Compute shape features of a 3D alpha-shape object
shape_features_3d(as)a named vector of numeric values or NULL if no non-singular vertices
an alpha-shape object of class ashape3d
Features are: - major.axis, minor.axis and least.axis: Lengths of the axes of the fitted ellipsoid - elongation: from 0 (line) to 1 (globular) - flatness: from 0 (flat) to 1 (spherical) - max.feret.diameter: Maximum Feret diameter - max.inscribed.radius: Radius of the largest inscribed sphere - sphericity: from 0 (not spherical) to 1 (perfect sphere) - concavity: fraction of the convex hull volume not in the object - volume - area: area of the surface of the alpha-shape