This page contains information about how to make shaders compatible with Unity’s lightmappers.
Meta Pass 提供纹理空间中的反照率值和发光值。这些值与实时渲染中使用的值是分开的，这意味着您可以使用 Meta Pass 来控制游戏对象从光照烘焙系统的角度观察的外观，而不会影响游戏对象在运行时的外观。这方面的一个很有用的例子是，您希望让悬崖上的绿色苔藓在光照贴图中生成夸张的绿色间接光，但又不想在着色器的实时通道中为地形重新着色。
Unity 的所有内置材质都具有 Meta Pass，并且标准着色器也包含 Meta Pass。如果使用了它们，则无需执行任何操作便会启用 Meta Pass。如果使用的是自定义着色器，您可以添加自己的 Meta Pass。
通过下面的着色器，可以指定仅由光照烘焙系统使用的反照率颜色和反照率纹理，而不会影响运行时的材质外观。在这个例子中，发光是从 UV 中获取的；但是可以使用任何值来控制发光。
Shader "Custom/metaPassShader"{
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
_GIAlbedoColor ("Color Albedo (GI)", Color)=(1,1,1,1)
_GIAlbedoTex ("Albedo (GI)",2D)="white"{}
}
SubShader {
// ------------------------------------------------------------------
// Extracts information for lightmapping, GI (emission, albedo, ...)
// This pass is not used during regular rendering.
Pass
{
Name "META"
Tags {"LightMode"="Meta"}
Cull Off
CGPROGRAM
#include"UnityStandardMeta.cginc"
sampler2D _GIAlbedoTex;
fixed4 _GIAlbedoColor;
float4 frag_meta2 (v2f_meta i): SV_Target
{
// We're interested in diffuse & specular colors
// and surface roughness to produce final albedo.
FragmentCommonData data = UNITY_SETUP_BRDF_INPUT (i.uv);
UnityMetaInput o;
UNITY_INITIALIZE_OUTPUT(UnityMetaInput, o);
fixed4 c = tex2D (_GIAlbedoTex, i.uv);
o.Albedo = fixed3(c.rgb * _GIAlbedoColor.rgb);
o.Emission = Emission(i.uv.xy);
return UnityMetaFragment(o);
}
#pragma vertex vert_meta
#pragma fragment frag_meta2
#pragma shader_feature _EMISSION
#pragma shader_feature _METALLICGLOSSMAP
#pragma shader_feature ___ _DETAIL_MULX2
ENDCG
}
Tags {"RenderType"="Opaque"}
LOD 200
CGPROGRAM
// Physically-based Standard lighting model, and enable shadows on all light types
#pragma surface surf Standard fullforwardshadows nometa
// 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 Unity 的所有默认 Meta Pass 中，光照烘焙系统均使用 Meta Pass 来处理漫射表面和金属性表面的反照率。光照贴图程序会处理漫射传输并在每次反弹时使用表面反照率。具有黑色（或几乎黑色）反照率的金属性表面不会反射任何光线。Meta 着色器 Pass 在渲染反照率时会让其偏向具有金属色调的更亮颜色。介电材质（木材、塑料、石材、混凝土、皮革、皮肤）具有白色镜面反射率。金属具有光谱镜面反射率。如果想要改变行为，则可以创建自定义的 Meta Pass。
注意：如果使用的是 Enlighten，则播放器中的 Meta Pass 不如 DynamicGI.SetEmissive 快，但更灵活，因为您可以使用多种颜色。
By default, shaders in Unity use monochrome transparency. This means that Unity uses the alpha channel of the material color or albedo texture to evaluate light transmission through the material.
During lightmapping, you can use custom RGB transparency instead. This means that Unity uses the values of a given texture to evaluate light transmission through the material. This is useful when you want color-based transparency that is independent of the material color or albedo texture; for example, if you want to bake lighting that simulates the behavior of a light shining through a stained glass window.
To use custom RGB transparency during lightmapping, add the following line to your ShaderLab code:
_TransparencyLM ("Transmissive Texture", 2D) = "white" {}
This creates a material property that appears in the Material Inspector with the name “Transmissive Texture”. Assign the desired texture to this field.