Specular from alpha in custom shader

Hi, I’ve been learning about writing my own shaders, particularly from Kenneth Lemmers youtube tutorials which are great.
I currently have a fake BRDF shader with diffuse and normal map included. But I’m having trouble adding specular from the diffuse texture’s alpha.
I know how to do it using the BlinnPhong surface but unsure with the current code I have here. I’ve removed any lines that attempt to use specular in the hopes to hopefully prevent confusion. :slight_smile:

Shader "Custom/PP_BRDF" 
		_ColorTint ("Colour Tint", Color) = (125,125,125,1)
		_MainTex ("Albedo", 2D) = "white" {}
		_BumpMap ("Normal Map", 2D) = "bump"{}
		_Ramp2D ("BRDF Ramp", 2D) = "gray" {}
		Tags { "RenderType"="Opaque" }
		LOD 200
		#pragma surface surf Ramp
		#pragma target 3.0

		float4 _ColorTint;
		sampler2D _MainTex;
		sampler2D _BumpMap;
		sampler2D _Ramp2D;


		struct Input 
			float4 color : Color;
			float2 uv_MainTex;
			float2 uv_BumpMap;
		half4 LightingRamp (SurfaceOutput s,half3 lightDir, half3 viewDir, half atten)
			float NdotL = dot(s.Normal, lightDir);
			float NdotE = dot(s.Normal, viewDir);
			//Diffuse Wrap
			float diff = (NdotL * 0.5) + 0.5;
			float2 brdfUV = float2(NdotE, diff);
			float3 BRDF = tex2D(_Ramp2D, brdfUV.xy).rgb;
			float4 c;
			c.rgb = BRDF;
			c.a = s.Alpha;
			return c;

		void surf (Input IN, inout SurfaceOutput o)
			half4 c = float4(0.5,0.5,0.5,1)/ tex2D (_MainTex, IN.uv_MainTex)/2;
			IN.color = _ColorTint;
			o.Albedo = tex2D(_MainTex, IN.uv_MainTex) * IN.color/c.rgb * 2;
			o.Alpha = c.a;
			o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
	FallBack "Diffuse"

Roughly, I think what you want is this:

half4 texColor = tex2D(_MainTex,IN.uv_MainTex);
o.Specular = texColor.a