Simple Planar Overhead Shadow Shader

Shader "Simple Planar Overhead Shadow" {
    Properties {
      _Color ("Color", Color) = (0,0,0,0)
      _GroundHeight ("Ground Height", Float) = 0
    }
    SubShader {
      Tags { "RenderType" = "Opaque" }
      CGPROGRAM
      #pragma surface surf Lambert vertex:vert
      struct Input {
          float2 uv_MainTex;
      };
      float4 _Color;
      float _GroundHeight;

      void vert (inout appdata_full v) {
        // transform into worldspace
		float4 world_space_vertex = mul( _Object2World, v.vertex );

		world_space_vertex.y = _GroundHeight;

		// transform back into local space
		v.vertex = mul( _World2Object, world_space_vertex );

      }

      void surf (Input IN, inout SurfaceOutput o) {
          o.Albedo = _Color;
      }
      ENDCG
    }
    Fallback "Diffuse"
  }