0. 简介

• 参考RectMask2D还有一些网上的例子，写了一个可以剪裁例子系统的2DMask。
• Unity版本：2017.3.0

1. ParticleRectMask2D

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

public class ParticleRectMask2D : UnityEngine.EventSystems.UIBehaviour, UnityEngine.UI.IClipper
{
    private const string CLIP_RECT = "_ClipRect";

    private RectTransform m_RectTransform;
    public RectTransform rectTransform
    {
        get { return m_RectTransform ?? (m_RectTransform = GetComponent<RectTransform>()); }
    }

    private readonly Vector3[] m_worldCorners = new Vector3[4];
    public Rect worldRect
    {
        get
        {
            rectTransform.GetWorldCorners(m_worldCorners);
            return new Rect(m_worldCorners[0].x, m_worldCorners[0].y, m_worldCorners[2].x - m_worldCorners[0].x, m_worldCorners[2].y - m_worldCorners[0].y);
        }
    }

    private Rect m_LastRect;
    
    protected override void OnEnable()
    {
        base.OnEnable();
        UnityEngine.UI.ClipperRegistry.Register(this);

        m_LastRect = worldRect;
        ModifyMaterial(m_LastRect);
    }

    protected override void OnDisable()
    {
        base.OnDisable();
        UnityEngine.UI.ClipperRegistry.Unregister(this);

        m_LastRect = Rect.zero;
        ModifyMaterial(m_LastRect);
    }

    private void ModifyMaterial(Rect rect)
    {
        ParticleSystem[] particles = gameObject.GetComponentsInChildren<ParticleSystem>();
        foreach (ParticleSystem particle in particles)
        {
            {
                Material m = particle.GetComponent<Renderer>().material;
                m.SetVector(CLIP_RECT, new Vector4(rect.xMin, rect.yMin, rect.xMax, rect.yMax));              
            }
        }
    }

    public virtual void PerformClipping()
    {
        Rect rect = worldRect;
        if (rect != m_LastRect)
        {            
            m_LastRect = rect;
            ModifyMaterial(m_LastRect);
        }
    }

}

2. Custom/Particles/Alpha Blended Premultiply Mask

// Upgrade NOTE: replaced '_Object2World' with 'unity_ObjectToWorld'

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "Custom/Particles/Alpha Blended Premultiply Mask" {
Properties {
    _MainTex ("Particle Texture", 2D) = "white" {}
    _InvFade ("Soft Particles Factor", Range(0.01,3.0)) = 1.0
    _ClipRect("ClipRect",Vector) = (0,0,0,0) // Clip
    [Enum(Enable, 1, Disable, 0)] _ClipStatus("ClipStatus", float) = 1.0 // Clip
}

Category {
    Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" "PreviewType"="Plane" }
    Blend One OneMinusSrcAlpha
    ColorMask RGB
    Cull Off Lighting Off ZWrite Off

    SubShader {
        Pass {

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0
            #pragma multi_compile_particles

            #include "UnityCG.cginc"
            #include "UnityUI.cginc" // Clip

            sampler2D _MainTex;
            fixed4 _TintColor;
            float4 _ClipRect; // Clip
            float _ClipStatus; // Clip

            struct appdata_t {
                float4 vertex : POSITION;
                fixed4 color : COLOR;
                float2 texcoord : TEXCOORD0;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct v2f {
                float4 vertex : SV_POSITION;
                fixed4 color : COLOR;
                float2 texcoord : TEXCOORD0;
                #ifdef SOFTPARTICLES_ON
                float4 projPos : TEXCOORD1;
                #endif
                float3 worldPosition : TEXCOORD2; // Clip
                UNITY_VERTEX_OUTPUT_STEREO
            };

            float4 _MainTex_ST;

            v2f vert (appdata_t v)
            {
                v2f o;
                // o.worldPosition = v.vertex 这么写好像也没问题
                o.worldPosition = mul(unity_ObjectToWorld, v.vertex); // Clip
                UNITY_SETUP_INSTANCE_ID(v);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
                o.vertex = UnityObjectToClipPos(v.vertex);
                #ifdef SOFTPARTICLES_ON
                o.projPos = ComputeScreenPos (o.vertex);
                COMPUTE_EYEDEPTH(o.projPos.z);
                #endif
                o.color = v.color;
                o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);
                return o;
            }

            UNITY_DECLARE_DEPTH_TEXTURE(_CameraDepthTexture);
            float _InvFade;

            fixed4 frag (v2f i) : SV_Target
            {
                #ifdef SOFTPARTICLES_ON
                float sceneZ = LinearEyeDepth (SAMPLE_DEPTH_TEXTURE_PROJ(_CameraDepthTexture, UNITY_PROJ_COORD(i.projPos)));
                float partZ = i.projPos.z;
                float fade = saturate (_InvFade * (sceneZ-partZ));
                i.color.a *= fade;
                #endif

                if (_ClipStatus == 0)
                {
                    return i.color * tex2D(_MainTex, i.texcoord) * i.color.a;
                }
                else
                {
                    // clip
                    fixed4 col = i.color * tex2D(_MainTex, i.texcoord) * i.color.a;
                    if (UnityGet2DClipping(i.worldPosition.xy, _ClipRect) == 0)
                    {
                        col = 0;
                    }
                    return col;
                }
            }
            ENDCG
        }
    }
}
}

3. 说明

ClipRect：剪裁区域。ClipStatus：粒子是否参与剪裁。

粒子系统设置材质的位置

默认创建的材质，修改Shape为Circle，就是一个比较方便测试的2D特效

ClipStatus = Enable

ClipStatus = Disable

4. 代码

https://pan.baidu.com/s/13kbB1uI3FPtLFQEUSCEsew