使用stencil实现mask功能

Shader "Custom/StencilMask"
{
    Properties
    {
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
        _StencilRef("StencilRef", Float) = 0
    }

    SubShader
    {
        Tags { "RenderType" = "Transparent" }
        Blend SrcAlpha OneMinusSrcAlpha

        Pass
        {
            Stencil
            {
                Ref[_StencilRef]
                Comp Always
                Pass Replace
            }

            CGPROGRAM

            #pragma vertex vert
            #pragma fragment frag

            #include "Lighting.cginc"

            sampler2D _MainTex;
            float4    _MainTex_ST;

            struct a2v
            {
                float4 vertex: POSITION;
                float4 texcoord: TEXCOORD0;
            };

            struct v2f
            {
                float4 pos: SV_POSITION;
                float2 uv: TEXCOORD0;
            };

            v2f vert(a2v v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                fixed4 color = tex2D(_MainTex, i.uv).rgba;

                //只有白色部分的stencil有效
                half3 delta = abs(color.rgb - fixed3(0.0, 0.0, 0.0));
                if (length(delta) < 1)
                {
                    discard;
                }

                //用于控制是否显示mask图片
                color.a = 0.5;

                return color;
            }

            ENDCG
        }
    }

    FallBack off
}