using UnityEngine;
using UnityEngine.UI;

public class Script_05_11 : MonoBehaviour
{
    //需要聚合的对象(例子中的Unity图标)
    public Image target;
    //Canvas对象
    public Canvas canvas;

    private Vector4 m_Center;
    private Material m_Material;
    private float m_Diameter; // 直径
    private float m_Current =0f;

    Vector3[] corners = new Vector3[4]; 

    void Awake () 
    {

        target.rectTransform.GetWorldCorners (corners); //计算四个角的坐标 左下 左上 右上 右下
        m_Diameter = Vector2.Distance (WordToCanvasPos(canvas,corners [0]), WordToCanvasPos(canvas,corners [2])) / 2f;

        float x =corners [0].x + ((corners [3].x - corners [0].x) / 2f);
        float y =corners [0].y + ((corners [1].y - corners [0].y) / 2f);

        Vector3 center = new Vector3 (x, y, 0f);
        Vector2 position = Vector2.zero;
        //坐标转换到相对canvas下的局部坐标
        RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, center, canvas.GetComponent<Camera>(), out position);




        center = new Vector4 (position.x,position.y,0f,0f);
        m_Material = GetComponent<Image>().material;
        m_Material.SetVector ("_Center", center);



        (canvas.transform as RectTransform).GetWorldCorners (corners);
        for (int i = 0; i < corners.Length; i++) {
            m_Current = Mathf.Max(Vector3.Distance (WordToCanvasPos(canvas,corners [i]), center),m_Current);
        }

        m_Material.SetFloat ("_Silder", m_Current);
    }


    float yVelocity = 0f;
    void Update () {
        float value = Mathf.SmoothDamp(m_Current, m_Diameter, ref yVelocity, 0.3f);
        if (!Mathf.Approximately (value, m_Current)) {//两个数是否相近
            m_Current = value;
            m_Material.SetFloat ("_Silder", m_Current);
        }
    }

    void OnGUI(){
        if(GUILayout.Button("Test")){
            Awake ();
        }
    }


    Vector2 WordToCanvasPos(Canvas canvas,Vector3 world){
        Vector2 position = Vector2.zero;
        RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, world, canvas.GetComponent<Camera>(), out position);
        return position;
    }
}

Shader "UI/Default_Mask"
{
    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
 
 
    //-------------------add----------------------
      _Center("Center", vector) = (0, 0, 0, 0)
      _Silder ("_Silder", Range (0,1000)) = 1000 // sliders
    //-------------------add----------------------
    }
 
    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_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;
            //-------------------add----------------------
            float _Silder;
            float2 _Center;
            //-------------------add----------------------
            v2f vert(appdata_t IN)
            {
                v2f OUT;
                UNITY_SETUP_INSTANCE_ID(IN);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
                OUT.worldPosition = IN.vertex;
                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
                //-------------------add----------------------
                   color.a*=(distance(IN.worldPosition.xy,_Center.xy) > _Silder);
                   color.rgb*= color.a;
                   //-------------------add----------------------
                return color;
            
            }
        ENDCG
        }
    }
}

将所有的坐标都转换到相对于Canvas父物体下的局部坐标，统一在同一个坐标系中计算距离。计算目标焦点的直径和中心点等，根据距离修改遮罩的透明度。