Guide Mask Shader

Shader "UI/GuideMask"
{
    Properties
    {
        [PreRendererData] _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

        _Center("Center", vector) = (0, 0, 0, 0)
        _Slider("Slider", Range(0, 1500)) = 1500

    }
    SubShader
    {
        Tags
        { 
            "RenderType"="Opaque" 
            "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 [unit_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_ALPHACLIP

            struct appdata_t
            {
                float4 vertext : 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;

            float _Slider;
            float2 _Center;

            v2f vert (appdata_t IN)
            {
                v2f OUT;
                UNITY_SETUP_INSTANCE_ID(IN);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
                OUT.worldPosition = IN.vertext;
                OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);
                OUT.texcoord = IN.texcoord;
                OUT.color = IN.color * _Color;
                return OUT;
            }

            sampler2D _MainTex;

            fixed4 frag (v2f IN) : SV_Target
            {
                half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
                
                color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);

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

                color.a *= (distance(IN.worldPosition.xy, _Center.xy) > _Slider);
                color.rgb *= color.a;

                return color;
            }
            ENDCG
        }
    }
}

The above shader is from others. It means to create an oval white shape on a gray mask. The oval shape size is controlled by the _Slider parameter.

To create an rectangle shape mask, let’s add a _RectSize to control the width and height for the rectangle.

_Center("Center", vector) = (0, 0, 0, 0)
_RectSize("RectSize", vector) = (0, 0, 0, 0)
_Slider("Slider", Range(1, 5)) = 1

float2 size = _RectSize.xy * _Slider;

float2 d = abs(IN.worldPosition - _Center.xy) - (size);
float2 d2 = min(max(d.x, d.y), 0.0) + length(max(d, 0.0));
float d3 = smoothstep(0.0, 5.0, d2);
color.a *= d3;