请帮我用unity 实现 再RenderTexture上画线的shader ，要求线有头尾效果，可橡皮擦，写出测试代码

以下是实现在RenderTexture上画线的Shader，并且具有头尾效果和橡皮擦功能的代码：

Shader "Custom/LineShader"
{
    Properties
    {
        _Color ("Color", Color) = (1,1,1,1)
        _Thickness ("Thickness", Range(0.01, 0.1)) = 0.05
        _Erase ("Erase", Range(0,1)) = 0
        _MainTex ("Texture", 2D) = "white" {}
    }
 
    SubShader
    {
        Tags {"Queue"="Transparent" "RenderType"="Opaque"}
        LOD 100
 
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
 
            struct appdata_t
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };
 
            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };
 
            sampler2D _MainTex;
            float4 _MainTex_ST;
            float4 _Color;
            float _Thickness;
            float _Erase;
 
            v2f vert (appdata_t v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                return o;
            }
 
            fixed4 frag (v2f i) : SV_Target
            {
                float4 col = tex2D(_MainTex, i.uv) * _Color;
                if (_Erase == 1) col = float4(0,0,0,0);
 
                // 计算线条的宽度
                float2 delta = float2(ddx(i.uv.x), ddy(i.uv.y)) * _ScreenParams.zw * _Thickness;
                float2 line = float2(1,1) - saturate(abs(i.uv - 0.5) * 2 - delta);
                col *= line.x * line.y;
 
                // 计算线头和线尾
                float2 head = saturate(i.uv * 20) * saturate((i.uv - 0.95) * 20);
                float2 tail = saturate((1 - i.uv) * 20) * saturate((0.05 - i.uv) * 20);
                col *= pow(head.x * head.y + tail.x * tail.y, 2.0);
 
                return col;
            }
            ENDCG
        }
    }
}

测试代码：

using UnityEngine;
using System.Collections;
 
public class DrawOnRenderTexture : MonoBehaviour {
 
    public Shader lineShader;
    public Color color = Color.white;
    public float thickness = 0.05f;
    public bool erase = false;
 
    private RenderTexture renderTexture;
    private Material lineMaterial;
    private Vector2? lastPoint = null;
 
    void Start () {
        // 创建一个256x256的RenderTexture
        renderTexture = new RenderTexture(256, 256, 0);
        renderTexture.Create();
 
        // 创建一个材质球，并将Shader设置为我们自定义的LineShader
        lineMaterial = new Material(lineShader);
        lineMaterial.SetTexture("_MainTex", renderTexture);
        lineMaterial.SetColor("_Color", color);
        lineMaterial.SetFloat("_Thickness", thickness);
        lineMaterial.SetFloat("_Erase", erase ? 1 : 0);
    }
 
    void Update () {
        // 当鼠标左键按下时，开始绘制
        if (Input.GetMouseButtonDown(0)) {
            lastPoint = Input.mousePosition;
        }
 
        // 当鼠标左键抬起时，停止绘制
        if (Input.GetMouseButtonUp(0)) {
            lastPoint = null;
        }
 
        // 当鼠标左键按下且移动时，绘制线条
        if (Input.GetMouseButton(0)) {
            if (lastPoint.HasValue) {
                // 计算线条的起点和终点
                Vector2 startPoint = lastPoint.Value / Screen.width;
                Vector2 endPoint = Input.mousePosition / Screen.width;
 
                // 在RenderTexture上绘制线条
                RenderTexture.active = renderTexture;
                GL.Clear(true, true, Color.clear);
                GL.PushMatrix();
                lineMaterial.SetPass(0);
                GL.LoadOrtho();
                GL.Begin(GL.QUADS);
                GL.TexCoord2(0, 0); GL.Vertex3(startPoint.x, startPoint.y, 0);
                GL.TexCoord2(0, 1); GL.Vertex3(startPoint.x, endPoint.y, 0);
                GL.TexCoord2(1, 1); GL.Vertex3(endPoint.x, endPoint.y, 0);
                GL.TexCoord2(1, 0); GL.Vertex3(endPoint.x, startPoint.y, 0);
                GL.End();
                GL.PopMatrix();
                RenderTexture.active = null;
 
                // 更新起点
                lastPoint = endPoint * Screen.width;
            }
        }
    }
 
    void OnGUI () {
        // 在屏幕上显示RenderTexture的内容
        GUI.DrawTexture(new Rect(0, 0, Screen.width, Screen.height), renderTexture, ScaleMode.ScaleToFit, false, 0);
    }
}

这个代码会在屏幕上创建一个256x256的RenderTexture，并且绑定到材质球上。当鼠标左键按下并移动时，会在RenderTexture上绘制线条，并且在屏幕上显示RenderTexture的内容。通过修改Shader的参数，可以实现不同的线条效果。