使用代码创建纯白图片，图片大小要与image组件大小相同
使用OnDrag触摸的时候将触摸点周围的像素都改为透明
使用shader判断两张图，只要有一张图像素点透明的地方就都透明
shader代码

Shader "Hidden/Draw"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _DrawTex ("drawtex", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType" = "Transparent" }
        Blend SrcAlpha OneMinusSrcAlpha
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

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

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _MainTex;
            sampler2D _DrawTex;

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = tex2D(_MainTex, i.uv);
                fixed4 drawcol = tex2D(_DrawTex, i.uv);
                col.a = drawcol.a == 0 ? 0 : col.a;
                // just invert the colors
                return col;
            }
            ENDCG
        }
    }
}

脚本代码

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class Drawonimage : MonoBehaviour,IDragHandler
{
    public Texture tu;
    public int r = 5;
    Texture2D texture;
    Vector2 wh;
    public void OnDrag(PointerEventData eventData)
    {
        Vector3 pos = transform.InverseTransformPoint(eventData.position);
        Debug.Log(pos);
        Vector2Int v2 = new Vector2Int((int)(pos.x + wh.x / 2), (int)(pos.y + wh.y / 2));
        int wbegin = v2.x - r;
        wbegin = wbegin > 0 ? wbegin : 0;
        int wend = v2.x + r;
        wend = wend < (int)wh.x ? wend : (int)wh.x;
        int hbegin = v2.y - r;
        hbegin = hbegin > 0 ? hbegin : 0;
        int hend = v2.y + r;
        hend = hend < (int)wh.y ? hend : (int)wh.y;

        for (int x = wbegin; x < wend; x++)
        {
            for (int y = hbegin; y < hend; y++)
            {
                if (Vector2.Distance(v2,new Vector2(x,y)) <= r)
                {
                    texture.SetPixel(x, y, new Color(1, 1, 1, 0));
                }
            }
        }
        texture.Apply();
    }

    // Start is called before the first frame update
    void Start()
    {
        wh = GetComponent<RectTransform>().sizeDelta;
        texture = new Texture2D((int)wh.x, (int)wh.y);
        for (int x = 0; x < wh.x; x++)
        {
            for (int y = 0; y < wh.y; y++)
            {
                texture.SetPixel(x, y, Color.white);
            }
        }
        texture.Apply();
        Material material = new Material(Shader.Find("Hidden/Draw"));
        material.SetTexture("_MainTex", tu);
        material.SetTexture("_DrawTex", texture);
        GetComponent<Image>().material = material;
    }

    // Update is called once per frame
    void Update()
    {
        
    }
}