Merge branch 'UpdateAreaLightImpl'

Assets/ScriptableRenderLoop/HDRenderLoop/Material/Lit/Lit.hlsl

 
     // Area light specific
     // UVs for sampling the LUTs
-    // TODO: Test with fastAcos
-    float theta = acos(dot(bsdfData.normalWS, V));
+    float theta = FastACos(dot(bsdfData.normalWS, V));
     // Scale and bias for the current precomputed table - the constant use here are the one that have been use when the table in LtcData.DisneyDiffuse.cs and LtcData.GGX.cs was use
     float2 uv = 0.0078125 + 0.984375 * float2(bsdfData.perceptualRoughness, theta * INV_HALF_PI);
 
         }
 
     #ifndef DIFFUSE_LAMBERT_BRDF
-        // TODO: verify that we do not need to multiply by PI.
         ltcValue *= preLightData.ltcDisneyDiffuseMagnitude;
     #endif
 
         ltcValue *= lightData.specularScale;
         specularLighting = fresnelTerm * lightData.color * ltcValue;
     }
-
-    // TODO: current area light code doesn't take into account artist attenuation radius!
-
 #endif
 }
 

Assets/ScriptableRenderLoop/ShaderLibrary/AreaLighting.hlsl

 float IntegrateEdge(float3 v1, float3 v2)
 {
     float cosTheta = dot(v1, v2);
-    // TODO: Explain the 0.9999 <= precision is important!
-    cosTheta = Clamp(cosTheta, -0.9999, 0.9999);
-
-    // TODO: Experiment with fastAcos
-    float theta = acos(cosTheta);
-    float res = cross(v1, v2).z * theta / sin(theta);
+    // Clamp to avoid artifacts. This particular constant gives the best results.
+    cosTheta    = Clamp(cosTheta, -0.9999, 0.9999);
+    float theta = FastACos(cosTheta);
+    float res   = cross(v1, v2).z * theta / sin(theta);
 
     return res;
 }

Assets/ScriptableRenderLoop/ShaderLibrary/Common.hlsl

 
 #define MERGE_NAME(X, Y) X##Y
 
+// Acos in 14 cycles.
-    float res = -0.156583 * x + HALF_PI;
-    res *= sqrt(1.0 - x);
-    return (inX >= 0) ? res : PI - res;
+    float res = (0.0468878 * x + -0.203471) * x + 1.570796; // p(x)
+    res *= sqrt(1.0f - x);
+
+    return (inX >= 0) ? res : PI - res; // Undo range reduction
 }
 
 // Same cost as Acos + 1 FR