One simply runs the test via pytest. To execute this specific test method one can do 'pytest Tests/varLib/models_test.py::test_modeling_error'.

* use pytest markers to mark specific test as 'slow'

So that one can optionally deselect tests marked with specific marker by passing -m option (e.g. to deselect 'slow' tests, pytest -m 'not slow' ...).

https://docs.pytest.org/en/stable/mark.html#registering-marks
https://docs.pytest.org/en/stable/example/parametrize.html#set-marks-or-test-id-for-individual-parametrized-test

* [varLib/models_test] Comment out non-test code

Co-authored-by: Cosimo Lupo <clupo@google.com>

Tests/varLib/models_test.py

@@ -34,6 +34,40 @@ def test_supportScalar():
     assert supportScalar({'wght':2.5}, {'wght':(0,2,4)}) == 0.75
 
 
+@pytest.mark.parametrize(
+    "numLocations, numSamples", [
+        pytest.param(127, 509, marks=pytest.mark.slow),
+        (31, 251),
+    ]
+)
+def test_modeling_error(numLocations, numSamples):
+    # https://github.com/fonttools/fonttools/issues/2213
+    locations = [{'axis': float(i)/numLocations} for i in range(numLocations)]
+    masterValues = [100. if i else 0. for i in range(numLocations)]
+
+    model = VariationModel(locations)
+
+    for i in range(numSamples):
+        loc = {'axis': float(i)/numSamples}
+        scalars = model.getScalars(loc)
+
+        deltas_float = model.getDeltas(masterValues)
+        deltas_round = model.getDeltas(masterValues, round=round)
+
+        expected = model.interpolateFromDeltasAndScalars(deltas_float, scalars)
+        actual = model.interpolateFromDeltasAndScalars(deltas_round, scalars)
+
+        err = abs(actual - expected)
+        assert err <= .5, (i, err)
+
+        # This is how NOT to round deltas.
+        #deltas_late_round = [round(d) for d in deltas_float]
+        #bad = model.interpolateFromDeltasAndScalars(deltas_late_round, scalars)
+        #err_bad = abs(bad - expected)
+        #if err != err_bad:
+        #    print("{:d}	{:.2}	{:.2}".format(i, err, err_bad))
+
+
 class VariationModelTest(object):
 
     @pytest.mark.parametrize(

setup.cfg

@@ -52,6 +52,8 @@ filterwarnings =
 	ignore:writePlist:DeprecationWarning:plistlib_test
 	ignore:some_function:DeprecationWarning:fontTools.ufoLib.utils
 	ignore::DeprecationWarning:fontTools.varLib.designspace
+markers = 
+    slow: marks tests as slow (deselect with '-m "not slow"')
 
 [tool:interrogate]
 ignore-semiprivate = true