阴影生成原理

以平行光为例，把相机移动到光源位置，计算阴影映射纹理（shadowmap），这张shadowmap本质上是一张深度图，它记录了从该光源的位置出发、能看到的场景中距离它最近的表面位置（深度信息）。渲染时中把顶点位置变换到光源空间下，得到它在光源空间中的三维位置信息。然后，我们使用xy分量对shadowmap进行采样，得到shadowmap中该位置的深度信息。如果该深度值小于该顶点的深度值（通常由z分量得到），那么说明该点位于阴影中。如图，将顶点P转换到光源空间，得到（Px, Py, Pz），Pz = 0.9，用（Px, Py）对shadowmap采样，得到C点的深度0.4，0.4 < 0.9，所以P点在阴影中

需要注意的细节

使用内置阴影的shader大致框架如下，这里略去一些不重要的部分

Shader "MyCustom/BuildinShadow"
{Properties{}SubShader{Tags { "RenderType"="Opaque" }Pass{Tags {"LightMode"="ForwardBase"}CGPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile_fwdbase#include "UnityCG.cginc"#include "Lighting.cginc"#include "AutoLight.cginc"struct appdata{float4 vertex : POSITION;};struct v2f{float4 pos : SV_POSITION;SHADOW_COORDS(x) //x表示第几个纹理};v2f vert (appdata v){v2f o;//...TRANSFER_SHADOW(o);return o;}fixed4 frag (v2f i) : SV_Target{//...//fixed atten = 1.0;//fixed shadow = SHADOW_ATTENUATION(i);//return fixed4(ambient + (diffuse + specular) * atten * shadow, 1.0);UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);return fixed4(ambient + (diffuse + specular) * atten, 1.0);}ENDCG}Pass{Tags {"LightMode"="ForwardAdd"}Blend One OneCGPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile_fwdadd// #pragma multi_compile_fwdadd_fullshadows#include "UnityCG.cginc"#include "Lighting.cginc"#include "AutoLight.cginc"struct appdata{float4 vertex : POSITION;};struct v2f{float4 pos : SV_POSITION;SHADOW_COORDS(x) //x表示第几个纹理};v2f vert (appdata v){v2f o;//...TRANSFER_SHADOW(o);return o;}fixed4 frag (v2f i) : SV_Target{//...UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);return fixed4((diffuse + specular) * atten, 1.0);}ENDCG}// 自己实现阴影，额外添加一个Pass，这个Pass是为了更新光源的阴影映射纹理//Pass//{//    Tags {"LightMode"="ShadowCaster"}//}}Fallback "Specular"
}

需要注意的细节

• 光源设置阴影类型，Hard Shadows或者Soft Shadows
• 物体的Mesh Renderer上设置Cast Shadows和Receive Shadoes，Plane或一些半透明物体需要设置双面投射阴影，即Cast Shadows设置为Two Sided
• 使用内置阴影需要在shader结尾加上 Fallback “Specular”，Specular的Fallback调用了VertexLit，VertexLit中有个Pass渲染光源的阴影映射纹理，或是摄像机的深度纹理。如果要自己实现阴影，需要额外写一个Pass，并把LightMode设置为ShadowCaster
• Base Pass计算平行光和环境光，申明编译指令#pragma multi_compile_fwdbase，Additional Pass计算其他类型的光源，如果点光源或聚光灯也要计算阴影，需要申明编译指令#pragma multi_compile_fwdadd_fullshadows 代替 #pragma multi_compile_fwdadd
• Additional Pass需要设置混合 Blend One One
• 引用内置文件 #include “AutoLight.cginc”，计算阴影时所用的宏都是在这个文件中声明的
• SHADOW_COORDS，TRANSFER_SHADOW和SHADOW_ATTENUATION这三个宏是计算阴影时的“三剑客”，SHADOW_COORDS就是声明一个用于对阴影纹理采样的坐标，TRANSFER_SHADOW将顶点从模型空间转换到光源空间后存储到_ShadowCoord中，SHADOW_ATTENUATION使用阴影坐标采样阴影贴图，也可以使用UNITY_LIGHT_ATTENUATION代替SHADOW_ATTENUATION，它用于计算光照衰减和阴影，接受3个参数，将光照衰减和阴影值相乘后的结果存储到第一个参数中，使用UNITY_LIGHT_ATTENUATION，使得Base Pass和Additional Pass的代码得以统一
• 这些宏中会使用上下文变量来进行计算，所以变量名称需要和宏中使用的名称一致，a2v（或appdata）结构体中的顶点坐标变量名必须是vertex，顶点着色器的输入结构体a2v（或appdata）必须命名为v，v2f中的顶点位置变量必须命名为pos

完整的Shader

Shader "MyCustom/BuildinShadow"
{Properties{_MainTex            ("Texture",            2D)             = "white" {}_Diffuse            ("Diffuse",            Color)          = (0.5, 0.5, 0.5, 1)_Specular           ("Specular",           Color)          = (1, 1, 1, 1)_SpecularPower      ("SpecularPower",      Range(0, 150))  = 20_SpecularIntensity  ("SpecularIntensity",  Range(0, 10))   = 1.5}SubShader{CGINCLUDEfloat _specular(float3 worldViewDir, float3 worldLightDir, float3 worldNormal, float power, float intensity){float3 halfVector = normalize(worldLightDir + worldViewDir);float hdotn = max(0, dot(halfVector, worldNormal));float specular = pow(hdotn, power) * intensity;return specular;}ENDCGTags { "RenderType"="Opaque" }LOD 100Pass{Tags {"LightMode"="ForwardBase"}CGPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile_fwdbase#include "UnityCG.cginc"#include "Lighting.cginc"#include "AutoLight.cginc"struct appdata{float4 vertex : POSITION;float3 normal : NORMAL;float2 uv     : TEXCOORD0;};struct v2f{float4 pos          : SV_POSITION;float2 uv           : TEXCOORD0;float3 worldNormal  : TEXCOORD1;float3 worldPos     : TEXCOORD2;SHADOW_COORDS(3)};sampler2D _MainTex;float4 _MainTex_ST;float4 _Diffuse;float4 _Specular;float _SpecularPower;float _SpecularIntensity;v2f vert (appdata v){v2f o;o.pos = UnityObjectToClipPos(v.vertex);o.uv = TRANSFORM_TEX(v.uv, _MainTex);o.worldNormal = UnityObjectToWorldNormal(v.normal);o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;TRANSFER_SHADOW(o);return o;}fixed4 frag (v2f i) : SV_Target{float3 worldLightDir = UnityWorldSpaceLightDir(i.worldPos);float3 worldViewDir = normalize(UnityWorldSpaceViewDir(i.worldPos));fixed4 texColor = tex2D(_MainTex, i.uv);fixed3 albedo = texColor.rgb * _Diffuse.rgb;// 环境光float3 ambient = UNITY_LIGHTMODEL_AMBIENT.xyz * albedo;// 漫反射float3 diffuse = _LightColor0.rgb * albedo * max(0, dot(i.worldNormal, worldLightDir));// 高光float3 specular = _LightColor0.rgb * _Specular.rgb * _specular(worldViewDir, worldLightDir, i.worldNormal, _SpecularPower, _SpecularIntensity);// atten为阴影和衰减的乘积，UNITY_LIGHT_ATTENUATION针对不同类型的光源进行计算UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);float4 finalColor = 1;finalColor.rgb = ambient + (diffuse + specular) * atten;return finalColor;}ENDCG}Pass{Tags {"LightMode"="ForwardAdd"}Blend One OneCGPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile_fwdadd// 其他光源使用阴影，使用下面的指令替换掉 multi_compile_fwdadd// #pragma multi_compile_fwdadd_fullshadows#include "UnityCG.cginc"#include "Lighting.cginc"#include "AutoLight.cginc"struct appdata{float4 vertex : POSITION;float3 normal : NORMAL;float2 uv     : TEXCOORD0;};struct v2f{float4 vertex       : SV_POSITION;float2 uv           : TEXCOORD0;float3 worldNormal  : TEXCOORD1;float3 worldPos     : TEXCOORD2;SHADOW_COORDS(3)};sampler2D _MainTex;float4 _MainTex_ST;float4 _Diffuse;float4 _Specular;float _SpecularPower;float _SpecularIntensity;v2f vert (appdata v){v2f o;o.vertex = UnityObjectToClipPos(v.vertex);o.uv = TRANSFORM_TEX(v.uv, _MainTex);o.worldNormal = UnityObjectToWorldNormal(v.normal);o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;TRANSFER_SHADOW(o);return o;}fixed4 frag (v2f i) : SV_Target{float3 worldLightDir = UnityWorldSpaceLightDir(i.worldPos);float3 worldViewDir = normalize(UnityWorldSpaceViewDir(i.worldPos));// 漫反射float3 diffuse = _LightColor0.rgb * _Diffuse.rgb * max(0, dot(i.worldNormal, worldLightDir));// 高光float3 specular = _LightColor0.rgb * _Specular.rgb * _specular(worldViewDir, worldLightDir, i.worldNormal, _SpecularPower, _SpecularIntensity);UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);float4 finalColor = 1;finalColor.rgb = (diffuse + specular) * atten;return finalColor;}ENDCG}}Fallback "Specular"
}

通过Frame Debug观察阴影的渲染过程，UpdateDepthTexture，即更新摄像机的深度纹理；RenderShadowmap，即渲染得到平行光的阴影映射纹理；CollectShadows，即根据深度纹理和阴影映射纹理得到屏幕空间的阴影图；最后绘制渲染结果

透明物体的阴影

需要注意的细节

• Tags设置 “Queue”=“AlphaTest” ，“RenderType”=“TransparentCutout”
• Cull Off 关闭剔除
• FallBack “Transparent/Cutout/VertexLit” 它含有透明度测试功能的ShadowCaster Pass
• 为了保证命名统一，必须使用_Cutoff来命名透明度阈值
• Mesh Renderer组件中的Cast Shadows属性设置为Two Sided，强制Unity在计算阴影映射纹理时计算所有面的深度信息

完整的shader

Shader "MyCustom/AlphaTestShadow" 
{Properties {_MainTex ("Main Tex",	   2D)			 = "white" {}_Color   ("Color Color", Color)			 = (1, 1, 1, 1)_Cutoff  ("Alpha Cutoff",  Range(0, 1))  = 0.5}SubShader {Tags { "Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout" }Pass {Tags { "LightMode"="ForwardBase" }Cull OffCGPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile_fwdbase#include "Lighting.cginc"#include "AutoLight.cginc"sampler2D _MainTex;float4 _MainTex_ST;fixed4 _Color;fixed _Cutoff;struct a2v{float4 vertex	: POSITION;float3 normal	: NORMAL;float4 uv		: TEXCOORD0;};struct v2f{float4 pos			: SV_POSITION;float3 worldNormal  : TEXCOORD0;float3 worldPos		: TEXCOORD1;float2 uv			: TEXCOORD2;SHADOW_COORDS(3)};v2f vert(a2v v){v2f o;o.pos = UnityObjectToClipPos(v.vertex);o.worldNormal = UnityObjectToWorldNormal(v.normal);o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;o.uv = TRANSFORM_TEX(v.uv, _MainTex);TRANSFER_SHADOW(o);return o;}fixed4 frag(v2f i) : SV_Target{fixed3 worldLightDir = normalize(UnityWorldSpaceLightDir(i.worldPos));fixed4 texColor = tex2D(_MainTex, i.uv);clip(texColor.a - _Cutoff);fixed3 albedo = texColor.rgb * _Color.rgb;// 环境光fixed3 ambient = UNITY_LIGHTMODEL_AMBIENT.xyz * albedo;// 漫反射fixed3 diffuse = _LightColor0.rgb * albedo * max(0, dot(i.worldNormal, worldLightDir));UNITY_LIGHT_ATTENUATION(atten, i, i.worldPos);return fixed4(ambient + diffuse * atten, 1.0);}ENDCG}}FallBack "Transparent/Cutout/VertexLit"
}

参考

《Shader 入门精要》
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