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LineRenderer线渲染器主要是用于在3D中渲染线段，在这里要注意LineRenderer渲染出的线段的两个端点是3D世界中的点，即它是属于世界坐标（World Space）中的。

这里我设置了一组单选按钮，通过单选按钮控制线条颜色和粗细。为了方便查看线条，我新建了一个Plane作为背景，根据鼠标按下时的位置，来持续绘制线段

以下为主要代码

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

public class MsPaint : MonoBehaviour
{
    //线条颜色
    private Color paintColor = Color.red;
    //线条粗细
    private float paintSize = 0.1f;

    //用来存储鼠标位置
    private List<Vector3> paintPos = new List<Vector3>();

    private bool isPressed;//鼠标是否长按
    private LineRenderer ren;

    private Vector3 lastPos;
    //线条材质
    public Material material;


    #region 单选框点击事件
    public void OnRedChange(bool isOn)
    {
        if (isOn)
        {
            paintColor = Color.red;
        }
    }
    public void OnGreenChange(bool isOn)
    {
        if (isOn)
        {
            paintColor = Color.green;
        }
    }
    public void OnBlueChange(bool isOn)
    {
        if (isOn)
        {
            paintColor = Color.blue;
        }
    }

    public void OnPaint1(bool isOn)
    {
        if (isOn)
        {
            paintSize = 0.1f;
        }
    }

    public void OnPaint2(bool isOn)
    {
        if (isOn)
        {
            paintSize = 0.2f;
        }
    }
    public void OnPaint4(bool isOn)
    {
        if (isOn)
        {
            paintSize = 0.4f;
        }
    }

    #endregion  


    private void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            if (EventSystem.current.IsPointerOverGameObject())
            {
                //判断是否点击UI
                return;
            }

            //创建一个空物体，给他动态添加LineRenderer组件
            GameObject line = new GameObject();
            line.transform.SetParent(transform);
            ren = line.AddComponent<LineRenderer>();

            ren.material = material;//设置材质
            ren.startColor = paintColor;//设置颜色
            ren.endColor = paintColor;

            ren.startWidth = paintSize;//设置线的宽度
            ren.endWidth = paintSize;

            ren.numCapVertices = 2;//设置端点圆滑度
            ren.numCornerVertices = 2;//设置拐角圆滑度，顶点越多越圆滑

            lastPos = GetMousePosition();//获取鼠标在世界坐标中的位置
            paintPos.Add(lastPos);
            ren.positionCount = paintPos.Count;//设置构成线条的点的数量
            ren.SetPositions(paintPos.ToArray());

            isPressed = true;
        }
        if (isPressed)
        {
            //鼠标长按开始绘制,两点距离大于0.1开始添加点
            if (Vector3.Distance(lastPos, GetMousePosition()) > 0.1f)
            {
                paintPos.Add(GetMousePosition());
                lastPos = GetMousePosition();
            }
            ren.positionCount = paintPos.Count;
            ren.SetPositions(paintPos.ToArray());
        }

        if (Input.GetMouseButtonUp(0))
        {
            isPressed = false;
            paintPos.Clear();
            ren = null;
        }
    }

    /// <summary>
    /// 获取鼠标位置，将屏幕坐标转为世界坐标
    /// </summary>
    /// <returns></returns>
    private Vector3 GetMousePosition()
    {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        bool isCollider = Physics.Raycast(ray, out hit);
        if (isCollider)
        {
            return hit.point - new Vector3(0, 0, 0.01f);//避免遮挡
        }
        return Vector3.zero;
    }


}