Cg Programming/Unity/Two-Sided Smooth Surfaces

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Rendering of Cayley's nodal cubic surface using different colors on the two sides of the surface.

This tutorial covers two-sided per-pixel lighting (i.e. two-sided Phong shading).

Here we combine the per-pixel lighting discussed in Section “Smooth Specular Highlights” with the two-sided lighting discussed in Section “Two-Sided Surfaces”.

Shader Coder[edit]

The required changes to the code of Section “Smooth Specular Highlights” are: new properties for the back material, duplication of the passes with front-face culling for one copy and back-face culling for the other copy, and negation of the surface normal vector for the rendering of the back faces. It's actually quite straightforward. The code looks like this:

Shader "Cg two-sided per-pixel lighting" {
   Properties {
      _Color ("Diffuse Material Color", Color) = (1,1,1,1) 
      _SpecColor ("Specular Material Color", Color) = (1,1,1,1) 
      _Shininess ("Shininess", Float) = 10
      _BackColor ("Back Material Diffuse Color", Color) = (1,1,1,1) 
      _BackSpecColor ("Back Material Specular Color", Color) 
         = (1,1,1,1) 
      _BackShininess ("Back Material Shininess", Float) = 10
   }
   SubShader {
      Pass {    
         Tags { "LightMode" = "ForwardBase" } 
            // pass for ambient light and first light source
         Cull Back // render only front faces
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         #include "UnityCG.cginc"
         uniform float4 _LightColor0; 
            // color of light source (from "Lighting.cginc")
 
         // User-specified properties
         uniform float4 _Color; 
         uniform float4 _SpecColor; 
         uniform float _Shininess;
         uniform float4 _BackColor; 
         uniform float4 _BackSpecColor; 
         uniform float _BackShininess;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 posWorld : TEXCOORD0;
            float3 normalDir : TEXCOORD1;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            float4x4 modelMatrix = _Object2World;
            float4x4 modelMatrixInverse = _World2Object; 
 
            output.posWorld = mul(modelMatrix, input.vertex);
            output.normalDir = normalize(
               mul(float4(input.normal, 0.0), modelMatrixInverse).xyz);
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            float3 normalDirection = normalize(input.normalDir);
 
            float3 viewDirection = normalize(
               _WorldSpaceCameraPos - input.posWorld.xyz);
            float3 lightDirection;
            float attenuation;
 
            if (0.0 == _WorldSpaceLightPos0.w) // directional light?
            {
               attenuation = 1.0; // no attenuation
               lightDirection = normalize(_WorldSpaceLightPos0.xyz);
            } 
            else // point or spot light
            {
               float3 vertexToLightSource = 
                  _WorldSpaceLightPos0.xyz - input.posWorld.xyz;
               float distance = length(vertexToLightSource);
               attenuation = 1.0 / distance; // linear attenuation 
               lightDirection = normalize(vertexToLightSource);
            }
 
            float3 ambientLighting = 
               UNITY_LIGHTMODEL_AMBIENT.rgb * _Color.rgb;
 
            float3 diffuseReflection = 
               attenuation * _LightColor0.rgb * _Color.rgb
               * max(0.0, dot(normalDirection, lightDirection));
 
            float3 specularReflection;
            if (dot(normalDirection, lightDirection) < 0.0) 
               // light source on the wrong side?
            {
               specularReflection = float3(0.0, 0.0, 0.0); 
                  // no specular reflection
            }
            else // light source on the right side
            {
               specularReflection = attenuation * _LightColor0.rgb 
                  * _SpecColor.rgb * pow(max(0.0, dot(
                  reflect(-lightDirection, normalDirection), 
                  viewDirection)), _Shininess);
            }
 
            return float4(ambientLighting + diffuseReflection 
               + specularReflection, 1.0);
         }
 
         ENDCG
      }
 
      Pass {    
         Tags { "LightMode" = "ForwardAdd" } 
            // pass for additional light sources
         Blend One One // additive blending 
         Cull Back // render only front faces
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         #include "UnityCG.cginc"
         uniform float4 _LightColor0; 
            // color of light source (from "Lighting.cginc")
 
         // User-specified properties
         uniform float4 _Color; 
         uniform float4 _SpecColor; 
         uniform float _Shininess;
         uniform float4 _BackColor; 
         uniform float4 _BackSpecColor; 
         uniform float _BackShininess;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 posWorld : TEXCOORD0;
            float3 normalDir : TEXCOORD1;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            float4x4 modelMatrix = _Object2World;
            float4x4 modelMatrixInverse = _World2Object; 
 
            output.posWorld = mul(modelMatrix, input.vertex);
            output.normalDir = normalize(
               mul(float4(input.normal, 0.0), modelMatrixInverse).xyz);
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            float3 normalDirection = normalize(input.normalDir);
 
            float3 viewDirection = normalize(
               _WorldSpaceCameraPos - input.posWorld.xyz);
            float3 lightDirection;
            float attenuation;
 
            if (0.0 == _WorldSpaceLightPos0.w) // directional light?
            {
               attenuation = 1.0; // no attenuation
               lightDirection = normalize(_WorldSpaceLightPos0.xyz);
            } 
            else // point or spot light
            {
               float3 vertexToLightSource = 
                  _WorldSpaceLightPos0.xyz - input.posWorld.xyz;
               float distance = length(vertexToLightSource);
               attenuation = 1.0 / distance; // linear attenuation 
               lightDirection = normalize(vertexToLightSource);
            }
 
            float3 diffuseReflection = 
               attenuation * _LightColor0.rgb * _Color.rgb
               * max(0.0, dot(normalDirection, lightDirection));
 
            float3 specularReflection;
            if (dot(normalDirection, lightDirection) < 0.0) 
               // light source on the wrong side?
            {
               specularReflection = float3(0.0, 0.0, 0.0); 
                  // no specular reflection
            }
            else // light source on the right side
            {
               specularReflection = attenuation * _LightColor0.rgb 
                  * _SpecColor.rgb * pow(max(0.0, dot(
                  reflect(-lightDirection, normalDirection), 
                  viewDirection)), _Shininess);
            }
 
            return float4(diffuseReflection 
               + specularReflection, 1.0);
               // no ambient lighting in this pass
         }
 
         ENDCG
      }
 
      Pass {    
         Tags { "LightMode" = "ForwardBase" } 
            // pass for ambient light and first light source
         Cull Front // render only back faces
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         #include "UnityCG.cginc"
         uniform float4 _LightColor0; 
            // color of light source (from "Lighting.cginc")
 
         // User-specified properties
         uniform float4 _Color; 
         uniform float4 _SpecColor; 
         uniform float _Shininess;
         uniform float4 _BackColor; 
         uniform float4 _BackSpecColor; 
         uniform float _BackShininess;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 posWorld : TEXCOORD0;
            float3 normalDir : TEXCOORD1;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            float4x4 modelMatrix = _Object2World;
            float4x4 modelMatrixInverse = _World2Object; 
 
            output.posWorld = mul(modelMatrix, input.vertex);
            output.normalDir = normalize(
               mul(float4(-input.normal, 0.0), modelMatrixInverse).xyz);
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            float3 normalDirection = normalize(input.normalDir);
 
            float3 viewDirection = normalize(
               _WorldSpaceCameraPos - input.posWorld.xyz);
            float3 lightDirection;
            float attenuation;
 
            if (0.0 == _WorldSpaceLightPos0.w) // directional light?
            {
               attenuation = 1.0; // no attenuation
               lightDirection = normalize(_WorldSpaceLightPos0.xyz);
            } 
            else // point or spot light
            {
               float3 vertexToLightSource = 
                  _WorldSpaceLightPos0.xyz - input.posWorld.xyz;
               float distance = length(vertexToLightSource);
               attenuation = 1.0 / distance; // linear attenuation 
               lightDirection = normalize(vertexToLightSource);
            }
 
            float3 ambientLighting = 
               UNITY_LIGHTMODEL_AMBIENT.rgb * _BackColor.rgb;
 
            float3 diffuseReflection = 
               attenuation * _LightColor0.rgb * _BackColor.rgb
               * max(0.0, dot(normalDirection, lightDirection));
 
            float3 specularReflection;
            if (dot(normalDirection, lightDirection) < 0.0) 
               // light source on the wrong side?
            {
               specularReflection = float3(0.0, 0.0, 0.0); 
                  // no specular reflection
            }
            else // light source on the right side
            {
               specularReflection = attenuation * _LightColor0.rgb 
                  * _BackSpecColor.rgb * pow(max(0.0, dot(
                  reflect(-lightDirection, normalDirection), 
                  viewDirection)), _BackShininess);
            }
 
            return float4(ambientLighting + diffuseReflection 
               + specularReflection, 1.0);
         }
 
         ENDCG
      }
 
      Pass {    
         Tags { "LightMode" = "ForwardAdd" } 
            // pass for additional light sources
         Blend One One // additive blending 
         Cull Front // render only back faces
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         #include "UnityCG.cginc"
         uniform float4 _LightColor0; 
            // color of light source (from "Lighting.cginc")
 
         // User-specified properties
         uniform float4 _Color; 
         uniform float4 _SpecColor; 
         uniform float _Shininess;
         uniform float4 _BackColor; 
         uniform float4 _BackSpecColor; 
         uniform float _BackShininess;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 posWorld : TEXCOORD0;
            float3 normalDir : TEXCOORD1;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            float4x4 modelMatrix = _Object2World;
            float4x4 modelMatrixInverse = _World2Object; 
 
            output.posWorld = mul(modelMatrix, input.vertex);
            output.normalDir = normalize(
               mul(float4(-input.normal, 0.0), modelMatrixInverse).xyz);
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            float3 normalDirection = normalize(input.normalDir);
 
            float3 viewDirection = normalize(
               _WorldSpaceCameraPos - input.posWorld.xyz);
            float3 lightDirection;
            float attenuation;
 
            if (0.0 == _WorldSpaceLightPos0.w) // directional light?
            {
               attenuation = 1.0; // no attenuation
               lightDirection = normalize(_WorldSpaceLightPos0.xyz);
            } 
            else // point or spot light
            {
               float3 vertexToLightSource = 
                  _WorldSpaceLightPos0.xyz - input.posWorld.xyz;
               float distance = length(vertexToLightSource);
               attenuation = 1.0 / distance; // linear attenuation 
               lightDirection = normalize(vertexToLightSource);
            }
 
            float3 diffuseReflection = 
               attenuation * _LightColor0.rgb * _BackColor.rgb
               * max(0.0, dot(normalDirection, lightDirection));
 
            float3 specularReflection;
            if (dot(normalDirection, lightDirection) < 0.0) 
               // light source on the wrong side?
            {
               specularReflection = float3(0.0, 0.0, 0.0); 
                  // no specular reflection
            }
            else // light source on the right side
            {
               specularReflection = attenuation * _LightColor0.rgb 
                  * _BackSpecColor.rgb * pow(max(0.0, dot(
                  reflect(-lightDirection, normalDirection), 
                  viewDirection)), _BackShininess);
            }
 
            return float4(diffuseReflection 
               + specularReflection, 1.0);
               // no ambient lighting in this pass
         }
 
         ENDCG
      }
 
   }
   Fallback "Specular"
}

Summary[edit]

Congratulations, you have reached the end of this short tutorial. We have seen:

  • How two-sided surfaces can be rendered with per-pixel lighting.

Further reading[edit]

If you still want to know more

< Cg Programming/Unity

Unless stated otherwise, all example source code on this page is granted to the public domain.