UnrealEngineUWP/Engine/Shaders/IESLightProfilesCommon.usf

// Copyright 1998-2014 Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	IESLightProfiles.usf: Common shader code for IES light profiles
=============================================================================*/

// IES light profiles
Texture2D		IESTexture;
SamplerState	IESTextureSampler;

// Apply 1D IES light profile texture
float ComputeLightProfileMultiplier(float3 WorldPosition, float3 LightPosition, float3 LightDirection)
{
#if USE_IES_PROFILE
	float3 NegLightDirection = normalize(WorldPosition - LightPosition);

	// -1..1
	float DotProd = dot(NegLightDirection, LightDirection);
	// -PI..PI (this distortion could be put into the texture but not without quality loss or more memory)
	float Angle = asin(DotProd);
	// 0..1
	float NormAngle = Angle / PI + 0.5f;

	return Texture2DSampleLevel(IESTexture, IESTextureSampler, float2(NormAngle, 0), 0).r;
#else 
	return 1.0f;
#endif
}