unity 已知一条直线，一个点，计算点与直线的交点

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using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CalculationIntersection1 : MonoBehaviour
{
    public Transform a; //A1
    public Transform b; //A2
    public Transform c; //B
    public Transform d;

    public Vector2 GetIntersection(Vector2 lineAStart, Vector2 lineAEnd, Vector2 lineBStart, Vector2 lineBEnd)
    {
        float x1 = lineAStart.x, y1 = lineAStart.y;
        float x2 = lineAEnd.x, y2 = lineAEnd.y;

        float x3 = lineBStart.x, y3 = lineBStart.y;
        float x4 = lineBEnd.x, y4 = lineBEnd.y;

        //两向量相互垂直，返回0
        if (x1 == x2 && x3 == x4 && x1 == x3)
        {
            return Vector2.zero;
        }

        //两向量相互平行。返回0
        if (y1 == y2 && y3 == y4 && y1 == y3)
        {
            return Vector2.zero;
        }

        //两向量相互垂直，返回0
        if (x1 == x2 && x3 == x4)
        {
            return Vector2.zero;
        }

        //两向量相互平行。返回0
        if (y1 == y2 && y3 == y4)
        {
            return Vector2.zero;
        }
        float x, y;

        if (x1 == x2)
        {
            float m2 = (y4 - y3) / (x4 - x3);
            float c2 = -m2 * x3 + y3;

            x = x1;
            y = c2 + m2 * x1;
        }
        else if (x3 == x4)
        {
            float m1 = (y2 - y1) / (x2 - x1);
            float c1 = -m1 * x1 + y1;

            x = x3;
            y = c1 + m1 * x3;
        }
        else
        {
            float m1 = (y2 - y1) / (x2 - x1);
            float c1 = -m1 * x1 + y1;
            float m2 = (y4 - y3) / (x4 - x3);
            float c2 = -m2 * x3 + y3;
            x = (c1 - c2) / (m2 - m1);
            y = c2 + m2 * x;
        }

        if (IsInsideLine(lineAStart, lineAEnd, x, y) &&
            IsInsideLine(lineBStart, lineBEnd, x, y))
        {
            return new Vector2(x, y);
        }
        return Vector2.zero;
    }
    //交点是否在线以内
    private bool IsInsideLine(Vector3 start, Vector3 end, float x, float y)
    {
        return ((x >= start.x && x <= end.x)
            || (x >= end.x && x <= start.x))
            && ((y >= start.y && y <= end.y)
                || (y >= end.y && y <= start.y));
    }

    /// <summary>
    /// 判断点位是否在一条直线上
    /// </summary>
    /// <param name="Pos1"></param>
    /// <param name="Pos2"></param>
    /// <param name="Pos3"></param>
    /// <returns></returns>
    public bool PosInLine(Vector3 Pos1, Vector3 Pos2, Vector3 Pos3)
    {
        Vector3 dirA = (Pos1 - Pos2).normalized;
        Vector3 dirB = new Vector3(
            Mathf.Abs(dirA.x),
            Mathf.Abs(dirA.y),
            Mathf.Abs(dirA.z)
            ).normalized;

        Vector3 dirC = (Pos3 - Pos1).normalized;
        Vector3 dirD = new Vector3(
         Mathf.Abs(dirC.x),
         Mathf.Abs(dirC.y),
         Mathf.Abs(dirC.z)
         ).normalized;

        if (dirB == dirD)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    private void OnDrawGizmos()
    {
        Gizmos.color = Color.red;
        Gizmos.DrawLine(a.position, b.position);
        Gizmos.color = Color.yellow;
        Gizmos.DrawLine(a.position, c.position);

        Vector3 dirAB = (b.position - a.position).normalized;
        Vector3 verticalABDir = (Vector3.Dot(Vector3.up, dirAB) * (-dirAB) + Vector3.up).normalized;   //直线的垂线
        Vector3 horizontalABDir = Vector3.Cross(dirAB, verticalABDir).normalized;
        Vector3 aa = c.position - horizontalABDir * 10;
        Vector3 bb = c.position + horizontalABDir * 10;

        Vector2 pos = GetIntersection(new Vector2(a.position.x, a.position.z),
          new Vector2(b.position.x, b.position.z),
          new Vector2(c.position.x, c.position.z),
          new Vector2(aa.x, aa.z));

        if (pos == Vector2.zero)
        {
            Gizmos.DrawLine(c.position, bb);
            Vector2 pos1 = GetIntersection(new Vector2(a.position.x, a.position.z),
          new Vector2(b.position.x, b.position.z),
          new Vector2(c.position.x, c.position.z),
          new Vector2(bb.x, bb.z));
           d.position = new Vector3(pos1.x, 0, pos1.y);
        }
        else
        {
            Gizmos.DrawLine(c.position, aa);
            d.position = new Vector3(pos.x, 0, pos.y);
        }
       Debug.Log(PosInLine(a.position,b.position,c.position));
    }
}