fix: correct contraction index formula by removing the squared area term and updating associated tests and documentation

Author: nicola-corbellini

docs/user_guide/theoretical_framework/low_level/contraction_expansion.md

@@ -17,10 +17,10 @@ These shape metrics are used to track structural changes relative to a movement
 - **Robustness**: Relies on Convex Hull computation which gracefully handles outlier limbs but can fail mathematically if all points form a flat plane (degeneracy). 
 
 ### Algorithm Details & Mathematics
-The index is the ratio of the squared surface area of the enclosed convex hull to the surface area of the axis-aligned bounding box (AABB).
+The index is the ratio of the surface area of the enclosed convex hull to the surface area of the axis-aligned bounding box (AABB).
 
 $$ 
-Index = \frac{Area_{hull}^2}{Area_{bbox}}
+Index = \frac{Area_{hull}}{Area_{bbox}}
 $$
 
 where $Area_{hull}$ is the internal surface area derived purely from the outermost perimeter of points, and $Area_{bbox}$ is the theoretical bounding rectangle/prism containing all movement.

pyeyesweb/low_level/contraction_expansion.py

@@ -45,7 +45,7 @@ class BoundingBoxFilledArea(StaticFeature):
     Mathematical formulation involves computing the ratio:
 
     $$
-    Index = \frac{Area_{hull}^2}{Area_{bbox}}
+    Index = \frac{Area_{hull}}{Area_{bbox}}
     $$
 
     where $Area_{hull}$ is the surface area of the 3D convex hull enclosing the points,

tests/unit_tests/test_low_level_features.py

@@ -42,7 +42,7 @@ def test_bounding_box_filled_area():
 
     assert result.is_valid is True
     assert hasattr(result, "contraction_index")
-    assert np.isclose(result.contraction_index, 6.0)
+    assert np.isclose(result.contraction_index, 1.0)
 
 
 def test_ellipsoid_sphericity():