Unity shader flat shadow

Shader "Custom/PlanarShadow"
{
	Properties
	{
		_MainTex("Texture", 2D) = "white" {}
		_ShadowInvLen("ShadowInvLen", float) = 6.36 //0.4449261
		_ShadowFalloff("ShadowFalloff", float) = 0.49
		_ShadowPlane("ShadowPlane",vector) = (0, 1.0, 0, 0.1)
		_ShadowFadeParams("ShadowFadeParams",vector) = (1.13, 1.5, 0.7, 0)  

	}

	SubShader
	{
		Tags{ "RenderType" = "Opaque" "Queue" = "Geometry+10" }
		LOD 100

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			// make fog work
			#pragma multi_compile_fog
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				UNITY_FOG_COORDS(1)
				float4 vertex : SV_POSITION;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;

			v2f vert(appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				UNITY_TRANSFER_FOG(o,o.vertex);
				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				fixed4 col = tex2D(_MainTex, i.uv);
				UNITY_APPLY_FOG(i.fogCoord, col);
				return col;
			}
			ENDCG
		}

		Pass
		{
			Blend SrcAlpha  OneMinusSrcAlpha
			ZWrite Off
			Cull Back
			ColorMask RGB

			Stencil
			{
				Ref 0
				Comp Equal
				WriteMask 255
				ReadMask 255
				Pass Invert
				Fail Keep
				ZFail Keep
			}

			CGPROGRAM

			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc"

			float4 _ShadowPlane;
			float _ShadowInvLen;
			float4 _ShadowFadeParams;
			float _ShadowFalloff;

			struct appdata
			{
				float4 vertex : POSITION;
			};

			struct v2f
			{
				float4 vertex : SV_POSITION;
				float3 xlv_TEXCOORD0 : TEXCOORD0;
				float3 xlv_TEXCOORD1 : TEXCOORD1;
			};

			v2f vert(appdata v)
			{
				v2f o;
				float3 lightdir = normalize(_WorldSpaceLightPos0.xyz);
				float3 worldpos = mul(unity_ObjectToWorld, v.vertex).xyz;
				float distance = (_ShadowPlane.w - dot(_ShadowPlane.xyz, worldpos)) / dot(_ShadowPlane.xyz, lightdir.xyz);
				worldpos = worldpos + distance * lightdir.xyz;
				o.vertex = mul(unity_MatrixVP, float4(worldpos, 1.0));
				float3 center = mul(unity_ObjectToWorld,float4(0,0,0,1)).xyz;
				o.xlv_TEXCOORD0 = center;
				o.xlv_TEXCOORD1 = worldpos;
				return o;
			}

			float4 frag(v2f i) : SV_Target
			{
				float3 posToPlane_2 = (i.xlv_TEXCOORD0 - i.xlv_TEXCOORD1);
				//float a = (pow((1.0 - clamp(((sqrt(dot(posToPlane_2, posToPlane_2)) * _ShadowInvLen) - _ShadowFadeParams.x), 0.0, 1.0)), _ShadowFadeParams.y) * _ShadowFadeParams.z);
				float a = 1.0 - saturate(distance(i.xlv_TEXCOORD0, i.xlv_TEXCOORD1) * _ShadowFalloff);
				return float4(0,0,0,a);
			}
			ENDCG
		}
	}
}