unity5中引入了基于物理着色(PBS)的Standard shader。由于这种着色器通过调节参数和贴图可逼真模拟各种硬质表面，所以不必再像unity4时代那样需要对各种质感材质单独编写着色器，而且能得到更好的效果(参考：http://docs.unity3d.com/Manual/shader-StandardShader.html)。这种“万能着色器”仿佛给人一种不再需要自己编写着色器的假象，但做游戏跟做虚拟现实不一样，除了真实性，还要追求趣味性和艺术夸张。所以老古语不过时：没有使用自定义着色器的游戏，不是好游戏。
但自己实现PBS是很困难的，如果我们想既继承Standard shader的PBS特性又加入自己的定制效果，最好我们的自定义shader能在Standard shader的基础上进行编写，即实现自定义PBS着色器（custom PBS shader）。
由于是新东西，资料不全，google了一整天也没能找到现成方法，unity官方文档中对此完全没有作说明（在surface shader自定义光照模型 部分只给了不带PBS的例子），unity论坛里有多个帖子问到类似问题，但都没有满意解答。最后在下面两个连接里找到了一点儿线索：
http://forum.unity3d.com/threads/for-those-in-u5-beta-is-pbr-really-that-good.283867/page-3#post-1886525
http://blogs.unity3d.com/2014/10/29/physically-based-shading-in-unity-5-a-primer/ （文章下面 MIG 的提问）
此线索是：“
from the release notes for beta 12:
- Shaders: Surface shaders can use physically based shading now; the same as Standard shader uses.
- Use "Standard" lighting function, and "SurfaceOutputStandard" output structure.
- Do an #include "UnityPBSLighting.cginc" in your shader to get it.
- Default options require shader model 3.0, so add a "#pragma target 3.0" too.
”
然后又结合了UnityPBSLighting.cginc中的源代码（注1），当然，只是从UnityPBSLighting.cginc中拷贝一些代码出来（而不是修改它），最后终于把custom PBS shader试验成功了。
注1：UnityPBSLighting.cginc这个文件在哪儿？有三个途径获得：
（1），在 http://docs.unity3d.com/Manual/SL-SurfaceShaderLighting.html 中写道：“file inside Unity (
下图第一个球用的是Standard shader，第二个球用的是“将法线当作颜色值”的自定义shader（不带PBS），第三个球是今天的试验成果：在Standard shader的PBS基础上添加了“将法线当作颜色值”效果的杂交shader。
第一个球的shader用的是unity(version 5.0.1f1 Personal)里新建shader时生成的默认shader:
Shader "Custom/NewShader" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
}
SubShader {
Tags { "RenderType"="Opaque" }
LOD 200
CGPROGRAM
// Physically based Standard lighting model, and enable shadows on all light types
#pragma surface surf Standard fullforwardshadows
// Use shader model 3.0 target, to get nicer looking lighting
#pragma target 3.0
sampler2D _MainTex;
struct Input {
float2 uv_MainTex;
};
half _Glossiness;
half _Metallic;
fixed4 _Color;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
}
ENDCG
}
FallBack "Diffuse"
}
在此基础上参照UnityPBSLighting.cginc中的源代码将光照模型以自定义光照模型的形式暴露出来，得到下面等价shader:
（这里需要注意的是，一般自定义光照模型只要实现一个 Lightning+自定义光照模型名称 的函数即可，但是对于PBS shader来说，要实现自定义光照模型还要多写一个 Lightning+自定义光照模型名称_GI 的函数。）
Shader "Custom/customPBS" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
}
SubShader {
Tags { "RenderType"="Opaque" "MyReplaceTag"="Other"}
LOD 200
CGPROGRAM
// Physically based Standard lighting model, and enable shadows on all light types
#pragma surface surf MyCustomStandard fullforwardshadows
// Use shader model 3.0 target, to get nicer looking lighting
#pragma target 3.0
#include "UnityPBSLighting.cginc"
inline void LightingMyCustomStandard_GI (
SurfaceOutputStandard s,
UnityGIInput data,
inout UnityGI gi)
{
gi = UnityGlobalIllumination (data, s.Occlusion, s.Smoothness, s.Normal);
}
inline half4 LightingMyCustomStandard (SurfaceOutputStandard s, half3 viewDir, UnityGI gi)
{
s.Normal = normalize(s.Normal);
half oneMinusReflectivity;
half3 specColor;
s.Albedo = DiffuseAndSpecularFromMetallic (s.Albedo, s.Metallic, /*out*/ specColor, /*out*/ oneMinusReflectivity);
// shader relies on pre-multiply alpha-blend (_SrcBlend = One, _DstBlend = OneMinusSrcAlpha)
// this is necessary to handle transparency in physically correct way - only diffuse component gets affected by alpha
half outputAlpha;
s.Albedo = PreMultiplyAlpha (s.Albedo, s.Alpha, oneMinusReflectivity, /*out*/ outputAlpha);
half4 c = UNITY_BRDF_PBS (s.Albedo, specColor, oneMinusReflectivity, s.Smoothness, s.Normal, viewDir, gi.light, gi.indirect);
c.rgb += UNITY_BRDF_GI (s.Albedo, specColor, oneMinusReflectivity, s.Smoothness, s.Normal, viewDir, s.Occlusion, gi);
c.a = outputAlpha;
return c;
}
sampler2D _MainTex;
struct Input {
float2 uv_MainTex;
};
half _Glossiness;
half _Metallic;
fixed4 _Color;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
}
ENDCG
}
FallBack "Diffuse"
}
上面shader和默认shader效果完全一样，但是由于暴露出来光照模型（即上面的LightingMyCustomStandard函数），便使得我们可以在光照模型层次上对其进行修改，实现出自己的变种shader效果。
例如下面shader，就是在LightingMyCustomStandard中插入一行代码，实现前面图中第三个球的效果：
Shader "Custom/customPBSAndShowNormalAsColor" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
}
SubShader {
Tags { "RenderType"="Opaque" }
LOD 200
CGPROGRAM
// Physically based Standard lighting model, and enable shadows on all light types
#pragma surface surf MyCustomStandard fullforwardshadows
// Use shader model 3.0 target, to get nicer looking lighting
#pragma target 3.0
#include "UnityPBSLighting.cginc"
inline void LightingMyCustomStandard_GI (
SurfaceOutputStandard s,
UnityGIInput data,
inout UnityGI gi)
{
gi = UnityGlobalIllumination (data, s.Occlusion, s.Smoothness, s.Normal);
}
inline half4 LightingMyCustomStandard (SurfaceOutputStandard s, half3 viewDir, UnityGI gi)
{
s.Normal = normalize(s.Normal);
half oneMinusReflectivity;
half3 specColor;
s.Albedo = DiffuseAndSpecularFromMetallic (s.Albedo, s.Metallic, /*out*/ specColor, /*out*/ oneMinusReflectivity);
// shader relies on pre-multiply alpha-blend (_SrcBlend = One, _DstBlend = OneMinusSrcAlpha)
// this is necessary to handle transparency in physically correct way - only diffuse component gets affected by alpha
half outputAlpha;
s.Albedo = PreMultiplyAlpha (s.Albedo, s.Alpha, oneMinusReflectivity, /*out*/ outputAlpha);
half4 c = UNITY_BRDF_PBS (s.Albedo, specColor, oneMinusReflectivity, s.Smoothness, s.Normal, viewDir, gi.light, gi.indirect);
c.rgb += UNITY_BRDF_GI (s.Albedo, specColor, oneMinusReflectivity, s.Smoothness, s.Normal, viewDir, s.Occlusion, gi);
c.a = outputAlpha;
c.rgb*=s.Normal *1.5;//added by wantnon
return c;
}
sampler2D _MainTex;
struct Input {
float2 uv_MainTex;
};
half _Glossiness;
half _Metallic;
fixed4 _Color;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
}
ENDCG
}
FallBack "Diffuse"
}
当然，如果不需要在光照模型的层次上对unity自带的PBS shader进行定制，是没有必要这么麻烦的。
比如就拿上面这个customPBS+ShowNormalAsColor来说，其实没必要深入到光照模型层次上去实现，下面shader就可以实现基本相同的效果：
(其中蓝字是在前面Custom/NewShader基础上新增的语句)
Shader "Custom/NewShaderAndShowNormalAsColor" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
}
SubShader {
Tags { "RenderType"="Opaque" }
LOD 200
CGPROGRAM
// Physically based Standard lighting model, and enable shadows on all light types
#pragma surface surf Standard fullforwardshadows
// Use shader model 3.0 target, to get nicer looking lighting
#pragma target 3.0
sampler2D _MainTex;
struct Input {
float2 uv_MainTex;
float3 worldNormal;//ref: http://wiki.unity3d.com/index.php?title=Shader_Code
};
half _Glossiness;
half _Metallic;
fixed4 _Color;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
c.rgb*=IN.worldNormal;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
}
ENDCG
}
FallBack "Diffuse"
}