Unity NGUI UIPanel下对粒子的剪裁

using System;
using UnityEngine;

[RequireComponent(typeof(UIPanel))]
public class ParticleSystemClipper : MonoBehaviour
{
    const string ShaderName = "Bleach/Particles Additive Area Clip";
    const float ClipInterval = 0.5f;

    UIPanel m_targetPanel;
    Shader m_shader;

    void Start()
    {
        // find panel
        m_targetPanel = GetComponent<UIPanel>();

        if (m_targetPanel == null)
            throw new ArgumentNullException("Cann‘t find the right UIPanel");
        if (m_targetPanel.clipping != UIDrawCall.Clipping.SoftClip)
            throw new InvalidOperationException("Don‘t need to clip");

        m_shader = Shader.Find(ShaderName);

        if (!IsInvoking("Clip"))
            InvokeRepeating("Clip", 0, ClipInterval);
    }

    Vector4 CalcClipArea()
    {
        var clipRegion = m_targetPanel.finalClipRegion;
        Vector4 nguiArea = new Vector4()
        {
            x = clipRegion.x - clipRegion.z / 2,
            y = clipRegion.y - clipRegion.w / 2,
            z = clipRegion.x + clipRegion.z / 2,
            w = clipRegion.y + clipRegion.w / 2
        };

        var uiRoot = LuaUtils.UIRoot.GetComponent<UIRoot>();
        var pos = m_targetPanel.transform.position - uiRoot.transform.position;
        float h = 2;
        float temp = h / uiRoot.manualHeight;

        return new Vector4()
        {
            x = pos.x + nguiArea.x * temp,
            y = pos.y + nguiArea.y * temp,
            z = pos.x + nguiArea.z * temp,
            w = pos.y + nguiArea.w * temp
        };
    }

    void Clip()
    {
        Vector4 clipArea = CalcClipArea();
        var particleSystems = this.GetComponentsInChildren<ParticleSystem>();

        for (int i = 0; i < particleSystems.Length; i++)
        {
            var ps = particleSystems[i];
            var mat = ps.renderer.material;

            if (mat.shader.name != ShaderName)
                mat.shader = m_shader;

            mat.SetVector("_Area", clipArea);
        }
    }

    void OnDestroy()
    {
        CancelInvoke("Clip");
    }
}

Shader "Bleach/Particles Additive Area Clip"
{
    Properties
    {
        _TintColor ("Tint Color", Color) = (0.5,0.5,0.5,0.5)
        _MainTex ("Particle Texture", 2D) = "white" {}
        _Area ("Area", Vector) = (0,0,1,1)
    }

    Category
    {
        Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
        Blend SrcAlpha One
        AlphaTest Greater .01
        ColorMask RGB
        Cull Off
        Lighting Off
        ZWrite Off
        Fog { Color (0,0,0,0) }
        
        SubShader 
        {
            Pass
            {
                CGPROGRAM
                #pragma vertex vert
                #pragma fragment frag
                #pragma multi_compile_particles

                #include "UnityCG.cginc"

                sampler2D _MainTex;
                fixed4 _TintColor;
                float4 _Area;
                
                struct appdata_t
                {
                    float4 vertex : POSITION;
                    fixed4 color : COLOR;
                    float2 texcoord : TEXCOORD0;
                };

                struct v2f
                {
                    float4 vertex : SV_POSITION;
                    fixed4 color : COLOR;
                    float2 texcoord : TEXCOORD0;
                    float2 worldPos : TEXCOORD1;
                };
                
                float4 _MainTex_ST;

                v2f vert (appdata_t v)
                {
                    v2f o;
                    o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                    o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);
                    o.color = v.color;
                    o.worldPos = mul(_Object2World, v.vertex).xy;
                    return o;
                }

                fixed4 frag (v2f i) : SV_Target
                {
                    bool inArea = i.worldPos.x >= _Area.x && i.worldPos.x <= _Area.z && i.worldPos.y >= _Area.y && i.worldPos.y <= _Area.w;
                    return inArea? 2.0f * i.color * _TintColor * tex2D(_MainTex, i.texcoord) : fixed4(0,0,0,0);
                }
                ENDCG 
            }
        }    
    }
}