Vector3.ProjectOnPlane

Parameters

Returns

Vector3 The vector that results from projection of vector on the plane.

Description

Projects a vector onto a plane.

This method resolves the input vector onto the plane and returns the resulting vector. The new vector is orthogonal to planeNormal and parallel to the plane. Note that planeNormal does not need to be normalized.

// This example rotates a cube above a tilted plane. As the cube rotates, the cube's position vector is projected onto the plane and rendered as a line. 

using UnityEngine;

public class ProjectOnPlaneExampleUpdate: MonoBehaviour
{
    GameObject groundPlane;
    GameObject rotObject;
    LineRenderer line;
    

    void Start ()
    {
        // Create the plane
        groundPlane = GameObject.CreatePrimitive(PrimitiveType.Plane);
        groundPlane.transform.Rotate(-30, 10, 0);

        // Create the item to rotate    
        rotObject = GameObject.CreatePrimitive(PrimitiveType.Cube);
        rotObject.transform.position = new Vector3(5,5,0);
        line = rotObject.AddComponent<LineRenderer>();
    }

    void Update()
    {
        // Set the rotation origin
        Vector3 origin = Vector3.zero;

        // Rotate the object above the plane
        rotObject.transform.RotateAround(origin, Vector3.up, 20 * Time.deltaTime);

        // Project the location of the cube onto the plane
        Vector3 projected = Vector3.ProjectOnPlane(rotObject.transform.position, groundPlane.transform.up);

        // Draw the projected vector as a line
        line.SetPosition(0, origin);
        line.SetPosition(1, projected);
        line.startWidth = 0.1f;  
    
    }
}

Additional resources: Project, Reflect.