How do I sample a shadowmap in a custom shader?

For the sake of anyone else who is trying to write a fragment shader that receives shadows, I figured it out.

You must do these things:

1. '#include "AutoLight.cginc"'

2. '#include "Lighting.cginc"'

3. Add "Tags {"LightMode" = "ForwardBase"}

4. '#pragma multi_compile_fwdbase'

5. Add the Unity macros to your VSOut struct, VS and PS: LIGHTING_COORDS, TRANSFER_VERTEX_TO_FRAGMENT, LIGHT_ATTENUATION.

None of this is documented in the Unity manual.

To access the primary directional light color, unity_LightColor[0] works as long as you don't add "Tags {"LightMode" = "ForwardBase"}". If you do add that line, then it doesn't work: use _LightColor0.rgb instead. Why? Who knows.. probably makes sense to someone with access to the Unity source code. Which means no one.

Good luck!

-Peter

 Shader "Custom/PeterShader2" {
 Properties
 {
     _MainTex ("Base (RGB)", 2D) = "white" {}
 }

 CGINCLUDE

 #include "UnityCG.cginc"
 #include "AutoLight.cginc"
 #include "Lighting.cginc"

 uniform sampler2D _MainTex;

 ENDCG

 SubShader
 {
     Tags { "RenderType"="Opaque" }
     LOD 200

     Pass
     {
         Lighting On

         Tags {"LightMode" = "ForwardBase"}

         CGPROGRAM

         #pragma vertex vert
         #pragma fragment frag
         #pragma multi_compile_fwdbase

         struct VSOut
         {
             float4 pos        : SV_POSITION;
             float2 uv        : TEXCOORD1;
             LIGHTING_COORDS(3,4)
         };

         VSOut vert(appdata_tan v)
         {
             VSOut o;
             o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
             o.uv = v.texcoord.xy;

             TRANSFER_VERTEX_TO_FRAGMENT(o);

             return o;
         }

         float4 frag(VSOut i) : COLOR
         {
             float3 lightColor = _LightColor0.rgb;
             float3 lightDir = _WorldSpaceLightPos0;
             float4 colorTex = tex2D(_MainTex, i.uv.xy * float2(25.0f));
             float  atten = LIGHT_ATTENUATION(i);
             float3 N = float3(0.0f, 1.0f, 0.0f);
             float  NL = saturate(dot(N, lightDir));

             float3 color = colorTex.rgb * lightColor * NL * atten;
             return float4(color, colorTex.a);
         }

         ENDCG
     }
 } 
 FallBack "Diffuse"
 }