Unity Shader实现裁切效果

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

public class DiscardTest : MonoBehaviour {
 private Material m;
 private void Start()
 {
 m = GetComponent<MeshRenderer>().material;
 }
 void Update () {
 m.SetFloat("_YFactor", transform.position.y);
 }
}

Shader "Unlit/3DdiscardTest"
{
 Properties
 {
 _MainTex ("Texture", 2D) = "white" {}
 //裁切的范围(根据实际物体大小而定，通过C#赋值)
 _DiscardFactor("DiscardFactor",Range(-0.51,0.55)) = 0.0
 //切口光的颜色
 _LightColor("LightColor",Color) = (1,1,1,1)
 //光的宽度
 _LightWidth("LightWidth",Range(0.0,0.1))=0.05
 }
 SubShader
 {
 Tags { "RenderType"="Opaque" }
 //不关闭背面剔除的话看不到物体内侧
 Cull off
 Pass
 {
 CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag 
 #include "UnityCG.cginc"
 struct v2f
 {
 float2 uv : TEXCOORD0;
 float4 vertex : SV_POSITION;
 float3 worldPos:TEXCOORD1;
 };

 sampler2D _MainTex;
 float _YFactor;
 float _DiscardFactor;
 float4 _LightColor;
 float _LightWidth;
 
 v2f vert (appdata_base v)
 {
 v2f o;
 o.vertex = UnityObjectToClipPos(v.vertex);
 o.worldPos= mul(unity_ObjectToWorld, v.vertex).xyz;
 o.uv = v.texcoord;
 return o;
 }
 
 fixed4 frag (v2f i) : SV_Target
 { 
 float factor = _YFactor-i.worldPos.y + _DiscardFactor;
 //_YFactor-i.worldPos.y=0时像素为物体中心，大于0像素在下面，小于0像素在上面，_DiscardFactor越小，factor越小，下面保留的就越少
 if (factor<0)
 {
 discard;
 }
 float4 color = tex2D(_MainTex, i.uv);
 //factor<0的部分已经被裁减，剩下的部分在加个边界做切面的描边
 if (factor < _LightWidth)
 {
 return _LightColor;
 }
 return color;
 //优化了上面if的写法，等价，但是看起来比较绕
 //float lerpFactor=saturate(sign(_LightWidth -factor));
 //return color * (1 - lerpFactor) + lerpFactor * _LightColor;
 }
 ENDCG
 }
 }
}