垂線　perpendicular line

suisen.cs

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

public class PerpendicularTest : MonoBehaviour
{

    [SerializeField] private GameObject _startPos;
    [SerializeField] private GameObject _endPos;
    [SerializeField] private GameObject _point;

    //https://sleepygamersmemo.blogspot.com/2019/03/perpendicular-foot-point.html

    void Start()
    {
        
    }

    void Update()
    {
        
        Debug.DrawLine(
            _startPos.transform.position,
            _endPos.transform.position,
            Color.red
        );

        //startPos,endPosベクトルにpointからひいた直行ベクトルの  交点P
        var p = PerpendicularFootPoint(
            _startPos.transform.position,
            _endPos.transform.position,
            _point.transform.position
        );

        Debug.DrawLine(
            _point.transform.position,
            p,
            Color.green
        );        

    }

    Vector3 PerpendicularFootPoint(Vector3 a, Vector3 b, Vector3 p)
    {
        return a + Vector3.Project(p - a, b - a);
    }


    // 点Pから直線ABに下ろした垂線の足の座標を返す
    Vector2 PerpendicularFootPoint(Vector2 a, Vector2 b, Vector2 p)
    {
        Vector2 ab = (b - a).normalized;
        return a + Vector2.Dot(p - a, ab) * ab;
    }    

}