Getting Surface Luminance in a Shader

Unity Engine
1

Hi all. I’m just wondering if someone can advise me how to access the per-pixel diffuse luminance value for a shader? I have the following shader, using a forward-lit lighting model. I intend to limit scene lighting to a single directional source, which may make it easier to solve?

Here is my shader:

Shader "JRPG/GradientMap"
{
	
	Properties
	{
		_MainTex ("Base (RGBA)", 2D) = "white" {}
		_GradientMap ("Base (RGB)", 2D) = "white" {}
	}
	
	SubShader
	{
		Tags { "RenderType"="Opaque" }
		LOD 200
		
		CGPROGRAM
		
		#pragma surface surf Lambert

		sampler2D _MainTex;
		sampler2D _GradientMap;

		struct Input
		{
			float2 uv_MainTex;
		};

		void surf (Input IN, inout SurfaceOutput o)
		{
			float surfaceLum = 1;
			float gradientVal = clamp(tex2D(_MainTex, IN.uv_MainTex).r * surfaceLum, 0, 1);
			float gradientSel = tex2D(_MainTex, IN.uv_MainTex).g;

			half4 c = tex2D(_GradientMap, float2(gradientVal, gradientSel));
			
			o.Albedo = c.rgb;
			o.Alpha = c.a;
		}
		
		ENDCG
	} 
	
	FallBack "Diffuse"

}

What I’d like to do is substitute the line:

float surfaceLum = 1;

with:

float surfaceLum = perPixelDiffuseLuminanceValueSomehow;

I’m also hoping to eventually improve it to support multiple lights (in a Deferred model) and specularity. But I’m a bit boggle at shaders at the moment.

2

Did you eventually got this working?