keywords: Math, check if a point is inside a rectangle or triangle

Rectangle

https://math.stackexchange.com/questions/190111/how-to-check-if-a-point-is-inside-a-rectangle

Triangle

Check whether a point is within a 3D Triangle
https://math.stackexchange.com/questions/4322/check-whether-a-point-is-within-a-3d-triangle

Irregular 2D Polygon

my favorite way to see if a point is inside or outside a path, is using its winding number.
Dont cry because it came to an end, smile because it happened. ― Gabriel Garcí­a Márquez