确保摄像机有Physics Raycaster组件

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

public class testDrawLine : MonoBehaviour, IDragHandler, IEndDragHandler
{
    public Color color = Color.red;
    public float thickness = 0.02f;
    private List<Vector3> points = new List<Vector3>();
    private List<Vector3> normals = new List<Vector3>();
    private List<int> splits = new List<int>();//分割索引 
    static Material lineMaterial;


    static void CreateLineMaterial()
    {
        if (!lineMaterial)
        { // Unity has a built-in shader that is useful for drawing 
          // simple colored things. 
            var shader = Shader.Find("Hidden/Internal-Colored");
            lineMaterial = new Material(shader);
            lineMaterial.hideFlags = HideFlags.HideAndDontSave;
            // Turn on alpha blending 
            lineMaterial.SetInt("_SrcBlend", (int)UnityEngine.Rendering.BlendMode.SrcAlpha);
            lineMaterial.SetInt("_DstBlend", (int)UnityEngine.Rendering.BlendMode.OneMinusSrcAlpha);
            // Turn backface culling off 
            lineMaterial.SetInt("_Cull", (int)UnityEngine.Rendering.CullMode.Off);
            // Turn off depth writes 
            lineMaterial.SetInt("_ZWrite", 0);
        }
    }

    public void OnRenderObject() {
        
        CreateLineMaterial();
        lineMaterial.SetPass(0);
        GL.PushMatrix();
        GL.MultMatrix(transform.localToWorldMatrix);
        GL.Begin(GL.LINES);
        int index = 0;
        for (int i=1; i<points.Count; i++) {
            if (index < splits.Count && i == splits[index]){
                index++; continue;
            }

            GL.Color(color);
            var from = points[i-1] + normals[i-1] * thickness;
            GL.Vertex3(from.x, from.y, from.z);
            var worldNormal = transform.TransformDirection(normals[i]);
            var to = points[i] + worldNormal * thickness;
            Debug.Log(worldNormal);
            GL.Vertex3(to.x, to.y, to.z); }
            GL.End();
            GL.PopMatrix();
        }

    public void OnBeginDrag(PointerEventData eventData) { }

    public void OnDrag(PointerEventData eventData) {
        if (!eventData.pointerCurrentRaycast.isValid) {
            SplitPoints();
            return;
        }
        var localPosition = transform.InverseTransformPoint(eventData.pointerCurrentRaycast.worldPosition);
        points.Add(localPosition);
        var localNormal = transform.InverseTransformDirection(eventData.pointerCurrentRaycast.worldNormal);
        normals.Add(localNormal);
    }

    public void OnEndDrag(PointerEventData eventData) {
        SplitPoints();
    }

    void SplitPoints() {
        if (splits.Count == 0 && points.Count != 0 || splits[splits.Count - 1] != points.Count)
            splits.Add(points.Count);

    }

}

在3D物体上挂上这个组件脚本就可以用鼠标在物体上划线了。