Trying to make a masking shader and the areas that are supposed to be blank are culling the things behind it.

Hi,

I'm trying to make a shader smoke particles and the areas that are supposed to be blank are culling the things behind it. I don't really know what the technical name for this is so I haven't been able to trouble shoot it very successfully.

This is the shader code I'm using: // Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,)' with 'UnityObjectToClipPos()'

     // Unlit alpha-blended shader.
     // - no lighting
     // - no lightmap support
     // - no per-material color
      
     Shader "Unlit/TransparentAlphaMask" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _AlphaTex ("Alpha mask (R)", 2D) = "white" {}
         _SpeedX("speedX", float)=1.5
         _SpeedY("speedY", float)=1.5
         _Scale("Scale", range(0.005,0.5))=0.03
     }
      
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         LOD 100
      
         Blend SrcAlpha OneminusSrcAlpha
 
      
         Pass {
             CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
              
                 #include "UnityCG.cginc"
              
      
                 struct appdata_t {
                     float4 vertex : POSITION;
                     float2 texcoord : TEXCOORD0;
                 };
      
                 struct v2f {
                     float4 vertex : SV_POSITION;
                     half2 texcoord : TEXCOORD0;
                 };
      
                 sampler2D _MainTex;
                 sampler2D _AlphaTex;
              
                 float4 _MainTex_ST;
              
                 v2f vert (appdata_t v)
                 {
                     v2f o;
                     o.vertex = UnityObjectToClipPos(v.vertex);
                     o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
                     return o;
                 }
              
                 fixed4 frag (v2f i) : SV_Target
                 {
                     fixed4 col = tex2D(_MainTex, i.texcoord);
                     fixed4 col2 = tex2D(_AlphaTex, i.texcoord);
                  
                     return fixed4(col.r, col.g, col.b, col2.a);
                 }
             ENDCG
         }
     }
      
     }