synthetic surface and spm specific features

tests/node_tests/lateral_force_sim.py

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+import numpy as np
+import pytest
+from tests.node_tests._shared import make_field
+
+
+def test_output_shapes():
+    """Both forward and reverse outputs must have the same shape as the input."""
+    from backend.nodes.lateral_force_sim import LateralForceSim
+
+    node = LateralForceSim()
+    field = make_field(shape=(48, 64))
+
+    for direction in ("forward", "reverse", "both"):
+        fwd, rev = node.process(field, direction, 0.3, 1e-9, 10e-9)
+        assert fwd.data.shape == field.data.shape, f"forward shape mismatch for direction={direction}"
+        assert rev.data.shape == field.data.shape, f"reverse shape mismatch for direction={direction}"
+
+
+def test_flat_surface_uniform():
+    """A flat (constant) topography has zero slope everywhere, so the lateral
+    force should be spatially uniform (pure friction, no topographic component)."""
+    from backend.nodes.lateral_force_sim import LateralForceSim
+
+    node = LateralForceSim()
+    data = np.full((32, 32), 5e-9, dtype=np.float64)
+    field = make_field(data=data)
+
+    fwd, rev = node.process(field, "both", 0.3, 1e-9, 10e-9)
+
+    # All values in each output should be identical (uniform)
+    assert np.ptp(fwd.data) < 1e-20, "Forward lateral force is not uniform on flat surface"
+    assert np.ptp(rev.data) < 1e-20, "Reverse lateral force is not uniform on flat surface"
+
+
+def test_forward_reverse_different():
+    """For a non-flat surface with 'both' direction, forward and reverse
+    lateral force signals should differ (topographic artifact is direction-dependent)."""
+    from backend.nodes.lateral_force_sim import LateralForceSim
+
+    node = LateralForceSim()
+    # Create a steep ramp in x so there is a significant slope
+    ramp = np.tile(np.linspace(0.0, 500e-9, 64), (64, 1))
+    field = make_field(data=ramp)
+
+    fwd, rev = node.process(field, "both", 0.3, 1e-9, 10e-9)
+
+    # Forward and reverse differ due to slope-dependent asymmetry;
+    # use strict tolerance to detect the difference at nanoNewton scale.
+    assert not np.allclose(fwd.data, rev.data, atol=0, rtol=1e-6), (
+        "Forward and reverse should differ on a sloped surface"
+    )
+
+
+def test_finite_values():
+    """All output values must be finite (no NaN or inf) for typical inputs."""
+    from backend.nodes.lateral_force_sim import LateralForceSim
+
+    node = LateralForceSim()
+    field = make_field()  # random topography
+
+    for direction in ("forward", "reverse", "both"):
+        fwd, rev = node.process(field, direction, 0.3, 1e-9, 10e-9)
+        assert np.isfinite(fwd.data).all(), f"Non-finite values in forward output (direction={direction})"
+        assert np.isfinite(rev.data).all(), f"Non-finite values in reverse output (direction={direction})"