// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
 
Shader "Custom/Transparent Colored Eraser"
{
Properties
{
_MainTex ("Base (RGB), Alpha (A)", 2D) = "white" {}
_RendTex ("Base (RGB), Alpha (A)", 2D) = "white" {}
}
 
SubShader
{
LOD 200
 
Tags
{
"Queue" = "Transparent"
"IgnoreProjector" = "True"
"RenderType" = "Transparent"
}
 
Pass
{
Cull Off
Lighting Off
ZWrite Off
Fog { Mode Off }
Offset -1, -1
ColorMask RGB
AlphaTest Greater .01
Blend SrcAlpha OneMinusSrcAlpha
ColorMaterial AmbientAndDiffuse
 
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"
 
sampler2D _MainTex;
float4 _MainTex_ST;
sampler2D _RendTex;
 
struct appdata_t
{
float4 vertex : POSITION;
half4 color : COLOR;
float2 texcoord : TEXCOORD0;
};
 
struct v2f
{
float4 vertex : POSITION;
half4 color : COLOR;
float2 texcoord : TEXCOORD0;
};
 
v2f vert (appdata_t v)
{
v2f o;
o.vertex = UnityObjectToClipPos(v.vertex);
o.color = v.color;
o.texcoord = v.texcoord;
return o;
}
 
half4 frag (v2f IN) : COLOR
{
// Sample the texture
half4 col = tex2D(_MainTex, IN.texcoord) * IN.color;
half4 rnd = tex2D(_RendTex, IN.texcoord) * IN.color;
col.a = rnd.a;
return col;
}
ENDCG
}
}
}

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
 
public class UIEraserTexture : MonoBehaviour, IPointerDownHandler, IPointerUpHandler
{
 public RawImage image;
 public int brushScale = 4;
 
 Texture2D texRender;
 RectTransform mRectTransform;
 Canvas canvas;
 
 void Awake()
 {
  mRectTransform = GetComponent<RectTransform>();
  canvas = GameObject.Find("Canvas").GetComponent<Canvas>();
 }
 
 void Start()
 {
 
  texRender = new Texture2D(image.mainTexture.width, image.mainTexture.height, TextureFormat.ARGB32, true);
 
  Reset();
 
 }
 
 bool isMove = false;
 
 public void OnPointerDown(PointerEventData data)
 {
  //Debug.Log("OnPointerDown..." + data.position);
  start = ConvertSceneToUI(data.position);
  isMove = true;
 }
 
 public void OnPointerUp(PointerEventData data)
 {
  isMove = false;
  //Debug.Log("OnPointerUp..." + data.position);
  OnMouseMove(data.position);
  start = Vector2.zero;
 }
 
 void Update()
 {
  if (isMove)
  {
   OnMouseMove(Input.mousePosition);
  }
 }
 
 Vector2 start = Vector2.zero;
 Vector2 end = Vector2.zero;
 
 Vector2 ConvertSceneToUI(Vector3 posi)
 {
  Vector2 postion;
  if (RectTransformUtility.ScreenPointToLocalPointInRectangle(mRectTransform, posi, canvas.worldCamera, out postion))
  {
   return postion;
  }
  return Vector2.zero;
 }
 
 void OnMouseMove(Vector2 position)
 {
 
  end = ConvertSceneToUI(position);
 
  Draw(new Rect(end.x + texRender.width / 2, end.y + texRender.height / 2, brushScale, brushScale));
 
  if (start.Equals(Vector2.zero))
  {
   return;
  }
 
  Rect disract = new Rect((start + end).x / 2 + texRender.width / 2, (start + end).y / 2 + texRender.height / 2, Mathf.Abs(end.x - start.x), Mathf.Abs(end.y - start.y));
 
  for (int x = (int)disract.xMin; x < (int)disract.xMax; x++)
  {
   for (int y = (int)disract.yMin; y < (int)disract.yMax; y++)
   {
    Draw(new Rect(x, y, brushScale, brushScale));
   }
  }
 
  start = end;
 }
 
 void Reset()
 {
 
  for (int i = 0; i < texRender.width; i++)
  {
 
   for (int j = 0; j < texRender.height; j++)
   {
 
    Color color = texRender.GetPixel(i, j);
    color.a = 1;
    texRender.SetPixel(i, j, color);
   }
  }
 
  texRender.Apply();
  image.material.SetTexture("_RendTex", texRender);
 
 }
 
 void Draw(Rect rect)
 {
 
  for (int x = (int)rect.xMin; x < (int)rect.xMax; x++)
  {
   for (int y = (int)rect.yMin; y < (int)rect.yMax; y++)
   {
    if (x < 0 || x > texRender.width || y < 0 || y > texRender.height)
    {
     return;
    }
    Color color = texRender.GetPixel(x, y);
    color.a = 0;
    texRender.SetPixel(x, y, color);
   }
  }
 
  texRender.Apply();
  image.material.SetTexture("_RendTex", texRender);
 }
}