From 179bf03cafb024f5e9c1c035b6071ffaa30d9be0 Mon Sep 17 00:00:00 2001
From: Mathias Agopian <mathias@google.com>
Date: Mon, 25 Nov 2019 12:58:54 -0800
Subject: [PATCH] implement absorption

---
 samples/materials/sandboxLit.mat |  2 ++
 shaders/src/common_lighting.fs   |  5 ++++-
 shaders/src/light_indirect.fs    | 28 ++++++++++++++++++++++------
 shaders/src/material_inputs.fs   |  8 ++++++--
 shaders/src/shading_lit.fs       |  8 ++++++++
 5 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/samples/materials/sandboxLit.mat b/samples/materials/sandboxLit.mat
index abac5d3d74..0bf58f75a7 100644
--- a/samples/materials/sandboxLit.mat
+++ b/samples/materials/sandboxLit.mat
@@ -38,6 +38,8 @@ material {
 fragment {
     void material(inout MaterialInputs material) {
         prepareMaterial(material);
+        //material.absorption = -log(vec3(1.0, 0.7, 0.1)) / 4.0;
+        //material.thickness = 1.0;
         material.baseColor.rgb = materialParams.baseColor;
         material.roughness = materialParams.roughness;
         material.metallic = materialParams.metallic;
diff --git a/shaders/src/common_lighting.fs b/shaders/src/common_lighting.fs
index 259b703f64..5346ddc513 100644
--- a/shaders/src/common_lighting.fs
+++ b/shaders/src/common_lighting.fs
@@ -25,8 +25,10 @@ struct PixelParams {
     float anisotropy;
 #endif
 
-#if defined(SHADING_MODEL_SUBSURFACE)
+#if defined(SHADING_MODEL_SUBSURFACE) || defined(HAS_REFRACTION)
     float thickness;
+#endif
+#if defined(SHADING_MODEL_SUBSURFACE)
     vec3  subsurfaceColor;
     float subsurfacePower;
 #endif
@@ -38,6 +40,7 @@ struct PixelParams {
 #if defined(HAS_REFRACTION)
     float eta;
     float transmission;
+    vec3 absorption;
 #endif
 };
 
diff --git a/shaders/src/light_indirect.fs b/shaders/src/light_indirect.fs
index 1f138df766..10d8d21179 100644
--- a/shaders/src/light_indirect.fs
+++ b/shaders/src/light_indirect.fs
@@ -333,9 +333,13 @@ void evaluateSubsurfaceIBL(const PixelParams pixel, const vec3 diffuseIrradiance
 }
 
 #if defined(HAS_REFRACTION)
-void applyRefraction(const PixelParams pixel, const vec3 n, vec3 Fd, vec3 Fr, inout vec3 color) {
-    float eta = pixel.eta;
-    vec3 v = -shading_view;
+void applyRefraction(const PixelParams pixel, const vec3 n0, vec3 Fd, vec3 Fr, inout vec3 color) {
+    float eta0 = pixel.eta;
+    vec3 r = -shading_view;
+
+#if defined(MATERIAL_HAS_ABSORPTION) && !defined(MATERIAL_HAS_THICKNESS)
+    vec3 T = 1.0 - pixel.absorption;
+#endif
 
     // for a sphere, with local refraction (i.e. screen space)
     //const float thickness = 1.0;
@@ -354,9 +358,16 @@ void applyRefraction(const PixelParams pixel, const vec3 n, vec3 Fd, vec3 Fr, in
     //vec3 p = d * r;   // sample screen space here
 
     // for a sphere, with light at infinity (i.e. cubemap)
-    vec3 r = refract(v, n, eta);
-    vec3 n2 = -normalize(dot(-n, r) * r + n * 0.5);
-    r = refract(r, n2, 1.0 / eta);
+    r = refract(r, n0, eta0);
+
+#if defined(MATERIAL_HAS_ABSORPTION) && defined(MATERIAL_HAS_THICKNESS)
+    float d = pixel.thickness * dot(-n0, r);
+    vec3 T = exp(-pixel.absorption * d);
+#endif
+
+    vec3 n1 = -normalize(dot(-n0, r) * r + n0 * 0.5);
+    float eta1 = 1.0 / eta0;
+    r = refract(r, n1, eta1);
 
     // for a slab, with light at infinity (i.e. cubemap)
     //      r = refract(v, n, eta);
@@ -366,6 +377,11 @@ void applyRefraction(const PixelParams pixel, const vec3 n, vec3 Fd, vec3 Fr, in
     // when reading from the cubemap, we are not pre-exposed so we apply iblLuminance
     // which is not the case when we'll read from the screen-space buffer
     vec3 Ft = prefilteredRadiance(r, pixel.perceptualRoughness) * frameUniforms.iblLuminance;
+
+#if defined(MATERIAL_HAS_ABSORPTION)
+    Ft *= T;
+#endif
+
     Fr *= frameUniforms.iblLuminance;
     Fd *= frameUniforms.iblLuminance;
 
diff --git a/shaders/src/material_inputs.fs b/shaders/src/material_inputs.fs
index 58e441d6f9..b1a3907816 100644
--- a/shaders/src/material_inputs.fs
+++ b/shaders/src/material_inputs.fs
@@ -29,8 +29,10 @@ struct MaterialInputs {
     float anisotropy;
     vec3  anisotropyDirection;
 
-#if defined(SHADING_MODEL_SUBSURFACE)
+#if defined(SHADING_MODEL_SUBSURFACE) || defined(HAS_REFRACTION)
     float thickness;
+#endif
+#if defined(SHADING_MODEL_SUBSURFACE)
     float subsurfacePower;
     vec3  subsurfaceColor;
 #endif
@@ -92,8 +94,10 @@ void initMaterial(out MaterialInputs material) {
     material.anisotropyDirection = vec3(1.0, 0.0, 0.0);
 #endif
 
-#if defined(SHADING_MODEL_SUBSURFACE)
+#if defined(SHADING_MODEL_SUBSURFACE) || defined(HAS_REFRACTION)
     material.thickness = 0.5;
+#endif
+#if defined(SHADING_MODEL_SUBSURFACE)
     material.subsurfacePower = 12.234;
     material.subsurfaceColor = vec3(1.0);
 #endif
diff --git a/shaders/src/shading_lit.fs b/shaders/src/shading_lit.fs
index 266f3ed962..878c102d2c 100644
--- a/shaders/src/shading_lit.fs
+++ b/shaders/src/shading_lit.fs
@@ -91,6 +91,14 @@ void getCommonPixelParams(const MaterialInputs material, inout PixelParams pixel
 #else
     pixel.transmission = 1.0;
 #endif
+#if defined(MATERIAL_HAS_ABSORPTION)
+    pixel.absorption = saturate(material.absorption);
+#else
+    pixel.absorption = vec3(0.0);
+#endif
+#if defined(MATERIAL_HAS_THICKNESS)
+    pixel.thickness = max(0.0, material.thickness);
+#endif
 #endif
 }