Multiple pass Surface Shader

Questions & Answers
1

I am working on a multiple pass surface shader based on the Unity builtin VertexLit shader…

Basically, what I am trying to do is let the shader do its thing in its own passes, and then add a second pass to layer some extra color on top of what color was originally there.

I’ve gotten that far, however the problem is, my surface shader pass is ignoring transparency – returning an alpha of zero should cause colors from the the original VertexLit shader to bleed through, right? Apparently not!

I am very new to any form of advanced shaders, and clearly there is something here I don’t understand. Does anyone care to give a bit of clarity to a noob? Thankyou!

Here is the code for the aforementioned shader: – I’ve noted the part of the code that stores my “secondary pass”

Shader "VertexLit Toon" {
Properties {
	_RimPower ("Rim Power", Float) = 2.5
	_Color ("Main Color", Color) = (1,1,1,1)
	_SpecColor ("Spec Color", Color) = (1,1,1,1)
	_Emission ("Emissive Color", Color) = (0,0,0,0)
	_Shininess ("Shininess", Range (0.01, 1)) = 0.7
	_MainTex ("Base (RGB)", 2D) = "white" {}
}

// 2/3 texture stage GPUs
SubShader {
	Tags { "RenderType"="Opaque" }
	LOD 100
	
	// Non-lightmapped
	Pass {
		Tags { "LightMode" = "Vertex" }
		
		Material {
			Diffuse [_Color]
			Ambient [_Color]
			Shininess [_Shininess]
			Specular [_SpecColor]
			Emission [_Emission]
		} 
		Lighting On
		SeparateSpecular On
		SetTexture [_MainTex] {
			Combine texture * primary DOUBLE, texture * primary
		} 
	}
	
	// Lightmapped, encoded as dLDR
	Pass {
		Tags { "LightMode" = "VertexLM" }
		
		BindChannels {
			Bind "Vertex", vertex
			Bind "normal", normal
			Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
			Bind "texcoord", texcoord1 // main uses 1st uv
		}
		
		SetTexture [unity_Lightmap] {
			matrix [unity_LightmapMatrix]
			constantColor [_Color]
			combine texture * constant
		}
		SetTexture [_MainTex] {
			combine texture * previous DOUBLE, texture * primary
		}
	}
	
	// Lightmapped, encoded as RGBM
	Pass {
		Tags { "LightMode" = "VertexLMRGBM" }
		
		BindChannels {
			Bind "Vertex", vertex
			Bind "normal", normal
			Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
			Bind "texcoord1", texcoord1 // unused
			Bind "texcoord", texcoord2 // main uses 1st uv
		}
		
		SetTexture [unity_Lightmap] {
			matrix [unity_LightmapMatrix]
			combine texture * texture alpha DOUBLE
		}
		SetTexture [unity_Lightmap] {
			constantColor [_Color]
			combine previous * constant
		}
		SetTexture [_MainTex] {
			combine texture * previous QUAD, texture * primary
		}
	}
	
	// Pass to render object as a shadow caster
	Pass {
		Name "ShadowCaster"
		Tags { "LightMode" = "ShadowCaster" }
		
		Fog {Mode Off}
		ZWrite On ZTest Less Cull Off
		Offset 1, 1

CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma multi_compile_shadowcaster
#pragma fragmentoption ARB_precision_hint_fastest
#include "UnityCG.cginc"

struct v2f { 
	V2F_SHADOW_CASTER;
};

v2f vert( appdata_base v )
{
	v2f o;
	TRANSFER_SHADOW_CASTER(o)
	return o;
}

float4 frag( v2f i ) : COLOR
{
	SHADOW_CASTER_FRAGMENT(i)
}
ENDCG

	}
	
	// Pass to render object as a shadow collector
	Pass
	{
		Name "ShadowCollector"
		Tags { "LightMode" = "ShadowCollector" }
		
		Fog {Mode Off}
		ZWrite On ZTest Less

		CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				#pragma fragmentoption ARB_precision_hint_fastest
				#pragma multi_compile_shadowcollector
				
				#define SHADOW_COLLECTOR_PASS
				#include "UnityCG.cginc"
				
				struct appdata {
					float4 vertex : POSITION;
				};
				
				struct v2f {
					V2F_SHADOW_COLLECTOR;
				};
				
				v2f vert (appdata v)
				{
					v2f o;
					TRANSFER_SHADOW_COLLECTOR(o)
					return o;
				}
				
				half4 frag (v2f i) : COLOR
				{
					SHADOW_COLLECTOR_FRAGMENT(i)
				}
		ENDCG
	}

   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //########## BEGIN SECONDARY SURFACE SHADER PASS ############
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
   //###########################################################
	
	CGPROGRAM
		#pragma surface surf Lambert
		
		struct Input
		{
		  float4 color : COLOR;
		};
		
		
		void surf (Input IN, inout SurfaceOutput o) {
		  o.Albedo = 0;
		  o.Alpha = 0;
		}
	ENDCG
	
	
}

// 1 texture stage GPUs
SubShader {
	Tags { "RenderType"="Opaque" }
	LOD 100

	// Non-lightmapped
	Pass {
		Tags { "LightMode" = "Vertex" }
		
		Material {
			Diffuse [_Color]
			Ambient [_Color]
			Shininess [_Shininess]
			Specular [_SpecColor]
			Emission [_Emission]
		} 
		Lighting On
		SeparateSpecular On
		SetTexture [_MainTex] {
			Combine texture * primary DOUBLE, texture * primary
		} 
	}	
	// Lightmapped, encoded as dLDR
	Pass {
		Tags { "LightMode" = "VertexLM" }

		BindChannels {
			Bind "Vertex", vertex
			Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
		}		
		SetTexture [unity_Lightmap] {
			matrix [unity_LightmapMatrix]
			constantColor [_Color]
			combine texture * constant
		}
	}
	Pass {
		Tags { "LightMode" = "VertexLM" }
		ZWrite Off
		Blend DstColor Zero
		SetTexture [_MainTex] {
			combine texture
		}
	}
}
}
3

I have to admit I got slightly lost trying to follow your code, but your description of “let the shader do its thing in its own passes, and then add a second pass to layer some extra color on top of what color was originally there.” makes it sound like you’re looking for a final colour modifier?

See Unity - Manual: Surface Shader examples - near the bottom.

2

It’s impossible to have a multiple pass surface shader. You have to write a CG vertex/fragment shader for that purpose.