一、搭好场景

1. 创建一个plan
2. 摄像机调成正交摄像机
3. 创建三个cube，附上材质
4. 再创建一个cube，找个色板，
5. 创建一个脚本，挂在非失活物体上
6. 创建一个slider，放在旁边
   

二、写代码

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

public class Draw : MonoBehaviour {
    LineRenderer line;
    Material mat;
    public Slider slider;
    int num = 0;//总共画画点数
    Color c;
	// Use this for initialization
	void Start () {
        slider.value = 0.1f;
	}
                // Update is called once per frame
    void Update () {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast(ray,out hit))
        {
            if (Input.GetMouseButtonDown(0))
            {
                if (c==null)
                {
                    return;
                }
                GameObject obj = new GameObject();
                line= obj.AddComponent<LineRenderer>();
                line.material.color= c;
                line.widthMultiplier = slider.value;//宽度
                line.SetPosition(0,hit.point);
                line.SetPosition(1, hit.point);
                num = 0;
            }
            if (Input.GetMouseButton(0))
            {
                num++;
                line.positionCount = num;
                line.SetPosition(num - 1, hit.point+Vector3.up*0.2f);

            }
            if (Input.GetMouseButtonDown(1))
            {
                StartCoroutine(ChangeColor());
            }
        }
	}
    IEnumerator ChangeColor()
    {
        yield return new WaitForEndOfFrame();
        Texture2D texture2D = new Texture2D(Screen.width,Screen.height,TextureFormat.RGB24,true);
        texture2D.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
        texture2D.Apply();
        c = texture2D.GetPixel((int)Input.mousePosition.x, (int)Input.mousePosition.y);
    }
}

三、效果