[IES] Use photometric profile to describe light intensity distribution.

[roeas]

Moving Frostbite to Physically Based Rendering 3.0. Photometric lights

When creating a new light profile, the spherical photometric function is reconstructed and sampled to fill a 2D texture with a spherical parametrization (θ, cos(φ)). We store normalized values scaled by the inverse of the maximum intensity in order to handle both masked and unmasked usage.

// Normalize the profile by its maximum intensity
float getIESProfileAttenuation(float3 L, ShadowLightInfo light)
{
    // Sample direction into light space
    float3 iesSampleDirection = mul(light. worldToLight, -L);

    // Cartesian to spherical
    // Texture encoded with cos( phi ), scale from -1 - >1 to 0 - >1
    float phiCoord = (iesSampleDirection.z * 0.5f) + 0.5f;
    float theta = atan2 (iesSampleDirection.y, iesSampleDirection.x);
    float thetaCoord = theta * FB_INV_TWO_PI ;
    float3 texCoord = float3(thetaCoord, phiCoord, light. lightIndex);
    float iesProfileScale = iesTexture.SampleLevel(sampler, texCoord, 0).r;

    return iesProfileScale ;
}

...

att *= getAngleAtt(L, lightForward, lightAngleScale, lightAngleOffset);
att *= getIESProfileAttenuation(L, light);
// lightColor depends on option.
// Non masked : lightColor = color * MaxCandelas
// Masked(for point light with luminous power): lightColor = color * phi / (4 * PI)
float3 luminance = BSDF(...) * saturate(dot(N, L)) * lightColor * att;

[roeas]

https://docs.unrealengine.com/5.0/en-US/using-ies-light-profiles-in-unreal-engine/

[roeas]

We can get result like these by ies feature.