本教程涵盖了半透明表面。
这是几个关于光照教程中的其中之一，它超出了Phone反射模型的范围。但是，这是基于章节“光滑镜面高光”中描述的逐像素光照的Phone反射模型。如果你还没有阅读过那个教程，建议你先看一下。
Phone反射模型并没有把半透明考虑进来，即光照会穿透材质。本教程是关于半透明表面的，也就是说这个表面允许光从一个面传到另一个面，比如纸张、衣服、塑料薄膜或者树叶。
漫射半透明度
我们将要区分光传输的两种类型：漫射半透明度和向前发散的半透明度，它们各自对应于Phone反射模型中的漫射和高光项。漫射半透明度是类似于Phone反射模型中漫反射项的光的漫射传输：这只是取决于表面法向量和指向光源方向的点乘 — 除非光源在背面我们就使用负向表面法向量，于是漫射半透明光照的等式就是这样：
这是许多半透明表面最常见的光照，比如纸张和树叶。
向前散射的半透明
一些半透明表面（比如塑料薄膜）几乎是透明的并且允许光线直接透过表面但只有一些向前的散射；也就是说，我们可以透过表面看到光源但图像会有些模糊。这个跟Phone反射模型的镜面项相似（查看章节“镜面高光”中的等式），除了我们用负的光线方向-L替换光线方向R以及指数
现在对应于前向散射光的明锐度。
当然，这个前向散射半透明度的模型并不总是准确的，但是它允许我们可以伪造效果并调整参数。
下面的实现基于章节“光滑镜面高光”，它是用Phone反射模型的逐像素光照表示的。这个实现允许渲染背面，并且使用内置Cg函数faceforward(n, v, ng)来翻转表面法向量，如果dot(v, ng) < 0就返回n，否则返回-n。这个方法通常会在轮廓处失败，它会导致一些像素的照明不正确。一个改进的版本会为在章节“双面光滑表面”提到的正面和背面使用不同的通道和颜色。
除了Phone反射模型的高光项，我们也要用以下代码计算漫射半透明和前向散射半透明的光照：
float3 diffuseTranslucency =
attenuation * _LightColor0.rgb
* _DiffuseTranslucentColor.rgb
* max(0.0, dot(lightDirection, -normalDirection));
float3 forwardTranslucency;
if (dot(normalDirection, lightDirection) > 0.0)
// light source on the wrong side?
{
forwardTranslucency = float3(0.0, 0.0, 0.0);
// no forward-scattered translucency
}
else // light source on the right side
{
forwardTranslucency = attenuation * _LightColor0.rgb
* _ForwardTranslucentColor.rgb * pow(max(0.0,
dot(-lightDirection, viewDirection)), _Sharpness);
}完整的着色器代码
完整的着色器代码为材质常量定义了着色器属性，并且为额外的光源添加了另一个有加性混合但没有环境光照的通道。
Shader "Cg translucent surfaces" {
Properties {
_Color ("Diffuse Material Color", Color) = (1,1,1,1)
_SpecColor ("Specular Material Color", Color) = (1,1,1,1)
_Shininess ("Shininess", Float) = 10
_DiffuseTranslucentColor ("Diffuse Translucent Color", Color)
= (1,1,1,1)
_ForwardTranslucentColor ("Forward Translucent Color", Color)
= (1,1,1,1)
_Sharpness ("Sharpness", Float) = 10
}
SubShader {
Pass {
Tags { "LightMode" = "ForwardBase" }
// pass for ambient light and first light source
Cull Off // show frontfaces and backfaces
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 _DiffuseTranslucentColor;
uniform float4 _ForwardTranslucentColor;
uniform float _Sharpness;
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);
normalDirection = faceforward(normalDirection,
-viewDirection, normalDirection);
// flip normal if dot(-viewDirection, normalDirection)>0
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);
}
// Computation of the Phong reflection model:
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);
}
// Computation of the translucent illumination:
float3 diffuseTranslucency =
attenuation * _LightColor0.rgb
* _DiffuseTranslucentColor.rgb
* max(0.0, dot(lightDirection, -normalDirection));
float3 forwardTranslucency;
if (dot(normalDirection, lightDirection) > 0.0)
// light source on the wrong side?
{
forwardTranslucency = float3(0.0, 0.0, 0.0);
// no forward-scattered translucency
}
else // light source on the right side
{
forwardTranslucency = attenuation * _LightColor0.rgb
* _ForwardTranslucentColor.rgb * pow(max(0.0,
dot(-lightDirection, viewDirection)), _Sharpness);
}
// Computation of the complete illumination:
return float4(ambientLighting
+ diffuseReflection + specularReflection
+ diffuseTranslucency + forwardTranslucency, 1.0);
}
ENDCG
}
Pass {
Tags { "LightMode" = "ForwardAdd" }
// pass for additional light sources
Cull Off
Blend One One // additive blending
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 _DiffuseTranslucentColor;
uniform float4 _ForwardTranslucentColor;
uniform float _Sharpness;
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);
normalDirection = faceforward(normalDirection,
-viewDirection, normalDirection);
// flip normal if dot(-viewDirection, normalDirection)>0
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);
}
// Computation of the Phong reflection model:
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);
}
// Computation of the translucent illumination:
float3 diffuseTranslucency =
attenuation * _LightColor0.rgb
* _DiffuseTranslucentColor.rgb
* max(0.0, dot(lightDirection, -normalDirection));
float3 forwardTranslucency;
if (dot(normalDirection, lightDirection) > 0.0)
// light source on the wrong side?
{
forwardTranslucency = float3(0.0, 0.0, 0.0);
// no forward-scattered translucency
}
else // light source on the right side
{
forwardTranslucency = attenuation * _LightColor0.rgb
* _ForwardTranslucentColor.rgb * pow(max(0.0,
dot(-lightDirection, viewDirection)), _Sharpness);
}
// Computation of the complete illumination:
return float4(diffuseReflection + specularReflection
+ diffuseTranslucency + forwardTranslucency, 1.0);
}
ENDCG
}
}
}总结
恭喜！你完成了半透明表面的教程，它非常常见但又不能用Phone反射模型来建模。我们学到了：
- 什么是半透明表面。
- 哪种半透明是最常见的（漫射半透明和前向散射半透明）。
- 如何实现漫射半透明和前向散射半透明。
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