How to write a transparent shader for a sprite that ignores transparent sprites behind it?

Question

I have multiple UI Sprites that have their alpha set so you can see what's behind them. I would like to make these UI Sprites ignore other UI Sprites.

So take this first picture. There are 4 UI Sprites that overlay over a part of each other.

Note: These sprites can appear in a random location and the number is unknown, so I thought a Shader would make sense for this.

I would like to still adjust their alpha, but I would like them to ignore each other so that they appear to be 1 image like below.

I tried modifying a Shader, but I couldn't work it out.

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

Shader "UI/Default"
{
    Properties
    {
        [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)

        _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

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

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

        Stencil
        {
            Ref [_Stencil]
            Comp [_StencilComp]
            Pass [_StencilOp]
            ReadMask [_StencilReadMask]
            WriteMask [_StencilWriteMask]
        }

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

        Pass
        {
            Name "Default"
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0

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

            #pragma multi_compile __ UNITY_UI_CLIP_RECT
            #pragma multi_compile __ UNITY_UI_ALPHACLIP

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

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

            fixed4 _Color;
            fixed4 _TextureSampleAdd;
            float4 _ClipRect;

            v2f vert(appdata_t v)
            {
                v2f OUT;
                UNITY_SETUP_INSTANCE_ID(v);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
                OUT.worldPosition = v.vertex;
                OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);

                OUT.texcoord = v.texcoord;

                OUT.color = v.color * _Color;
                return OUT;
            }

            sampler2D _MainTex;

            fixed4 frag(v2f IN) : SV_Target
            {
                half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;

                #ifdef UNITY_UI_CLIP_RECT
                color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
                #endif

                #ifdef UNITY_UI_ALPHACLIP
                clip (color.a - 0.001);
                #endif

                return color;
            }
        ENDCG
        }
    }
}

Answer 1

Just change your Stencil like below Image:

Stencil Op/Comparison Values

Comparison Functions

0 - Always
1 - Never
2 - Less
3 - Equal
4 - LEqual
5 - Greater
6 - NotEqual
7 - GEqual
8 - Always // (This is the default for the UI shaders so I suspect this one is technically the 'correct' Always, but any value beyond it will also count as Always)

Stencil Operations

0 - Keep
1 - Zero
2 - Replace
3 - IncrSat
4 - DecrSat
5 - Invert
6 - IncrWrap
7 - DecrWrap

References: unity forum & khronos

---

Adding Enum to Inspector

It's hard to remember above stencil number so you can use enum

change your properties like this:

[Enum(UnityEngine.Rendering.CompareFunction)] _StencilComp ("Stencil Comparison", Int) = 3
[Enum(UnityEngine.Rendering.StencilOp)] _StencilOp ("Stencil Operation", Int) = 3

useful link : configurableStencil shader

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Final Result