Given two points, $\underline{a}_1,\underline{a}_2 \in D$:
\[
   \underline{a}_1
   =
   r_1
   \begin{pmatrix}
      \cos{\phi}\\\sin{\phi}
   \end{pmatrix},
   \qquad
   \underline{a}_2
   =
   r_2
   \begin{pmatrix}
      \cos{\theta}\\\sin{\theta}
   \end{pmatrix},
\]
$0 \leq r_1,r_2 \lt 1$.