Unity script and shader to apply partial transparency to a sprite (based on Unitys standard sprite shader)

PartiallyTransparentSprite.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using Random = UnityEngine.Random;

public class PartiallyTransparentSprite : MonoBehaviour {

  public SpriteRenderer renderer;
  Material material;
  List<Color> holes= new List<Color>();

  void Start() {
    material = renderer.material;
  }
  
  void Update() {
    int i = 0;
    holes.ForEach((hole) => material.SetColor("_hole" + i++, hole));
  }

  public void Shot() {
    float u = Random.Range(.1f, .9f);
    float v = Random.Range(.1f, .9f);
    float radius = Random.Range(.1f, .2f);
    Color hole = new Color(u, v, radius, 0f);
    holes.Add(hole);
  }
}

PartiallyTransparentSprite.shader

Shader "Custom/PartiallyTransparentSprite"
{
  Properties
  {
    [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
    _Color ("Tint", Color) = (1,1,1,1)
    [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
    _Damaged ("Damaged", Float) = 0
  }

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

    Cull Off
    Lighting Off
    ZWrite Off
    Fog { Mode Off }
    Blend One OneMinusSrcAlpha

    Pass
    {
      CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        #pragma multi_compile DUMMY PIXELSNAP_ON
        #include "UnityCG.cginc"

        fixed4 _Color;
        sampler2D _MainTex;
        float PixelSnap;
        float _Damaged;

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

        struct v2f {
          float4 vertex   : SV_POSITION;
          fixed4 color    : COLOR;
          half2 texcoord  : TEXCOORD0;
        };

        v2f vert(appdata_t IN) {
            v2f OUT;
            OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
            OUT.texcoord = IN.texcoord;
            OUT.color = IN.color * _Color;
            #ifdef PIXELSNAP_ON
            OUT.vertex = UnityPixelSnap (OUT.vertex);
            #endif

            return OUT;
        }

        // shots
        uniform float4 _hole1;
        uniform float4 _hole2;
        uniform float4 _hole3;

        // checks, if x and y are inside the given hole
        float isInsideHole(float x, float y, float4 hole) {
          float radius = hole.z;

          if(pow(x - hole.x, 2) + pow(y - hole.y, 2) < pow(radius, 2))
            return 1.0;
          return 0.0;
        }

        fixed4 frag(v2f IN, v2f_img i) : SV_Target {
          fixed4 c;

          if(_Damaged) c = float4(1.0,1.0,1.0,1.0);
          else c = tex2D(_MainTex, IN.texcoord) * IN.color;

          float4 source = tex2D(_MainTex, i.uv);
          float x = i.uv.x;
          float y = i.uv.y;
          
          // check if pixel is inside a hole and render it transparent if so
          if(isInsideHole(x, y, _hole1)) { c.a = 0.0; c.rgb *= c.a; return c;}
          if(isInsideHole(x, y, _hole2)) { c.a = 0.0; c.rgb *= c.a; return c;}
          if(isInsideHole(x, y, _hole3)) { c.a = 0.0; c.rgb *= c.a; return c;}
          
          c.rgb *= c.a;

          return c;
        }
      ENDCG
    }
  }
}