I'm reading the book <<High-Dimensional Data Analysis with Low-Dimensional Models: Principles, Computation, and Applications>>. In page 120, the book defines the internal angle with:
The internal angle $\beta(F,G)$ of a face F of a polytope $G$ is the fraction of span$(G-x)$ occupied by $G-x$, where span(·) denotes the linear span, and $x$ is any point in relint(F).
Meanwhile, the book illustrates the definition by: Figure 3.18 Internal Angles of Convex Polytopes.
I'm confused about how to get the numerical values in Figure 3.18 (i.e., $\frac{1}{2},\frac{1}{4},\frac{1}{4}$), can somebody help me? Thanks a lot!