Using shadow texture to recieve shadows (on grass)

Hi everyone!

I've created some grass on a custom mesh terrain (NOT Unity terrain) and I want it to receive shadows from surrounding trees. Grass is created from a point cloud where each vertex becomes a billboard in geometry shader. Trees successfully cast shadows on terrain.

I've tried sampling Unity's shadow textures for light (since they have all that fancy good looking cascading stuff) to render shadows on grass but I have failed. The closest question to what I'm asking is this one: http://answers.unity3d.com/questions/442592/accessing-shadow-texture-in-shader.html For some reason neither of the answers given there helped me. I've tried both with SHADOW_COORDS and LIGHTING_COORDS (and other appropriate functions) but it didn't work. I always receive shadow attenuation equal to 1.

I've never created a multiple pass shader so if an answer is somewhere in there I am kindly begging for detailed info.

At the bottom is shortened grass billboard shader with a screenshot of a scene.(I deliberately exaggerated with light intensity and shadow strength so that it would be obvious). I'm hoping to achieve different attenuation for my grass in shadow and lit areas. As commented in a shader sample, red color means that attenuation is equal to 1 and every bit of my grass is red. (I reduced the contrast of the picture so it doesn't hurt your eyes)

How can I use shadow textures to sample light/shadows? Can it be done even though I have a geometry shader? Can I sample shadow texture in vertex shader and pass its value to fragment shader trough geometry shader?

Thank you

Shader and screenshot:

 Shader "Custom/GrassBillboardShortened" 
 {
     Properties 
     {
         /** properties **/
     }
 
     SubShader 
     {
         Tags { "Queue" = "Geometry" "RenderType"="Opaque" }
 
         Pass
         {
             Tags { "LightMode" = "ForwardBase"}
 
             CGPROGRAM
                 #pragma vertex vertexShader
                 #pragma fragment fragmentShader
                 #pragma geometry geometryShader
                 #pragma multi_compile_fwdbase
 
                 #include "UnityCG.cginc"
                 #include "AutoLight.cginc"
 
                 struct VS_INPUT
                 {
                     float4 vertex : POSITION;
                     /* .. and other variables used for creating billboard ... */
                 };
 
                 // I put shadow stuff in separate struct because of naming in some macros
                 struct VS_SHADOW
                 {
                     float4 pos : POSITION;
                     SHADOW_COORDS(0)
                 };
 
                 struct GS_INPUT
                 {
                     float4 worldPosition : POSITION;
                     /* .. and other variables like billboard size etc... */
                 };
 
                 struct FS_INPUT
                 {
                     float4 position    : SV_POSITION;
                     fixed2 uv_MainTexture : TEXCOORD0;
                     /* I used variable name _ShadowCoord since SHADOW_ATTENUATION uses it (according to "AutoLight.cginc") */
                     float4 _ShadowCoord : TEXCOORD1;
                 };
 
                 /* ... declarations of variables ... */
 
                 // Vertex Shader ------------------------------------------------
                 GS_INPUT vertexShader(VS_INPUT v)
                 {
                     GS_INPUT vOut;
                     
                     vOut.worldPosition =  mul(_Object2World, v.vertex);
 
                     // I put shadow stuff in separate struct
                     VS_SHADOW shadow;
                     shadow.pos = mul(UNITY_MATRIX_MVP, v.vertex);
 
                     /* I used this instead of TRANSFER_SHADOW(shadow) since I had to store shadow coords in a variable to pass them to
                      * geometry shader. When I use TRANSFER_SHADOW, I guess shadows coords are stored somewhere in _ShadowCoord variable
                      * according to "AutoLight.cginc" but for some reason I can't reference it with shadow._ShadowCoord */
                     /* It's not that I wanted to have it this way but every other attempt (like sampling attenuation here in vertex shader)
                      * also failed */
                     vOut.shadowCoords = ComputeScreenPos(shadow.pos); 
 
                     return vOut;
                 }
 
 
                 // Geometry Shader -----------------------------------------------------
                 [maxvertexcount(4)]
                 void geometryShader(point GS_INPUT p[1], inout TriangleStream<FS_INPUT> triStream)
                 {
                     /* ... bunch of code for creating billboard geometry - that works ... */
                     float4 v[4];    // 4 vertices for billboard are created 
                     float4x4 vpMatrix = mul(UNITY_MATRIX_MVP, _World2Object);
                     
                     FS_INPUT fIn;
                     fIn.position = mul(vpMatrix, v[0]);
                     fIn.uv_MainTexture = float2(1.0f, 0.0f);
                     fIn._ShadowCoord = p[0].shadowCoords;
                     
                     triStream.Append(fIn);
 
                     fIn.position =  mul(vpMatrix, v[1]);
                     fIn.uv_MainTexture = float2(1.0f, 1.0f);
                     fIn._ShadowCoord = p[0].shadowCoords;
 
                     triStream.Append(fIn);
 
                     fIn.position =  mul(vpMatrix, v[2]);
                     fIn.uv_MainTexture = float2(0.0f, 0.0f);
                     fIn._ShadowCoord = p[0].shadowCoords;
 
                     triStream.Append(fIn);
 
                     fIn.position =  mul(vpMatrix, v[3]);
                     fIn.uv_MainTexture = float2(0.0f, 1.0f);
                     fIn._ShadowCoord = p[0].shadowCoords;
 
                     triStream.Append(fIn);
                 }
 
 
                 // Fragment Shader -----------------------------------------------
                 float4 fragmentShader(FS_INPUT fIn) : COLOR
                 {
                     fixed4 color = tex2D(_MainTex, fIn.uv_MainTexture);
                     
                     if (color.a < _Cutoff) 
                         discard;
 
                     float atten = SHADOW_ATTENUATION(fIn);
                     if (atten > 0.99)
                         color = fixed4(1.0,0.0f,0.0f,1.0f);
                     else
                         color = fixed4(0.0,1.0,0.0,1.0f);
 
                     // I always get fully red color which means atten is always > 0.99
                     return color;
                 }
 
             ENDCG
         }
     } 
 
     FallBack "Diffuse"
 }