Epipolar scattering FX: added Ozone approximation that fixes yellow-green sky

1 files changed, 17 insertions, 0 deletions

diff --git a/PostProcess/EpipolarLightScattering/src/EpipolarLightScattering.cpp b/PostProcess/EpipolarLightScattering/src/EpipolarLightScattering.cpp
index 396f267..74ae2de 100644
--- a/PostProcess/EpipolarLightScattering/src/EpipolarLightScattering.cpp
+++ b/PostProcess/EpipolarLightScattering/src/EpipolarLightScattering.cpp
@@ -2231,6 +2231,7 @@ void EpipolarLightScattering::PrepareForNewFrame(FrameAttribs&
 
     // clang-format off
     bool bRecomputeSctrCoeffs = m_PostProcessingAttribs.bUseCustomSctrCoeffs    != PPAttribs.bUseCustomSctrCoeffs    ||
+                                m_PostProcessingAttribs.bUseOzoneApproximation  != PPAttribs.bUseOzoneApproximation  ||
                                 m_PostProcessingAttribs.fAerosolDensityScale    != PPAttribs.fAerosolDensityScale    ||
                                 m_PostProcessingAttribs.fAerosolAbsorbtionScale != PPAttribs.fAerosolAbsorbtionScale ||
                                 (PPAttribs.bUseCustomSctrCoeffs && 
@@ -2623,6 +2624,22 @@ void EpipolarLightScattering::ComputeScatteringCoefficients(IDeviceContext* pDev
         f4RayleighExtinctionCoeff = f4TotalRayleighSctrCoeff;
     }
 
+    if (m_PostProcessingAttribs.bUseOzoneApproximation && !m_PostProcessingAttribs.bUseCustomSctrCoeffs)
+    {
+        // As noted in [1], taking into account ozone particle absorption is essential to reproduce
+        // the blue of the zenith sky. Without ozone, the sky can appear too yellow overall, especially
+        // at sunset/sunrise. Ozone should be concentrated 32km up in the sky, however the author suggests
+        // to use the same distribution as the Rayleigh particle distribution, which allows simply adding
+        // ozone absorption to Rayleigh absorption.
+        //
+        // 1. Physically Based Sky, Atmosphere and Cloud Rendering in Frostbite, Sebastien Hillaire
+        //    (Physically-Based Shading in Theory and Practice, Siggraph 2016)
+
+        // Add Ozone absorption coefficients from [1] to total Rayleigh extinction coefficient
+        const float4 f4OzoneAbsorption = float4{3.426f, 8.298f, 0.356f, 0.f} * 6e-7f;
+        m_MediaParams.f4RayleighExtinctionCoeff += f4OzoneAbsorption;
+    }
+
     // Calculate angular and total scattering coefficients for Mie scattering:
     {
         float4& f4AngularMieSctrCoeff = m_MediaParams.f4AngularMieSctrCoeff;