Implemented Ashikhmin-Shirley diffuse term

cook-torrance/shaders/cookTorrance.frag

@@ -4,7 +4,7 @@
 #include "common/sRGB.h"
 #define D_BECKMANN
 #define G_COOK_TORRANCE
-#define F_DIFFUSE_NDL
+#define RD_ASHIKHMIN_SHIRLEY
 #include "brdf/cookTorrance.h"
 
 layout(constant_id = 0) const bool c_gammaCorrection = true;

framework/framework.vcxproj

@@ -138,6 +138,7 @@
     <ClInclude Include="debugOutputStream.h" />
     <ClInclude Include="graphicsApp.h" />
     <ClInclude Include="shaders\brdf\cookTorrance.h" />
+    <ClInclude Include="shaders\brdf\diffuse.h" />
     <ClInclude Include="shaders\brdf\distribution.h" />
     <ClInclude Include="shaders\brdf\geometric.h" />
     <ClInclude Include="shaders\brdf\phong.h" />

framework/framework.vcxproj.filters

@@ -135,6 +135,9 @@
     <ClInclude Include="shaders\common\luma.h">
       <Filter>Resource Files</Filter>
     </ClInclude>
+    <ClInclude Include="shaders\brdf\diffuse.h">
+      <Filter>Resource Files\brdf</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="arcball.cpp">

framework/shaders/brdf/cookTorrance.h

@@ -2,6 +2,7 @@
 #include "geometric.h"
 #include "distribution.h"
 #include "schlick.h"
+#include "diffuse.h"
 
 vec3 cookTorrance(vec3 n, vec3 l, vec3 v,
     vec3 f0, float roughness,
@@ -31,11 +32,10 @@ vec3 cookTorrance(vec3 n, vec3 l, vec3 v,
 #error geometric function not defined
 #endif
     vec3 spec = F * D * G/max(4. * NdL * NdV, 1e-6);
-#if defined(F_DIFFUSE_NDL)
-    vec3 Fd = fresnelSchlick(f0, NdL);
-#else // UE4 computes it as dot(h, v)
-    vec3 Fd = F;
+#if defined(RD_ASHIKHMIN_SHIRLEY)
+    vec3 diff = ashikhminShirleyDiffuse(albedo, luma709(f0), NdL, NdV);
+#else
+    vec3 diff = albedo/PI * (1. - luma709(F));
 #endif
-    vec3 diff =  albedo * (1. - luma709(Fd))/4.;
-    return Idiff * (max(NdL, 0.) * diff + spec);
+    return Idiff * max(NdL, 0.) * (diff + spec);
 }

framework/shaders/brdf/diffuse.h

@@ -0,0 +1,14 @@
+float halfcosPow5(float cosTheta)
+{
+    float x = 1. - cosTheta/2.;
+    float x_2 = x * x;
+    return x_2 * x_2 * x;
+}
+
+// https://pdfs.semanticscholar.org/808d/048d4331cf5dac5fd924b50249b7915c6d73.pdf
+vec3 ashikhminShirleyDiffuse(vec3 albedo, float f0, float NdL, float NdV)
+{
+    return 28./23. * albedo/PI * (1. - f0) *
+        (1. - halfcosPow5(NdL)) *
+        (1. - halfcosPow5(NdV));
+}