UnrealEngineUWP/Engine/Shaders/Private/ThinTranslucentCommon.ush

// Copyright Epic Games, Inc. All Rights Reserved.

#include "ShadingCommon.ush"

#pragma once

#define THIN_TRANSLUCENT_USE_DOTNV_THICKNESS 1

#define THIN_TRANSLUCENT_ACCURATE_FRESNEL 1

#if MATERIAL_SHADINGMODEL_THIN_TRANSLUCENT
void AccumulateThinTranslucentModel(inout float3 DualBlendColorAdd,
								inout float3 DualBlendColorMul,
								FMaterialPixelParameters MaterialParams,
								FGBufferData GBuffer,
								float3 DiffuseColor,
								float3 SpecularColor,
								float3 EmissiveColor,
								float TopMaterialCoverage)
{
	const float3 N = MaterialParams.WorldNormal;
	const float3 V = MaterialParams.CameraVector;

	const float NoV = saturate( abs( dot(N, V) ) + 1e-5 );

	// how much to multiply the background color by
	float3 Transmittance = float3(1.0,1.0,1.0);

	// how much to add for foreground color
	float3 SurfaceColor = float3(0.0f,0.0f,0.0f);

	float3 TransmittanceColor = GetThinTranslucentMaterialOutput0(MaterialParams);
	
	// color is for normalized thickness
#if THIN_TRANSLUCENT_USE_DOTNV_THICKNESS
	float PathLength = 1.0f/NoV;
#else
	float PathLength = 1.0f;
#endif	
	float3 NegativeAbsorptionCoefficient = log(TransmittanceColor);
	float3 ColorTopMaterialCoverage = exp(NegativeAbsorptionCoefficient * PathLength);

	// Light goes from background -> solid surface -> camera, and we need fresnel at both interactions.
	const float3 FresnelRatio = F_Schlick(GBuffer.SpecularColor, NoV);

	// FresnelRatio light is reflected back into the background, and the rest refracts into the surface.
	Transmittance = Transmittance * (1-FresnelRatio);

	// Light gets lost from absorption through the surface
	Transmittance = Transmittance * ColorTopMaterialCoverage;

	// Exiting the surface, FresnelRatio light is reflected back into the surface, and the rest refracts out of the surface.
#if THIN_TRANSLUCENT_ACCURATE_FRESNEL
	// Accurate calculation
	const float F0 = dot(float3(1.0f,1.0f,1.0f)/3.0f,GBuffer.SpecularColor.rgb); // air to glass F0
	const float NIor = DielectricF0ToIor(F0);
	const float ReverseNIor = 1.0/NIor; // glass to air IOR
	const float ReverseF0 = DielectricIorToF0(ReverseNIor); // glass to air F0
	const float3 ReverseFresnelRatio = F_Schlick(ReverseF0, NoV);
	Transmittance = Transmittance * (1-ReverseFresnelRatio);
#else
	// Approximation, assumes same amount refracts from air->material as material->air.
	Transmittance = Transmittance * (1-FresnelRatio);
#endif
	// We are treating the BaseColor and Emissive color as a layer on top of the absorbing media, but below specular layer.
	float3 DefaultLitColor = DiffuseColor + EmissiveColor;

	SurfaceColor += DefaultLitColor * TopMaterialCoverage;
	Transmittance *= (1.0f - TopMaterialCoverage);

	SurfaceColor += SpecularColor;

	DualBlendColorAdd = SurfaceColor;
	DualBlendColorMul = Transmittance;
}
#endif