在自定义Shader中应用Unity全局光照

上一篇是关于Unity全局光照系统的介绍和使用方法，但是用于测试效果的场景物体都是使用了Standard材质。大部分的builtin-shader都能够支持Unity的全局光照系统，和Lightmap和Light Probe配合工作。而使用surface shader编写的自定义shader不需要做任何额外处理也同样能够支持，这是因为Unity在通过surface shader生成vertex&fragment shader时替我们做了这些处理。但是今天要讨论的是使用vertex&fragment shader编写自定义shader时，如何让我们的shader支持Unity的全局光照系统。

从一个简单的Diffuse Shader开始

我们从一个简单的Diffuse Shader开始，逐步为它加添加代码让它能够支持全局光照。Shader如下：

Shader "Custom/Diffuse"
{
	Properties
	{
		_MainTex("Main Texture", 2D) = "white" {}
		_Color("Color Tint", Color) = (0.0, 0.0, 0.0, 0.0)
		[HDR]_Emission("Emission", Color) = (0.0, 0.0, 0.0, 0.0)
	}
	SubShader
	{
		Tags { "RenderType" = "Opaque" }

		// forward base pass
		Pass
		{
			Tags{ "LightMode" = "ForwardBase" }
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma multi_compile_fwdbase

			#include "UnityCG.cginc"
			#include "Lighting.cginc"
			#include "AutoLight.cginc"

			sampler2D _MainTex;
			float4 _MainTex_ST;
			float4 _Color;
			float4 _Emission;

			struct appdata
			{
				float4 vertex : POSITION;
				float3 normal : NORMAL;
				float2 uv : TEXCOORD0;// main texture uv
			};

			struct v2f
			{
				float4 pos : SV_POSITION;
				float2 uv : TEXCOORD0;
				float3 worldPos : TEXCOORD1;
				float3 worldNormal : TEXCOORD2;
				SHADOW_COORDS(3)
			};

			v2f vert(appdata v)
			{
				v2f o = (v2f)0;
				o.pos = UnityObjectToClipPos(v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
				o.worldNormal = UnityObjectToWorldNormal(v.normal);

				TRANSFER_SHADOW(o);
				return o;
			}

			float4 frag(v2f i) : SV_Target
			{
				float4 albedo = tex2D(_MainTex, i.uv) * _Color;

				// calculate direct diffuse lighting (Lambert)
				fixed3 worldNormal = normalize(i.worldNormal);
				fixed3 worldLight = normalize(UnityWorldSpaceLightDir(i.worldPos));
				float3 direct = _LightColor0.rgb * saturate(dot(worldNormal, worldLight));

				// calculate light attenuation and shadow
				UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);

				return float4(_Emission + albedo * direct * atten, 1.0);
			}


			ENDCG
		}

		// forward add pass
		Pass
		{
			Tags{ "LightMode" = "ForwardAdd" }
			Blend One One

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma multi_compile_fwdadd_fullshadows

			#include "UnityCG.cginc"
			#include "Lighting.cginc"
			#include "AutoLight.cginc"

			sampler2D _MainTex;
			float4 _MainTex_ST;
			float4 _Color;

			struct appdata
			{
				float4 vertex : POSITION;
				float3 normal : NORMAL;
				float2 uv : TEXCOORD0;// main texture uv
			};

			struct v2f
			{
				float4 pos : SV_POSITION;
				float2 uv : TEXCOORD0;
				float3 worldPos : TEXCOO