UE4 灯光shader:SpecularGGX记录

方便自己看看ShadingModels.ush

float3 SpecularGGX( float Roughness, float3 SpecularColor, BxDFContext Context, float NoL, FAreaLight AreaLight )
{
	float a2 = Pow4( Roughness );
	float Energy = EnergyNormalization( a2, Context.VoH, AreaLight );
	
	// Generalized microfacet specular
	float D = D_GGX( a2, Context.NoH ) * Energy;
	float Vis = Vis_SmithJointApprox( a2, Context.NoV, NoL );
	float3 F = F_Schlick( SpecularColor, Context.VoH );

	return (D * Vis) * F;
}

float EnergyNormalization( inout float a2, float VoH, FAreaLight AreaLight )
{
	if( AreaLight.SphereSinAlphaSoft > 0 )
	{
		// Modify Roughness
		a2 = saturate( a2 + Pow2( AreaLight.SphereSinAlphaSoft ) / ( VoH * 3.6 + 0.4 ) );
	}

	float Sphere_a2 = a2;
	float Energy = 1;
	if( AreaLight.SphereSinAlpha > 0 )
	{
		Sphere_a2 = New_a2( a2, AreaLight.SphereSinAlpha, VoH );
		Energy = a2 / Sphere_a2;
	}

	if( AreaLight.LineCosSubtended < 1 )
	{
#if 1
		float LineCosTwoAlpha = AreaLight.LineCosSubtended;
		float LineTanAlpha = sqrt( ( 1.0001 - LineCosTwoAlpha ) / ( 1 + LineCosTwoAlpha ) );
		float Line_a2 = New_a2( Sphere_a2, LineTanAlpha, VoH );
		Energy *= sqrt( Sphere_a2 / Line_a2 );
#else
		float LineCosTwoAlpha = AreaLight.LineCosSubtended;
		float LineSinAlpha = sqrt( 0.5 - 0.5 * LineCosTwoAlpha );
		float Line_a2 = New_a2( Sphere_a2, LineSinAlpha, VoH );
		Energy *= Sphere_a2 / Line_a2;
#endif
	}

	return Energy;
}

// GGX / Trowbridge-Reitz
// [Walter et al. 2007, "Microfacet models for refraction through rough surfaces"]
float D_GGX( float a2, float NoH )
{
	float d = ( NoH * a2 - NoH ) * NoH + 1;	// 2 mad
	return a2 / ( PI*d*d );					// 4 mul, 1 rcp
}

// Appoximation of joint Smith term for GGX
// [Heitz 2014, "Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs"]
float Vis_SmithJointApprox( float a2, float NoV, float NoL )
{
	float a = sqrt(a2);
	float Vis_SmithV = NoL * ( NoV * ( 1 - a ) + a );
	float Vis_SmithL = NoV * ( NoL * ( 1 - a ) + a );
	return 0.5 * rcp( Vis_SmithV + Vis_SmithL );
}

// [Schlick 1994, "An Inexpensive BRDF Model for Physically-Based Rendering"]
float3 F_Schlick( float3 SpecularColor, float VoH )
{
	float Fc = Pow5( 1 - VoH );					// 1 sub, 3 mul
	//return Fc + (1 - Fc) * SpecularColor;		// 1 add, 3 mad
	
	// Anything less than 2% is physically impossible and is instead considered to be shadowing
	return saturate( 50.0 * SpecularColor.g ) * Fc + (1 - Fc) * SpecularColor;
	
}