Question Shader discarding some pixels based on color + casting shadows even for discarded ones

Hi,

I have this shader that discards some pixels based on the alpha chanel of the color vertex.
I would like however to still cast shadow for those discarded. I thought adding an extra shadowcaster pass for my surface shader would do the job, but shadows are still not casted.

Here is the shader, any idea what I might be missing?

Thanks!

Code (CSharp):

1. Shader "Custom/ColorVertexDirt" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,1)
4.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
5.         _DirtTex ("Dirt Text (RGB)", 2D) = "white" {}
6.         _Glossiness("Smoothness", Range(0,1)) = 0.5
7.         _Metallic("Metallic", Range(0,1)) = 0.0
8.         _Blend("Blend", Range(0,1)) = 0.5
9.         _AlphaCut("_AlphaCut", Range(0,1)) = 0.3
10.         _OccludedColor("Occluded Color", Color) = (1,1,1,1)
11.     }
12.         SubShader{
13.  
14.         Pass
15.         {
16.             Name "ShadowCaster"
17.              Tags{ "Queue" = "Transparent" "LightMode" = "ShadowCaster"  }
18.  
19.             CGPROGRAM
20.             #pragma vertex vert
21.             #pragma fragment frag
22.             #pragma multi_compile_shadowcaster
23.             #include "UnityCG.cginc"
24.             struct v2f {
25.                 V2F_SHADOW_CASTER;
26.             };
27.  
28.             v2f vert(appdata_full v)
29.             {
30.                 v2f o;
31.                 TRANSFER_SHADOW_CASTER_NORMALOFFSET(o)
32.                 return o;
33.             }
34.  
35.             float4 frag(v2f i) : SV_Target
36.             {
37.                 SHADOW_CASTER_FRAGMENT(i)
38.             }
39.             ENDCG
40.  
41.         }
42.  
43.         Tags { "RenderType" = "Opaque" "Queue" = "Geometry+30"}
44.         LOD 200
45.         Cull Off
46.  
47.         CGPROGRAM
48.         // Physically based Standard lighting model, and enable shadows on all light types
49.         #pragma surface surf Standard  vertex:vert
50.         // Use shader model 3.0 target, to get nicer looking lighting
51.         #pragma target 4.0
52.  
53.         sampler2D _MainTex;
54.         sampler2D _DirtTex;
55.  
56.         struct Input {
57.             float2 uv_MainTex;
58.             float4 color: COLOR;
59.             float4 screenPos;
60.             float3 worldPos;
61.         };
62.  
63.         half _Glossiness;
64.         half _Metallic;
65.         fixed4 _Color;
66.         float _Blend;
67.         float _AlphaCut;
68.  
69.         void vert(inout appdata_full i, out Input o)
70.         {
71.             UNITY_INITIALIZE_OUTPUT(Input, o);
72.             o.screenPos = ComputeScreenPos(i.vertex);
73.         }
74.  
75.  
76.         void surf (Input IN, inout SurfaceOutputStandard o) {
77.             if (IN.color.a < 0.2) discard;
78.  
79.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color ;
80.  
81.             fixed normaladd = o.Normal.r + o.Normal.g + o.Normal.b;
82.             fixed4 dirt = tex2D (_DirtTex, IN.uv_MainTex)*0.5;
83.             fixed3 normalcolor = IN.color * 0.7 + IN.color * normaladd * 0.38 + o.Normal * 0.03 + _Color + unity_LightColor[0].rgb * 0.10;
84.  
85.             o.Albedo = normalcolor;
86.  
87.             if (dirt.a > _AlphaCut)
88.             {
89.  
90.                 o.Albedo = (dirt * _Blend + normalcolor * (1-_Blend));
91.             }
92.  
93.             o.Metallic = _Metallic;
94.             o.Smoothness = _Glossiness;
95.             o.Alpha = c.a;
96.  
97.         }
98.         ENDCG
99.  
100.        
101.     }
102.     FallBack "Diffuse"
103.     //Fallback "Transparent/VertexLit"
104. }
105.