UnrealEngineUWP/Engine/Shaders/DeferredLightingCommon.usf

// Copyright 1998-2015 Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	DeferredLightingCommon.usf: Common definitions for deferred lighting.
=============================================================================*/

#ifndef __DEFERRED_LIGHTING_COMMON__
#define __DEFERRED_LIGHTING_COMMON__

#include "DeferredShadingCommon.usf"
#include "DynamicLightingCommon.usf"
#include "BRDF.usf"
#include "MonteCarlo.usf"
#include "IESLightProfilesCommon.usf"
#include "ShadingModels.usf"

/** 
 * Data about a single light.
 * Putting the light data in this struct allows the same lighting code to be used between standard deferred, 
 * Where many light properties are known at compile time, and tiled deferred, where all light properties have to be fetched from a buffer.
 */
struct FDeferredLightData
{
	float4 LightPositionAndInvRadius;
	float4 LightColorAndFalloffExponent;
	float3 LightDirection;
	float4 SpotAnglesAndSourceRadius;
	float MinRoughness;
	float2 DistanceFadeMAD;
	float4 ShadowMapChannelMask;
	/** Whether to use inverse squared falloff. */
	bool bInverseSquared;
	/** Whether this is a light with radial attenuation, aka point or spot light. */
	bool bRadialLight;
	/** Whether this light needs spotlight attenuation. */
	bool bSpotLight;
	/** Whether the light should apply shadowing. */
	bool bShadowed;
};

/** Data about a single light to be shaded with the simple shading model, designed for speed and limited feature set. */
struct FSimpleDeferredLightData
{
	float4 LightPositionAndInvRadius;
	float4 LightColorAndFalloffExponent;
	/** Whether to use inverse squared falloff. */
	bool bInverseSquared;
};


#undef LIGHT_SOURCE_SHAPE
#define LIGHT_SOURCE_SHAPE 1

bool RayHitSphere( float3 R, float3 SphereCenter, float SphereRadius )
{
	float3 ClosestPointOnRay = max( 0, dot( SphereCenter, R ) ) * R;
	float3 CenterToRay = ClosestPointOnRay - SphereCenter;
	return dot( CenterToRay, CenterToRay ) <= Square( SphereRadius );
}

bool RayHitRect( float3 R, float3 RectCenter, float3 RectX, float3 RectY, float3 RectZ, float RectExtentX, float RectExtentY )
{
	// Intersect ray with plane
	float3 PointOnPlane = R * max( 0, dot( RectZ, RectCenter ) / dot( RectZ, R ) );

	bool InExtentX = abs( dot( RectX, PointOnPlane - RectCenter ) ) <= RectExtentX;
	bool InExtentY = abs( dot( RectY, PointOnPlane - RectCenter ) ) <= RectExtentY;
	return InExtentX && InExtentY;
}

float3 PointLightSpecularMIS( FScreenSpaceData ScreenSpaceData, FDeferredLightData LightData, float3 LightCenter, float3 V, float3 N, uint2 Random )
{
	FGBufferData GBuffer = ScreenSpaceData.GBuffer;
	float Roughness = GBuffer.Roughness;

	float NoV = saturate( dot( N, V ) );
	NoV = max( 0.001, NoV );
	
	const float SourceRadius = max( 1, LightData.SpotAnglesAndSourceRadius.z );

	const float DistanceSqr = dot( LightCenter, LightCenter );
	const float3 ConeAxis = normalize( LightCenter );
	const float ConeCos = sqrt( 1 - Square( SourceRadius ) / DistanceSqr );

	const float SampleColor = (1.0/PI) / Square(SourceRadius);

	float3 SpecularLighting = 0;

	const uint NumSamplesGGX = 16;
	const uint NumSamplesCone = 16;
	for( uint i = 0; i < NumSamplesGGX + NumSamplesCone; i++ )
	{
		bool bSampleBRDF = i < NumSamplesGGX;

		float3 L, H;
		if( bSampleBRDF )
		{
			float2 E = Hammersley( i, NumSamplesGGX, Random );
			H = TangentToWorld( ImportanceSampleGGX( E, Roughness ).xyz, N );
			L = 2 * dot( V, H ) * H - V;
		}
		else
		{
			float2 E = Hammersley( i, NumSamplesCone, Random );
			L = TangentToWorld( UniformSampleCone( E, ConeCos ).xyz, ConeAxis );
			H = normalize(V + L);
		}

		float NoL = saturate( dot(N, L) );
		float NoH = saturate( dot(N, H) );
		float VoH = saturate( dot(V, H) );
		
		if( NoL > 0 )
		{
			if( bSampleBRDF && !RayHitSphere( L, LightCenter, SourceRadius ) )
			{
				continue;
			}
			
			// Generalized microfacet specular
			float D = D_GGX( Roughness, NoH );
			float Vis = GeometricVisibility( Roughness, NoV, NoL, VoH );
			float3 F = Fresnel( GBuffer.SpecularColor, VoH );

			float ConePDF = 1.0 / ( 2 * PI * (1 - ConeCos) );
			float GGXPDF = D * NoH / (4 * VoH);

			if( bSampleBRDF )
			{
				float Weight = MISWeight( NumSamplesGGX, GGXPDF, NumSamplesCone, ConePDF );
				SpecularLighting += F * ( SampleColor * NoL * Vis * (4 * VoH / NoH) * Weight );
			}
			else
			{
				float Weight = MISWeight( NumSamplesCone, ConePDF, NumSamplesGGX, GGXPDF );
				SpecularLighting += F * ( SampleColor * NoL * Vis * D / ConePDF * Weight );
			}
		}
	}

	return SpecularLighting / (NumSamplesGGX + NumSamplesCone);
}

// Find representative incoming light direction and energy modification
float3 AreaLightSpecular( FDeferredLightData LightData, inout float3 LobeRoughness, inout float3 ToLight, inout float3 L, float3 V, half3 N )
{
	float3 LobeEnergy = 1;

	LobeRoughness = max( LobeRoughness, LightData.MinRoughness );
	float3 m = LobeRoughness * LobeRoughness;
	
	const float SourceRadius = LightData.SpotAnglesAndSourceRadius.z;
	const float SourceLength = LightData.SpotAnglesAndSourceRadius.w;

	// TODO early out for point lights
	
	float3 R = reflect( -V, N );
	float InvDistToLight = rsqrt( dot( ToLight, ToLight ) );

	BRANCH
	if( SourceLength > 0 )
	{
		// Energy conservation
		// asin(x) is angle to sphere, atan(x) is angle to disk, saturate(x) is free and in the middle
		float LineAngle = saturate( SourceLength * InvDistToLight );
		LobeEnergy *= m / saturate( m + 0.5 * LineAngle );

		// Closest point on line segment to ray
		float3 L01 = LightData.LightDirection * SourceLength;
		float3 L0 = ToLight - 0.5 * L01;
		float3 L1 = ToLight + 0.5 * L01;

#if 1
		// Shortest distance
		float a = Square( SourceLength );
		float b = dot( R, L01 );
		float t = saturate( dot( L0, b*R - L01 ) / (a - b*b) );
#else
		// Smallest angle
		float A = Square( SourceLength );
		float B = 2 * dot( L0, L01 );
		float C = dot( L0, L0 );
		float D = dot( R, L0 );
		float E = dot( R, L01 );
		float t = saturate( (B*D - 2*C*E) / (B*E - 2*A*D) );
#endif

		ToLight = L0 + t * L01;
	}

	BRANCH
	if( SourceRadius > 0 )
	{
		// Energy conservation
		// asin(x) is angle to sphere, atan(x) is angle to disk, saturate(x) is free and in the middle
		float SphereAngle = saturate( SourceRadius * InvDistToLight );
		LobeEnergy *= Square( m / saturate( m + 0.5 * SphereAngle ) );
		
		// Closest point on sphere to ray
		float3 ClosestPointOnRay = dot( ToLight, R ) * R;
		float3 CenterToRay = ClosestPointOnRay - ToLight;
		float3 ClosestPointOnSphere = ToLight + CenterToRay * saturate( SourceRadius * rsqrt( dot( CenterToRay, CenterToRay ) ) );
		ToLight = ClosestPointOnSphere;
	}

	L = normalize( ToLight );

	return LobeEnergy;
}

/** Returns 0 for positions closer than the fade near distance from the camera, and 1 for positions further than the fade far distance. */
float DistanceFromCameraFade(FScreenSpaceData ScreenSpaceData, FDeferredLightData LightData, float3 WorldPosition, float3 CameraPosition)
{
	// depth (non radial) based fading over distance
	float Fade = saturate(ScreenSpaceData.GBuffer.Depth * DeferredLightUniforms.DistanceFadeMAD.x + DeferredLightUniforms.DistanceFadeMAD.y);
	return Fade * Fade;
}

void GetShadowTerms(FScreenSpaceData ScreenSpaceData, FDeferredLightData LightData, float3 WorldPosition, float4 LightAttenuation, out float OpaqueShadowTerm, out float SSSShadowTerm)
{
	// Remapping the light attenuation buffer (see ShadowRendering.cpp)

	// LightAttenuation: Light function + per-object shadows in z, per-object SSS shadowing in w, 
	// Whole scene directional light shadows in x, whole scene directional light SSS shadows in y
	// Get static shadowing from the appropriate GBuffer channel
	float UsesStaticShadowMap = dot(LightData.ShadowMapChannelMask, float4(1, 1, 1, 1));
	float StaticShadowing = lerp(1, dot(ScreenSpaceData.GBuffer.PrecomputedShadowFactors, LightData.ShadowMapChannelMask), UsesStaticShadowMap);

	if (LightData.bRadialLight)
	{
		// Remapping the light attenuation buffer (see ShadowRendering.cpp)

		OpaqueShadowTerm = LightAttenuation.z * StaticShadowing;
		// SSS uses a separate shadowing term that allows light to penetrate the surface
		//@todo - how to do static shadowing of SSS correctly?
		SSSShadowTerm = LightAttenuation.w * StaticShadowing;
	}
	else
	{
		// Remapping the light attenuation buffer (see ShadowRendering.cpp)
		// Also fix up the fade between dynamic and static shadows
		// to work with plane splits rather than spheres.

		float DynamicShadowFraction = DistanceFromCameraFade(ScreenSpaceData, LightData, WorldPosition, View.ViewOrigin.xyz);
		// For a directional light, fade between static shadowing and the whole scene dynamic shadowing based on distance + per object shadows
		OpaqueShadowTerm = lerp(LightAttenuation.x, StaticShadowing, DynamicShadowFraction);
		// Fade between SSS dynamic shadowing and static shadowing based on distance
		SSSShadowTerm = min(lerp(LightAttenuation.y, StaticShadowing, DynamicShadowFraction), LightAttenuation.w);
		
		// combine with light function (MUL is correct, MIN would not be correct and likely to be slower)
		OpaqueShadowTerm *= LightAttenuation.z;
		SSSShadowTerm *= LightAttenuation.z;
	}
}

/** Calculates lighting for a given position, normal, etc with a fully featured lighting model designed for quality. */
float4 GetDynamicLighting(float3 WorldPosition, float3 CameraVector, float2 InUV, FScreenSpaceData ScreenSpaceData, uint ShadingModelID, FDeferredLightData LightData, float4 LightAttenuation, uint2 Random)
{
	FLightAccumulator LightAccumulator = (FLightAccumulator)0;

	float3 V = -CameraVector;
	float3 N = ScreenSpaceData.GBuffer.WorldNormal;
	float3 ToLight = LightData.LightDirection;
	float3 L = ToLight;	// no need to normalize
	float NoL = saturate( dot(N, L) );
	float DistanceAttenuation = 1;
	float LightRadiusMask = 1;
	float SpotFalloff = 1;

	if (LightData.bRadialLight)
	{
		ToLight = LightData.LightPositionAndInvRadius.xyz - WorldPosition;
		
		float DistanceSqr = dot( ToLight, ToLight );
		L = ToLight * rsqrt( DistanceSqr );

		if (LightData.bInverseSquared)
		{
			const float SourceLength = LightData.SpotAnglesAndSourceRadius.w;

			BRANCH
			if( SourceLength > 0 )
			{
				// Line segment irradiance
				float3 L01 = LightData.LightDirection * SourceLength;
				float3 L0 = ToLight - 0.5 * L01;
				float3 L1 = ToLight + 0.5 * L01;
				float LengthL0 = length( L0 );
				float LengthL1 = length( L1 );

				DistanceAttenuation = rcp( ( LengthL0 * LengthL1 + dot( L0, L1 ) ) * 0.5 + 1 );
				NoL = saturate( 0.5 * ( dot(N, L0) / LengthL0 + dot(N, L1) / LengthL1 ) );
			}
			else
			{
				// Sphere irradiance (technically just 1/d^2 but this avoids inf)
				DistanceAttenuation = 1 / ( DistanceSqr + 1 );
				NoL = saturate( dot( N, L ) );
			}

			// TODO optimize
			LightRadiusMask = Square( saturate( 1 - Square( DistanceSqr * Square(LightData.LightPositionAndInvRadius.w) ) ) );
		}
		else
		{
			DistanceAttenuation = 1;
			NoL = saturate( dot( N, L ) );
			
			LightRadiusMask = RadialAttenuation(ToLight * LightData.LightPositionAndInvRadius.w, LightData.LightColorAndFalloffExponent.w);	
		}

		if (LightData.bSpotLight)
		{
			SpotFalloff = SpotAttenuation(L, -LightData.LightDirection, LightData.SpotAnglesAndSourceRadius.xy);
		}
	}

	LightAccumulator.EstimatedCost += 0.3f;		// running the PixelShader at all has a cost

	BRANCH
	if (LightRadiusMask > 0 && SpotFalloff > 0)
	{
		float SurfaceShadow = 1;
		float SubsurfaceShadow = 1;

		BRANCH
		if (LightData.bShadowed)
		{
			GetShadowTerms(ScreenSpaceData, LightData, WorldPosition, LightAttenuation, SurfaceShadow, SubsurfaceShadow);

			// greatly reduces shadow mapping artifacts
			SurfaceShadow *= saturate(dot(N, L) * 6 - 0.2);
		}
		else
		{
			SurfaceShadow = ScreenSpaceData.AmbientOcclusion;
		}

		float SurfaceAttenuation	= (DistanceAttenuation * LightRadiusMask * SpotFalloff) * SurfaceShadow;
		float SubsurfaceAttenuation	= (DistanceAttenuation * LightRadiusMask * SpotFalloff) * SubsurfaceShadow;

		LightAccumulator.EstimatedCost += 0.3f;		// add the cost of getting the shadow terms

		{
			const float3 LightColor = LightData.LightColorAndFalloffExponent.rgb;

			const float ClearCoatRoughness	= ScreenSpaceData.GBuffer.CustomData.y;

			float3 LobeRoughness = float3(ClearCoatRoughness, ScreenSpaceData.GBuffer.Roughness, 1);
			float3 LobeEnergy = AreaLightSpecular(LightData, LobeRoughness, ToLight, L, V, N);

			// accumulate diffuse and specular
			{
#if 1	// for testing if there is a perf impact
				// correct screen space subsurface scattering
				float3 SurfaceLightingDiff = SurfaceShading(ScreenSpaceData.GBuffer, LobeRoughness, LobeEnergy, L, V, N, float2(1, 0));
				float3 SurfaceLightingSpec = SurfaceShading(ScreenSpaceData.GBuffer, LobeRoughness, LobeEnergy, L, V, N, float2(0, 1));
				LightAccumulator_Add(LightAccumulator, SurfaceLightingDiff, SurfaceLightingSpec, LightColor * (NoL * SurfaceAttenuation));
#else
				// wrong screen space subsurface scattering but potentially faster
				float3 SurfaceLighting = SurfaceShading(ScreenSpaceData.GBuffer, LobeRoughness, LobeEnergy, L, V, N, float2(1, 1));
				LightAccumulator_Add(LightAccumulator, SurfaceLighting, 0, LightColor * (NoL * SurfaceAttenuation));
#endif
			}

			// accumulate subsurface
			{
				float3 SubsurfaceLighting = SubsurfaceShading(ScreenSpaceData.GBuffer, L, V, N);

				LightAccumulator_Add(LightAccumulator, SubsurfaceLighting, 0, LightColor * SubsurfaceAttenuation);

				LightAccumulator.EstimatedCost += 0.4f;		// add the cost of the lighting computations (should sum up to 1 form one light)
			}
		}
	}

	return LightAccumulator_GetResult(LightAccumulator);
}

/** 
 * Calculates lighting for a given position, normal, etc with a simple lighting model designed for speed. 
 * All lights rendered through this method are unshadowed point lights with no shadowing or light function or IES.
 * A cheap specular is used instead of the more correct area specular, no fresnel.
 */
float3 GetSimpleDynamicLighting(float3 WorldPosition, float3 CameraVector, FScreenSpaceData ScreenSpaceData, FSimpleDeferredLightData LightData)
{
	float3 V = -CameraVector;
	float3 N = ScreenSpaceData.GBuffer.WorldNormal;
	float3 ToLight = LightData.LightPositionAndInvRadius.xyz - WorldPosition;
	float DistanceAttenuation = 1;
	
	float DistanceSqr = dot( ToLight, ToLight );
	float3 L = ToLight * rsqrt( DistanceSqr );
	float NoL = saturate( dot( N, L ) );

	if (LightData.bInverseSquared)
	{
		// Sphere falloff (technically just 1/d2 but this avoids inf)
		DistanceAttenuation = 1 / ( DistanceSqr + 1 );
	
		float LightRadiusMask = Square( saturate( 1 - Square( DistanceSqr * Square(LightData.LightPositionAndInvRadius.w) ) ) );
		DistanceAttenuation *= LightRadiusMask;
	}
	else
	{
		DistanceAttenuation = RadialAttenuation(ToLight * LightData.LightPositionAndInvRadius.w, LightData.LightColorAndFalloffExponent.w);
	}

	float3 OutLighting = 0;

	BRANCH
	if (DistanceAttenuation > 0)
	{
		const float3 LightColor = LightData.LightColorAndFalloffExponent.rgb;

		// Apply SSAO to the direct lighting since we're not going to have any other shadowing
		float Attenuation = DistanceAttenuation * ScreenSpaceData.AmbientOcclusion;

		OutLighting += LightColor * (NoL * Attenuation) * SimpleShading( ScreenSpaceData.GBuffer, ScreenSpaceData.GBuffer.Roughness, L, V, N );
	}

	return OutLighting;
}

#endif