From c3f7e737d48a87fa87e1cc230383cb24f86a74a0 Mon Sep 17 00:00:00 2001
From: assiduous <assiduous@diligentgraphics.com>
Date: Sat, 21 Dec 2024 17:06:47 -0800
Subject: [PATCH] Added PBR-neutral tone mapping (close #247)

---
 Components/src/ToneMapping.cpp                |  4 +++-
 .../ToneMapping/public/ToneMapping.fxh        | 22 +++++++++++++++++++
 .../public/ToneMappingStructures.fxh          |  3 +++
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Components/src/ToneMapping.cpp b/Components/src/ToneMapping.cpp
index 57514841..a3251094 100644
--- a/Components/src/ToneMapping.cpp
+++ b/Components/src/ToneMapping.cpp
@@ -86,7 +86,7 @@ bool ToneMappingUpdateUI(HLSL::ToneMappingAttribs& Attribs, float* AverageLogLum
 {
     bool AttribsChanged = false;
     {
-        std::array<const char*, 10> ToneMappingMode{};
+        std::array<const char*, TONE_MAPPING_MODE_COUNT> ToneMappingMode{};
         ToneMappingMode[TONE_MAPPING_MODE_NONE]         = "None";
         ToneMappingMode[TONE_MAPPING_MODE_EXP]          = "Exp";
         ToneMappingMode[TONE_MAPPING_MODE_REINHARD]     = "Reinhard";
@@ -97,6 +97,8 @@ bool ToneMappingUpdateUI(HLSL::ToneMappingAttribs& Attribs, float* AverageLogLum
         ToneMappingMode[TONE_MAPPING_ADAPTIVE_LOG]      = "Adaptive log";
         ToneMappingMode[TONE_MAPPING_AGX]               = "AgX";
         ToneMappingMode[TONE_MAPPING_AGX_CUSTOM]        = "AgX Custom";
+        ToneMappingMode[TONE_MAPPING_PBR_NEUTRAL]       = "PBR Neutral";
+        static_assert(TONE_MAPPING_MODE_COUNT == 11, "Please update ToneMappingMode array");
         if (ImGui::Combo("Tone Mapping Mode", &Attribs.iToneMappingMode, ToneMappingMode.data(), static_cast<int>(ToneMappingMode.size())))
             AttribsChanged = true;
     }
diff --git a/Shaders/PostProcess/ToneMapping/public/ToneMapping.fxh b/Shaders/PostProcess/ToneMapping/public/ToneMapping.fxh
index 2c9276ba..168e8810 100644
--- a/Shaders/PostProcess/ToneMapping/public/ToneMapping.fxh
+++ b/Shaders/PostProcess/ToneMapping/public/ToneMapping.fxh
@@ -170,6 +170,28 @@ float3 ToneMap(in float3 f3Color, ToneMappingAttribs Attribs, float fAveLogLum)
         f3ToneMappedColor = AgXEotf(f3ToneMappedColor);
         return f3ToneMappedColor;
     }
+#elif TONE_MAPPING_MODE == TONE_MAPPING_PBR_NEUTRAL
+    {
+        // https://modelviewer.dev/examples/tone-mapping
+        // https://github.com/KhronosGroup/ToneMapping/blob/main/PBR_Neutral/README.md#pbr-neutral-specification
+        float F90 = 0.04;      // Fresnel reflection at normal incidence
+        float Ks  = 0.8 - F90; // Highlight compression start
+        float Kd  = 0.15;      // Speed of desaturation
+        
+        float x      = min(f3Color.r, min(f3Color.g, f3Color.b));
+        float Offset = (x <= 2.0 * F90) ? (x - x * x / (4.0 * F90)) : F90;
+        float3 f3ToneMappedColor = f3Color - float3(Offset, Offset, Offset);
+
+        float Peak = max(f3ToneMappedColor.r, max(f3ToneMappedColor.g, f3ToneMappedColor.b));
+        if (Peak > Ks)
+        {
+            float NewPeak = 1.0 - (1.0 - Ks) * (1.0 - Ks) / (Peak + 1.0 - 2.0 * Ks);
+            f3ToneMappedColor *= NewPeak / Peak;
+            float g = 1.0 / (Kd * (Peak - NewPeak) + 1.0);
+            f3ToneMappedColor = lerp(float3(NewPeak, NewPeak, NewPeak), f3ToneMappedColor, g);
+        }
+        return f3ToneMappedColor; 
+    }
 #else
     {
         return f3Color;
diff --git a/Shaders/PostProcess/ToneMapping/public/ToneMappingStructures.fxh b/Shaders/PostProcess/ToneMapping/public/ToneMappingStructures.fxh
index e3fb47f2..e2922c66 100644
--- a/Shaders/PostProcess/ToneMapping/public/ToneMappingStructures.fxh
+++ b/Shaders/PostProcess/ToneMapping/public/ToneMappingStructures.fxh
@@ -18,6 +18,9 @@
 #define TONE_MAPPING_ADAPTIVE_LOG       7
 #define TONE_MAPPING_AGX                8
 #define TONE_MAPPING_AGX_CUSTOM         9
+#define TONE_MAPPING_PBR_NEUTRAL       10
+#define TONE_MAPPING_MODE_COUNT        11
+
 
 struct AgXAttribs
 {