Cg Programming/Unity/Two-Sided Surfaces双面表面

本教程涵盖了双面逐顶点光照。

这是Unity中关于基础光照系列教程的一部分。在本教程中，我们会扩展章节的“镜面高光”到渲染双面表面。如果你没有阅读“镜面高光”，这会是一个阅读它的非常好的机会。

双面光照

一个包含了位于中心点的代数曲面。它的渲染会为曲面的两面使用不同的颜色。

如上图所示的代数曲面，对曲面的两面应用不同的颜色有时还是有意义的。在章节“裁剪”中，我们已经看到有前面剔除和背面剔除的两个通道是如何被用来对一个网格的两面应用不同的着色器的。我们将会在这里使用相同的策略。

着色器代码

对于双面逐顶点光照的着色器代码其实就是章节“镜面高光”中代码的一个简单扩展。它要求两套材质参数（前面和背面），以及复制所有的通道—-一个从前面剔除拷贝，另一个从背面剔除拷贝。两份拷贝的着色器是等价的除了背面着色器使用了负的表面法向量和背面材质属性。

Shader "Cg two-sided per-vertex lighting" {
Properties {
_Color ("Front Material Diffuse Color", Color) = (1,1,1,1)
_SpecColor ("Front Material Specular Color", Color) = (1,1,1,1)
_Shininess ("Front Material 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" }
// 环境光和第一个光源的通道
Cull Back // 只渲染前面

CGPROGRAM

#pragma vertex vert
#pragma fragment frag

#include "UnityCG.cginc"
uniform float4 _LightColor0; // 光源颜色 (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 col : COLOR;
};

vertexOutput vert(vertexInput input)
{
vertexOutput output;

float4x4 modelMatrix = _Object2World;
float4x4 modelMatrixInverse = _World2Object;

float3 normalDirection = normalize(
mul(float4(input.normal, 0.0), modelMatrixInverse).xyz);
float3 viewDirection = normalize(_WorldSpaceCameraPos
- mul(modelMatrix, input.vertex).xyz);
float3 lightDirection;
float attenuation;

if (0.0 == _WorldSpaceLightPos0.w) // 方向光
{
attenuation = 1.0; // 没有衰减
lightDirection = normalize(_WorldSpaceLightPos0.xyz);
}
else // 点或聚光源
{
float3 vertexToLightSource = _WorldSpaceLightPos0.xyz
- mul(modelMatrix, input.vertex).xyz;
float distance = length(vertexToLightSource);
attenuation = 1.0 / distance; // 线性衰减
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);
}

output.col = float4(ambientLighting + diffuseReflection
+ specularReflection, 1.0);
output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
return output;
}

float4 frag(vertexOutput input) : COLOR
{
return input.col;
}

ENDCG
}

Pass {
Tags { "LightMode" = "ForwardAdd" }
// 额外光源的通道
Blend One One // 加性混合
Cull Back // 只渲染前面

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 col : COLOR;
};

vertexOutput vert(vertexInput input)
{
vertexOutput output;

float4x4 modelMatrix = _Object2World;
float4x4 modelMatrixInverse = _World2Object;

float3 normalDirection = normalize(
mul(float4(input.normal, 0.0), modelMatrixInverse).xyz);
float3 viewDirection = normalize(_WorldSpaceCameraPos
- mul(modelMatrix, input.vertex).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
- mul(modelMatrix, input.vertex).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);
}

output.col = float4(diffuseReflection
+ specularReflection, 1.0);
// no ambient contribution in this pass
output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
return output;
}

float4 frag(vertexOutput input) : COLOR
{
return input.col;
}

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 col : COLOR;
};

vertexOutput vert(vertexInput input)
{
vertexOutput output;

float4x4 modelMatrix = _Object2World;
float4x4 modelMatrixInverse = _World2Object;

float3 normalDirection = normalize(
mul(float4(-input.normal, 0.0), modelMatrixInverse).xyz);
float3 viewDirection = normalize(_WorldSpaceCameraPos
- mul(modelMatrix, input.vertex).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
- mul(modelMatrix, input.vertex).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);
}

output.col = float4(ambientLighting + diffuseReflection
+ specularReflection, 1.0);
output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
return output;
}

float4 frag(vertexOutput input) : COLOR
{
return input.col;
}

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 col : COLOR;
};

vertexOutput vert(vertexInput input)
{
vertexOutput output;

float4x4 modelMatrix = _Object2World;
float4x4 modelMatrixInverse = _World2Object;

float3 normalDirection = normalize(
mul(float4(-input.normal, 0.0), modelMatrixInverse).xyz);
float3 viewDirection = normalize(_WorldSpaceCameraPos
- mul(modelMatrix, input.vertex).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
- mul(modelMatrix, input.vertex).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);
}

output.col = float4(diffuseReflection
+ specularReflection, 1.0);
// no ambient contribution in this pass
output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
return output;
}

float4 frag(vertexOutput input) : COLOR
{
return input.col;
}

ENDCG
}

}
Fallback "Specular"
}

以上代码包含了4个通道，第一对通道渲染了前面，第二对通道使用了负向法向量和背面材质属性渲染了背面。每一对通道的第二个除了加性混合和缺少的环境颜色外跟第一个是一模一样的。

总结

恭喜，在一个冗长的着色器后你完成了本教程。我们学习了：

如何使用前面剔除和背面剔除对一个网格的两面应用不同的着色器。

对于背面三角形如何改变Phong光照的计算。

深入阅读

about using a fragment input parameter with semantic VFACE to distinguish between front-facing and back-facing triangles, see Unity’s documentation of shader semantics.

- 关于单面曲面的着色器版本，你可以阅读章节“镜面高光”。

- 关于Cg中的前面和背面三角形，你可以阅读章节“裁剪”。

- 关于使用带有语义VFACE的片元输入参数来区分前面和背面三角形，你可以阅读Unity着色器语义的文档。