A bit better: Variants of duality in geometric algebras with degenerate metrics

Multiplication by the pseudoscalar $\mathbf{I}$ has been traditionally used in geometric algebra to perform non-metric operations such as calculating coordinates and the regressive product. In algebras with degenerate metrics, such as euclidean PGA $P({\mathbb{R}}_{3,0,1}^{\ast })$, this approach breaks down, leading to a search for non-metric forms of duality. The article compares the dual coordinate map $J:G\to G^{\ast }$, a double algebra duality, and Hodge duality $H:G\to G$, a single algebra duality for this purpose. While the two maps are computationally identical, only $J$ is coordinate-free and provides direct support for geometric duality, whereby every geometric primitive appears twice, once as a point-based and once as a plane-based form, an essential feature not only of projective geometry but also of euclidean kinematics and dynamics. Our analysis concludes with a proposed duality-neutral software implementation, requiring a single bit field per multi-vector.