Unity Shader总结（八）——光源类型和光照衰减

光源类型

光源分为：平行光，点光源，聚光灯，面光源
光源属性有五个：位置，方向，颜色，强度，衰减
前向渲染中，Base部分计算环境光和自法光，Base只会处理一个逐像素的平行光，如果有多个平行光，其他的按逐顶点处理或者在Additional中按逐像素处理。

Shader "Unity Shaders Book/Chapter 9/Forward Rendering" {
    
    
	Properties {
    
    
		_Diffuse ("Diffuse", Color) = (1, 1, 1, 1)
		_Specular ("Specular", Color) = (1, 1, 1, 1)
		_Gloss ("Gloss", Range(8.0, 256)) = 20
	}
	SubShader {
    
    
		Tags {
    
     "RenderType"="Opaque" }
		
		Pass {
    
    
			// Pass for ambient light & first pixel light (directional light)
			Tags {
    
     "LightMode"="ForwardBase" }
		
			CGPROGRAM
			
			// Apparently need to add this declaration 
			#pragma multi_compile_fwdbase	
			
			#pragma vertex vert
			#pragma fragment frag
			
			#include "Lighting.cginc"
			
			fixed4 _Diffuse;
			fixed4 _Specular;
			float _Gloss;
			
			struct a2v {
    
    
				float4 vertex : POSITION;
				float3 normal : NORMAL;
			};
			
			struct v2f {
    
    
				float4 pos : SV_POSITION;
				float3 worldNormal : TEXCOORD0;
				float3 worldPos : TEXCOORD1;
			};
			
			v2f vert(a2v v) {
    
    
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);
				
				o.worldNormal = UnityObjectToWorldNormal(v.normal);
				
				o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
				
				return o;
			}
			
			fixed4 frag(v2f i) : SV_Target {
    
    
				fixed3 worldNormal = normalize(i.worldNormal);
				//_WorldSpaceLightPos0.xyz 方向
				fixed3 worldLightDir = normalize(_WorldSpaceLightPos0.xyz);
				
				fixed3 ambient = UNITY_LIGHTMODEL_AMBIENT.xyz;
				//_LightColor0.rgb 颜色和强度相乘的结果
			 	fixed3 diffuse = _LightColor0.rgb * _Diffuse.rgb * max(0, dot(worldNormal, worldLightDir));

			 	fixed3 viewDir = normalize(_WorldSpaceCameraPos.xyz - i.worldPos.xyz);
			 	fixed3 halfDir = normalize(worldLightDir + viewDir);
			 	fixed3 specular = _LightColor0.rgb * _Specular.rgb * pow(max(0, dot(worldNormal, halfDir)), _Gloss);
				//平行光没有衰减，因此值为1
				fixed atten = 1.0;
				
				return fixed4(ambient + (diffuse + specular) * atten, 1.0);
			}
			
			ENDCG
		}
	
		Pass {
    
    
			// Pass for other pixel lights
			Tags {
    
     "LightMode"="ForwardAdd" }
			
			Blend One One
		
			CGPROGRAM
			

			// Apparently need to add this declaration
			#pragma multi_compile_fwdadd
			
			#pragma vertex vert
			#pragma fragment frag
			
			#include "Lighting.cginc"
			#include "AutoLight.cginc"
			
			fixed4 _Diffuse;
			fixed4 _Specular;
			float _Gloss;
			
			struct a2v {
    
    
				float4 vertex : POSITION;
				float3 normal : NORMAL;
			};
			
			struct v2f {
    
    
				float4 pos : SV_POSITION;
				float3 worldNormal : TEXCOORD0;
				float3 worldPos : TEXCOORD1;
			};
			
			v2f vert(a2v v) {
    
    
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);
				
				o.worldNormal = UnityObjectToWorldNormal(v.normal);
				
				o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
				
				return o;
			}
			
			fixed4 frag(v2f i) : SV_Target {
    
    
				fixed3 worldNormal = normalize(i.worldNormal);
				#ifdef USING_DIRECTIONAL_LIGHT
					fixed3 worldLightDir = normalize(_WorldSpaceLightPos0.xyz);
				#else
					fixed3 worldLightDir = normalize(_WorldSpaceLightPos0.xyz - i.worldPos.xyz);
				#endif
				
				fixed3 diffuse = _LightColor0.rgb * _Diffuse.rgb * max(0, dot(worldNormal, worldLightDir));
				
				fixed3 viewDir = normalize(_WorldSpaceCameraPos.xyz - i.worldPos.xyz);
				fixed3 halfDir = normalize(worldLightDir + viewDir);
				fixed3 specular = _LightColor0.rgb * _Specular.rgb * pow(max(0, dot(worldNormal, halfDir)), _Gloss);
				
				#ifdef USING_DIRECTIONAL_LIGHT
					fixed atten = 1.0;
				#else
					#if defined (POINT)
				        float3 lightCoord = mul(unity_WorldToLight, float4(i.worldPos, 1)).xyz;
				        fixed atten = tex2D(_LightTexture0, dot(lightCoord, lightCoord).rr).UNITY_ATTEN_CHANNEL;
				    #elif defined (SPOT)
				        float4 lightCoord = mul(unity_WorldToLight, float4(i.worldPos, 1));
				        fixed atten = (lightCoord.z > 0) * tex2D(_LightTexture0, lightCoord.xy / lightCoord.w + 0.5).w * tex2D(_LightTextureB0, dot(lightCoord, lightCoord).rr).UNITY_ATTEN_CHANNEL;
				    #else
				        fixed atten = 1.0;
				    #endif
				#endif

				return fixed4((diffuse + specular) * atten, 1.0);
			}
			
			ENDCG
		}
	}
	FallBack "Specular"
}

如果不想多次调用Additional Pass，可以将一些光源设置为Not Important；
为了避免复杂的数学计算消耗性能，通常光照衰减使用衰减纹理进行查找，即_LightTexture0。查找的是纹理对角线上的纹理颜色值，（0,0）即与光源位置重合，（1,1）距离光源最远。
算衰减值首先把世界空间中的顶点坐标变换到光源空间；

 float4 lightCoord = mul(unity_WorldToLight, float4(i.worldPos, 1));

再使用这个坐标的模的平方（避免开方）对衰减纹理采样，得到衰减值;

fixed atten = tex2D(_LightTexture0,dot(lightCoord,lightCoord).rr).UNITY_ATTEN_CHANNEL;

宏UNITY_ATTEN_CHANNEL用来得到衰减纹理中衰减值所在的分量，以得到最终的衰减值；