|Analysis Situs ./features/check face norms|
To draw a normal field for the selected face, use the corresponding "Show face normals" context menu action. This function samples the parametric domain of the selected face and renders the normal field using the following conventions:
The color of the vector field (red or blue) is therefore used to represent the topological orientation of the face. The topological orientation is essentially a Boolean flag which allows to agree or not with the as-constructed geometric orientation of the host surface. The latter geometric orientation is the intristic property of the mathematical form which is defined by the underlying surface parameterization. In geometric modeling, the topological orientations are used to properly orient material without touching the geometric information.