UGUI灰度图shader

在项目中，我们经常会遇到要将一张图片变成灰色，这时候我们可以通过给图片添加一个灰度图的材质，网上找到过别人提供的shader，存在一个bug：在scrollview中遮罩失效，最后找到了解决方法。下面提供一个ugui上的shader，scrollview中已经正常使用了

Shader "custom/SpriteGray"
{
    Properties
    {
        [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
        _Color("Tint", Color) = (1,1,1,1)
        [MaterialToggle] PixelSnap("Pixel snap", Float) = 0

        // required for UI.Mask
        _StencilComp ("Stencil Comparison", Float) = 8
        _Stencil ("Stencil ID", Float) = 0
        _StencilOp ("Stencil Operation", Float) = 0
        _StencilWriteMask ("Stencil Write Mask", Float) = 255
        _StencilReadMask ("Stencil Read Mask", Float) = 255
        _ColorMask ("Color Mask", Float) = 15
    }

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

        Cull Off
        Lighting Off
        ZWrite Off
        Fog{ Mode Off }
        Blend One OneMinusSrcAlpha

        // required for UI.Mask
        Stencil
        {
            Ref [_Stencil]
            Comp [_StencilComp]
            Pass [_StencilOp] 
            ReadMask [_StencilReadMask]
            WriteMask [_StencilWriteMask]
        }
        ColorMask [_ColorMask]

        Pass
        {
            CGPROGRAM
    #pragma vertex vert
    #pragma fragment frag
    #pragma multi_compile DUMMY PIXELSNAP_ON
    #include "UnityCG.cginc"

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

            struct v2f
            {
                float4 vertex   : SV_POSITION;
                fixed4 color : COLOR;
                half2 texcoord  : TEXCOORD0;
            };

            fixed4 _Color;

            v2f vert(appdata_t IN)
            {
                v2f OUT;
                OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
                OUT.texcoord = IN.texcoord;
                OUT.color = IN.color * _Color;
        #ifdef PIXELSNAP_ON
                OUT.vertex = UnityPixelSnap(OUT.vertex);
        #endif

                return OUT;
            }

            sampler2D _MainTex;

            fixed4 frag(v2f IN) : SV_Target
            {
                fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
                c.rgb *= c.a;
                float gray = dot(c.xyz, float3(0.299, 0.587, 0.114));
                c.xyz = float3(gray, gray, gray);
                return c;
            }
            ENDCG
        }
    }
    FallBack "Diffuse"
}