搜了下大概有这几种方法

• 通过模板缓存
• 通过shader裁剪区域：案例一，案例二，案例三，三个案例都是类似的方法，需要在c#传入数据到shader
• 通过插件

某乎上的模板缓存方法link，（没有登录看不到全文，实现估计和上面差不多）

如果使用的是模板缓存的方法，给Viewport的image一个材质mat，使用mask.shader即可，然后修改粒子的shader。
mask.shader如下

Shader "UI/UIMask"
{
	Properties
	{
		[PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)		
		
		_StencilComp ("Stencil Comparison", Float) = 8
		_StencilRef("Stencil Ref", Float) = 1
		_StencilOp ("Stencil Operation", Float) = 0
		_StencilWriteMask ("Stencil Write Mask", Float) = 255
		_StencilReadMask ("Stencil Read Mask", Float) = 255

		_ColorMask ("Color Mask", Float) = 15

		//[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
	}

	SubShader
	{
		Tags
		{ 
			"Queue"="Transparent" 
			"IgnoreProjector"="True" 
			"RenderType"="Transparent" 
			"PreviewType"="Plane"
			"CanUseSpriteAtlas"="True"
		}	

		Cull Off
		Lighting Off
		ZWrite Off
		ZTest [unity_GUIZTestMode]
		Blend SrcAlpha OneMinusSrcAlpha
		ColorMask 0

		Stencil
		{
			Ref [_StencilRef]
			Comp Always
			Pass Replace
		}

		Pass
		{
		CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"
			#include "UnityUI.cginc"

			//#pragma multi_compile __ UNITY_UI_ALPHACLIP
			
			struct appdata_t
			{
				float4 vertex   : POSITION;
				float4 color    : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			struct v2f
			{
				float4 vertex   : SV_POSITION;
				fixed4 color    : COLOR;
				half2 texcoord  : TEXCOORD0;
				float4 worldPosition : TEXCOORD1;
			};
			
			fixed4 _Color;
			fixed4 _TextureSampleAdd;
			float4 _ClipRect;

			v2f vert(appdata_t IN)
			{
				v2f OUT;
				OUT.worldPosition = IN.vertex;
				OUT.vertex = UnityObjectToClipPos(IN.vertex);

				OUT.texcoord = IN.texcoord;
				
				#ifdef UNITY_HALF_TEXEL_OFFSET
				OUT.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
				#endif
				
				OUT.color = IN.color * _Color;
				return OUT;
			}

			sampler2D _MainTex;

			fixed4 frag(v2f IN) : SV_Target
			{
				half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;				
				
				color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
				
				//#ifdef UNITY_UI_ALPHACLIP
				//clip (color.a - 0.001);
				//#endif

				return color;
			}
		ENDCG
		}
	}
}