Unity Standard shader 裡面 全局光照Global Illumination（GI）
Standard 粗略的來看，其實分為兩個部分，一個是真正的BRDF，第二部分是UnityGI。
全局光照是在局部光照的基礎上，增加考慮物體與物體之間光線互動。是以說如果局部光照系統就是由光源+待渲染物體+視點組成的話，那麼全局光照系統就是由光源+各待渲染物體之間的反射光+待渲染物體+視點組成。
另外如果沒有全局光照技術，這些自發光的表面并不會真的着涼周圍的物體，而是它本身看起來更亮了而已。
##Unity GI
half4 fragForwardBaseInternal (VertexOutputForwardBase i)
{
FRAGMENT_SETUP(s)
UNITY_SETUP_INSTANCE_ID(i);
UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
UnityLight mainLight = MainLight ();
UNITY_LIGHT_ATTENUATION(atten, i, s.posWorld);
half occlusion = Occlusion(i.tex.xy);
UnityGI gi = FragmentGI (s, occlusion, i.ambientOrLightmapUV, atten, mainLight);
half4 c = UNITY_BRDF_PBS (s.diffColor, s.specColor, s.oneMinusReflectivity, s.smoothness, s.normalWorld, -s.eyeVec, gi.light, gi.indirect);
c.rgb += Emission(i.tex.xy);
UNITY_APPLY_FOG(i.fogCoord, c.rgb);
return OutputForward (c, s.alpha);
}
在UnityStandardCore.cginc裡面，簡單的概括 color = FragmentGI + UNITY_BRDF_PBS +Emission；
GI主要在UnityGI，另外還有在UNITY_BRDF_PBS 引用到。
FragmentGI 函數 計算global illumination，傳回 UnityGI
先看下 UnityGI，這個是個結構體。
在 UnityLightingCommon.cginc裡面有定義
struct UnityGI
{
UnityLight light;
UnityIndirect indirect;
};
在UnityStandardCore.cginc 裡面有四處定義了FragmentGI函數，最後還是下面的代碼處理傳回UnityGI
inline UnityGI FragmentGI (FragmentCommonData s, half occlusion, half4 i_ambientOrLightmapUV, half atten, UnityLight light, bool reflections)
{
UnityGIInput d;
d.light = light;
d.worldPos = s.posWorld;
d.worldViewDir = -s.eyeVec;
d.atten = atten;
#if defined(LIGHTMAP_ON) || defined(DYNAMICLIGHTMAP_ON)
d.ambient = 0;
d.lightmapUV = i_ambientOrLightmapUV;
#else
d.ambient = i_ambientOrLightmapUV.rgb;
d.lightmapUV = 0;
#endif
d.probeHDR[0] = unity_SpecCube0_HDR;
d.probeHDR[1] = unity_SpecCube1_HDR;
#if defined(UNITY_SPECCUBE_BLENDING) || defined(UNITY_SPECCUBE_BOX_PROJECTION)
d.boxMin[0] = unity_SpecCube0_BoxMin; // .w holds lerp value for blending
#endif
#ifdef UNITY_SPECCUBE_BOX_PROJECTION
d.boxMax[0] = unity_SpecCube0_BoxMax;
d.probePosition[0] = unity_SpecCube0_ProbePosition;
d.boxMax[1] = unity_SpecCube1_BoxMax;
d.boxMin[1] = unity_SpecCube1_BoxMin;
d.probePosition[1] = unity_SpecCube1_ProbePosition;
#endif
if(reflections)
{
Unity_GlossyEnvironmentData g = UnityGlossyEnvironmentSetup(s.smoothness, -s.eyeVec, s.normalWorld, s.specColor);
// Replace the reflUVW if it has been compute in Vertex shader. Note: the compiler will optimize the calcul in UnityGlossyEnvironmentSetup itself
#if UNITY_STANDARD_SIMPLE
g.reflUVW = s.reflUVW;
#endif
return UnityGlobalIllumination (d, occlusion, s.normalWorld, g);
}
else
{
return UnityGlobalIllumination (d, occlusion, s.normalWorld);
}
}
FragmentGI函數處理根據UnityGIInput，其中UnityGIInput也是結構體。開始時候對UnityGIInput進行指派。即是燈光+世界空間頂點坐标+觀察方向（視線的反方向）+衰減直接指派即可。随後是光照貼圖，在啟用了靜态光照貼圖或者動态光照貼圖的情況下，環境光為0，然後獲得光照貼圖的UV。否則的話，ambient直接使用VertexGIForward計算的rgb值。
後面是對反射探針的計算。
對是否反射調用合适 UnityGlobalIllumination函數。
struct UnityGIInput
{
UnityLight light; // pixel light, sent from the engine
float3 worldPos;
half3 worldViewDir;
half atten;
half3 ambient;
// interpolated lightmap UVs are passed as full float precision data to fragment shaders
// so lightmapUV (which is used as a tmp inside of lightmap fragment shaders) should
// also be full float precision to avoid data loss before sampling a texture.
float4 lightmapUV; // .xy = static lightmap UV, .zw = dynamic lightmap UV
#if defined(UNITY_SPECCUBE_BLENDING) || defined(UNITY_SPECCUBE_BOX_PROJECTION)
float4 boxMin[2];
#endif
#ifdef UNITY_SPECCUBE_BOX_PROJECTION
float4 boxMax[2];
float4 probePosition[2];
#endif
// HDR cubemap properties, use to decompress HDR texture
float4 probeHDR[2];
};
UnityGlobalIllumination 真正計算GlobalIllumination的函數
在UnityGlobalIllumination.cginc檔案裡面有四個UnityGlobalIllumination函數，主要是下面的代碼。
inline UnityGI UnityGlobalIllumination (UnityGIInput data, half occlusion, half3 normalWorld, Unity_GlossyEnvironmentData glossIn)
{
UnityGI o_gi = UnityGI_Base(data, occlusion, normalWorld);
o_gi.indirect.specular = UnityGI_IndirectSpecular(data, occlusion, glossIn);
return o_gi;
}
UnityGI_Base函數
inline UnityGI UnityGI_Base(UnityGIInput data, half occlusion, half3 normalWorld)
{
UnityGI o_gi;
ResetUnityGI(o_gi);
// Base pass with Lightmap support is responsible for handling ShadowMask / blending here for performance reason
#if defined(HANDLE_SHADOWS_BLENDING_IN_GI)
half bakedAtten = UnitySampleBakedOcclusion(data.lightmapUV.xy, data.worldPos);
float zDist = dot(_WorldSpaceCameraPos - data.worldPos, UNITY_MATRIX_V[2].xyz);
float fadeDist = UnityComputeShadowFadeDistance(data.worldPos, zDist);
data.atten = UnityMixRealtimeAndBakedShadows(data.atten, bakedAtten, UnityComputeShadowFade(fadeDist));
#endif
o_gi.light = data.light;
o_gi.light.color *= data.atten;
#if UNITY_SHOULD_SAMPLE_SH
o_gi.indirect.diffuse = ShadeSHPerPixel (normalWorld, data.ambient, data.worldPos);
#endif
#if defined(LIGHTMAP_ON)
// Baked lightmaps
half4 bakedColorTex = UNITY_SAMPLE_TEX2D(unity_Lightmap, data.lightmapUV.xy);
half3 bakedColor = DecodeLightmap(bakedColorTex);
#ifdef DIRLIGHTMAP_COMBINED
fixed4 bakedDirTex = UNITY_SAMPLE_TEX2D_SAMPLER (unity_LightmapInd, unity_Lightmap, data.lightmapUV.xy);
o_gi.indirect.diffuse = DecodeDirectionalLightmap (bakedColor, bakedDirTex, normalWorld);
#if defined(LIGHTMAP_SHADOW_MIXING) && !defined(SHADOWS_SHADOWMASK) && defined(SHADOWS_SCREEN)
ResetUnityLight(o_gi.light);
o_gi.indirect.diffuse = SubtractMainLightWithRealtimeAttenuationFromLightmap (o_gi.indirect.diffuse, data.atten, bakedColorTex, normalWorld);
#endif
#else // not directional lightmap
o_gi.indirect.diffuse = bakedColor;
#if defined(LIGHTMAP_SHADOW_MIXING) && !defined(SHADOWS_SHADOWMASK) && defined(SHADOWS_SCREEN)
ResetUnityLight(o_gi.light);
o_gi.indirect.diffuse = SubtractMainLightWithRealtimeAttenuationFromLightmap(o_gi.indirect.diffuse, data.atten, bakedColorTex, normalWorld);
#endif
#endif
#endif
#ifdef DYNAMICLIGHTMAP_ON
// Dynamic lightmaps
fixed4 realtimeColorTex = UNITY_SAMPLE_TEX2D(unity_DynamicLightmap, data.lightmapUV.zw);
half3 realtimeColor = DecodeRealtimeLightmap (realtimeColorTex);
#ifdef DIRLIGHTMAP_COMBINED
half4 realtimeDirTex = UNITY_SAMPLE_TEX2D_SAMPLER(unity_DynamicDirectionality, unity_DynamicLightmap, data.lightmapUV.zw);
o_gi.indirect.diffuse += DecodeDirectionalLightmap (realtimeColor, realtimeDirTex, normalWorld);
#else
o_gi.indirect.diffuse += realtimeColor;
#endif
#endif
o_gi.indirect.diffuse *= occlusion;
return o_gi;
}
ShadowMask陰影的衰減（烘焙陰影和實時陰影混合），SH的計算，烘焙的lightmap，平行光與非平行光lightmap，動态lightmap。最終傳回UnityGI結構，該結構包含light，color，indirect.diffuse參數。
其中ShadowMask陰影遮罩是Unity5.6版本的新特性。
UnityGI_IndirectSpecular 函數 間接高光
inline half3 UnityGI_IndirectSpecular(UnityGIInput data, half occlusion, Unity_GlossyEnvironmentData glossIn)
{
half3 specular;
#ifdef UNITY_SPECCUBE_BOX_PROJECTION
// we will tweak reflUVW in glossIn directly (as we pass it to Unity_GlossyEnvironment twice for probe0 and probe1), so keep original to pass into BoxProjectedCubemapDirection
half3 originalReflUVW = glossIn.reflUVW;
glossIn.reflUVW = BoxProjectedCubemapDirection (originalReflUVW, data.worldPos, data.probePosition[0], data.boxMin[0], data.boxMax[0]);
#endif
#ifdef _GLOSSYREFLECTIONS_OFF
specular = unity_IndirectSpecColor.rgb;
#else
half3 env0 = Unity_GlossyEnvironment (UNITY_PASS_TEXCUBE(unity_SpecCube0), data.probeHDR[0], glossIn);
#ifdef UNITY_SPECCUBE_BLENDING
const float kBlendFactor = 0.99999;
float blendLerp = data.boxMin[0].w;
UNITY_BRANCH
if (blendLerp < kBlendFactor)
{
#ifdef UNITY_SPECCUBE_BOX_PROJECTION
glossIn.reflUVW = BoxProjectedCubemapDirection (originalReflUVW, data.worldPos, data.probePosition[1], data.boxMin[1], data.boxMax[1]);
#endif
half3 env1 = Unity_GlossyEnvironment (UNITY_PASS_TEXCUBE_SAMPLER(unity_SpecCube1,unity_SpecCube0), data.probeHDR[1], glossIn);
specular = lerp(env1, env0, blendLerp);
}
else
{
specular = env0;
}
#else
specular = env0;
#endif
#endif
return specular * occlusion;
}
UnityGI_IndirectSpecular（UnityGLobalIllumination.cginc）計算間接高光，用probe相關的屬性計算。
通過調用Unity_GlossyEnvironment采樣Reflection Cube，計算HDR。
如果啟用了Box Projection，則通過BoxProjectedCubemapDirection計算變換後的方向。
UNITY_BRDF_PBS 裡面用到的GlobalIllumination
關于BRDF部分，來看下UnityStandardBRDF.cginc 裡面的 BRDF1_Unity_PBS函數
half3 color = diffColor * (gi.diffuse + light.color * diffuseTerm)
+ specularTerm * light.color * FresnelTerm (specColor, lh)
+ surfaceReduction * gi.specular * FresnelLerp (specColor, grazingTerm, nv);
light的顔色和 diffuse，以及specular 是GlobalIllumination傳進來的。
Image Based Lighting (IBL)
其中在材質上反應出周圍的環境也是PBS的重要組成部分。在光照模型中一般把周圍的環境當作一個大的光源來對待，不過環境光不同于實時光，而是作為間接光（indirect light）通過IBL（ Image Based Lighting）來實作。這裡也是優化Standard shader的一個比較重要的原因。
IBL一般通過環境光貼圖（environment map）來實作。Unity用reflection probe來儲存環境光貼圖，通過内置變量unity_SpecCube0，unity_SpecCube1通路。
IBL就是采樣兩次，用粗糙度做插值。這個地方可以做些優化。
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