Fix flaky optical flow test (compare numbers, not rendered image)

src/python/tests/optical_flow_test.py

@@ -1,10 +1,9 @@
 """
 Module for testing optical flow computations. Provides test cases for optical flow computation from pose landmarks data.
 """
-import tempfile
 from unittest import TestCase
 
-from matplotlib.testing.compare import compare_images
+import numpy as np
 
 from pose_format.numpy.representation.distance import DistanceRepresentation
 from pose_format.pose import Pose
@@ -18,30 +17,21 @@ class TestOpticalFlow(TestCase):
 
     def test_optical_flow(self):
         """
-        Tests optical flow from pose landmarks data and 
-        visualize the computed flow using a heatmap. 
-
-        Compares the generated heatmap with the reference image provided in `data/optical_flow.png` to validate the correctness of the omputed optical flow.
-
-        Raises:
-            AssertionError: If the computed optical flow visualization does not match the reference image.
+        Tests optical flow from pose landmarks data against a numeric reference.
+
+        Compares the computed flow with `data/optical_flow.npy`. The reference is
+        the raw numeric output (not a rendered image), so the test is independent
+        of matplotlib/freetype rendering differences across environments.
         """
         calculator = OpticalFlowCalculator(fps=30, distance=DistanceRepresentation())
 
         with open('tests/data/mediapipe.pose', 'rb') as f:
             pose = Pose.read(f.read())
             pose = pose.get_components(["POSE_LANDMARKS", "RIGHT_HAND_LANDMARKS", "LEFT_HAND_LANDMARKS"])
 
-        flow = calculator(pose.body.data)
-        flow = flow.squeeze(axis=1)
-        print(flow.shape)
-
-        # Matplotlib heatmap
-        import matplotlib.pyplot as plt
-        plt.imshow(flow.T)
-        plt.tight_layout()
-
-        fp = tempfile.NamedTemporaryFile()
-        plt.savefig(fp.name, format='png')
+        flow = calculator(pose.body.data).squeeze(axis=1)
 
-        self.assertTrue(compare_images('tests/data/optical_flow.png', fp.name, 0.001) is None)
+        expected = np.load('tests/data/optical_flow.npy')
+        self.assertEqual(flow.shape, expected.shape)
+        self.assertTrue(np.allclose(flow, expected),
+                        "Computed optical flow does not match the reference.")