# Definition:Crossing (Jordan Curve)/Parity

## Definition

Let $P$ be a polygon embedded in $\R^2$.

Let $q \in \R^2 \setminus \partial P$.

Let $\mathbf v \in R^2 \setminus \set {\mathbf 0}$ be a non-zero vector.

Let $\LL = \set {q + s \mathbf v: s \in \R_{\ge 0} }$ be a ray with start point $q$.

Let $\map N q$ be the number of crossings between $\LL$ and the boundary $\partial P$ of $P$.

Then the parity of $q$ is defined as:

$\map {\operatorname{par} } q := \map N q \bmod 2$.