Even–odd rule

The even–odd rule is an algorithm implemented in vector-based graphic software, like the PostScript language and Scalable Vector Graphics, which determines how a graphical shape with more than one closed outline will be filled. Unlike the nonzero-rule algorithm, this algorithm will alternatively color and leave uncolored shapes defined by nested closed paths irrespective of their winding.
The SVG defines the even–odd rule by saying:
The rule can be seen in effect in many vector graphic programs, where a crossing of an outline with itself causes shapes to fill in strange ways.
On a simple curve, the even–odd rule reduces to a decision algorithm for the point in polygon problem.
The SVG computer vector graphics standard may be configured to use the even–odd rule when drawing polygons, though it uses the non-zero rule by default.

Implementation

Below is a partial example implementation in Python, by using a ray to the right of the point being checked:

def is_point_in_path -> bool:
"""Determine if the point is on the path, corner, or boundary of the polygon
Args:
x -- The x coordinates of point.
y -- The y coordinates of point.
poly -- a list of tuples
Returns:
True if the point is in the path or is a corner or on the boundary"""
c = False
for i in range:
ax, ay = poly
bx, by = poly
if and :
# point is a corner
return True
if != :
slope = * - *
if slope 0:
# point is on boundary
return True
if != :
c = not c
return c