Search Unity

Calculating relative position with cross product

Discussion in 'Scripting' started by Sendatsu_Yoshimitsu, Jul 27, 2019.

  1. Sendatsu_Yoshimitsu

    Sendatsu_Yoshimitsu

    Joined:
    May 19, 2014
    Posts:
    673
    I'm trying to calculate if one line segment is to the left or right-hand side of another segment. My understanding is that, given that both segments are on the x,z axes, if I take the cross of one line's start and endpoints relative to the second line's direction, the sign of the result's y-value should always be negative if the second line is on the left, and positive if it's on the right:

    Code (CSharp):
    1.  
    2.    public class LineSegment
    3.     {
    4.         Vector3 p0; //start point
    5.         Vector3 p1; //end point
    6.  
    7.         public bool IsLeft(LineSegment lineSegment)
    8.         {
    9.             return Vector3.Cross(p1 - p0, lineSegment.p0).y <= 0 && Vector3.Cross(p1 - p0, lineSegment.p1).y <= 0;
    10.         }
    11.     }
    12.  
    However, in practice this always seems to return false; am I grossly mis-understanding how Vector3.Cross is supposed to be used?
     
    Last edited: Jul 27, 2019
  2. Madgvox

    Madgvox

    Joined:
    Apr 13, 2014
    Posts:
    829
    Cross Product takes two vectors, you are giving it a vector and a "point".

    You need to cross the vector of
    p1 - p0
    with the vector of
    other.p0 - p0
    to get the proper result.

    Code (CSharp):
    1. Vector3.Cross(p1 - p0, lineSegment.p0 - p0).y <= 0
     
  3. Sendatsu_Yoshimitsu

    Sendatsu_Yoshimitsu

    Joined:
    May 19, 2014
    Posts:
    673
    Oops, that was a really obvious mistake on my part- thank you for the clarification!!
     
unityunity