Revert "Cycles: optimize ensure_valid_reflection(), reduces render time by about 1%"

Both before and after can have artifacts with some normal maps, but this seems to give
worse artifacts on average which are not worth the minor performance increase.

This reverts commit 21bc1a99baa765d81c3203fd2e451681b8a7fd55.

Ref T88368, D10084

1 files changed, 53 insertions, 16 deletions

diff --git a/intern/cycles/kernel/shaders/stdcycles.h b/intern/cycles/kernel/shaders/stdcycles.h
index af7b645d9a2..dd604da68ce 100644
--- a/intern/cycles/kernel/shaders/stdcycles.h
+++ b/intern/cycles/kernel/shaders/stdcycles.h
@@ -84,30 +84,67 @@ closure color principled_hair(normal N,
 closure color henyey_greenstein(float g) BUILTIN;
 closure color absorption() BUILTIN;
 
-normal ensure_valid_reflection(normal Ng, normal I, normal N)
+normal ensure_valid_reflection(normal Ng, vector I, normal N)
 {
   /* The implementation here mirrors the one in kernel_montecarlo.h,
    * check there for an explanation of the algorithm. */
-  vector R;
-  float NI = dot(N, I);
-  float NgR, threshold;
-
-  if (NI > 0) {
-    R = (2 * NI) * N - I;
-    NgR = dot(Ng, R);
-    threshold = min(0.9 * dot(Ng, I), 0.01);
-    if (NgR >= threshold) {
-      return N;
+
+  float sqr(float x)
+  {
+    return x * x;
+  }
+
+  vector R = 2 * dot(N, I) * N - I;
+
+  float threshold = min(0.9 * dot(Ng, I), 0.01);
+  if (dot(Ng, R) >= threshold) {
+    return N;
+  }
+
+  float NdotNg = dot(N, Ng);
+  vector X = normalize(N - NdotNg * Ng);
+
+  float Ix = dot(I, X), Iz = dot(I, Ng);
+  float Ix2 = sqr(Ix), Iz2 = sqr(Iz);
+  float a = Ix2 + Iz2;
+
+  float b = sqrt(Ix2 * (a - sqr(threshold)));
+  float c = Iz * threshold + a;
+
+  float fac = 0.5 / a;
+  float N1_z2 = fac * (b + c), N2_z2 = fac * (-b + c);
+  int valid1 = (N1_z2 > 1e-5) && (N1_z2 <= (1.0 + 1e-5));
+  int valid2 = (N2_z2 > 1e-5) && (N2_z2 <= (1.0 + 1e-5));
+
+  float N_new_x, N_new_z;
+  if (valid1 && valid2) {
+    float N1_x = sqrt(1.0 - N1_z2), N1_z = sqrt(N1_z2);
+    float N2_x = sqrt(1.0 - N2_z2), N2_z = sqrt(N2_z2);
+
+    float R1 = 2 * (N1_x * Ix + N1_z * Iz) * N1_z - Iz;
+    float R2 = 2 * (N2_x * Ix + N2_z * Iz) * N2_z - Iz;
+
+    valid1 = (R1 >= 1e-5);
+    valid2 = (R2 >= 1e-5);
+    if (valid1 && valid2) {
+      N_new_x = (R1 < R2) ? N1_x : N2_x;
+      N_new_z = (R1 < R2) ? N1_z : N2_z;
+    }
+    else {
+      N_new_x = (R1 > R2) ? N1_x : N2_x;
+      N_new_z = (R1 > R2) ? N1_z : N2_z;
     }
   }
+  else if (valid1 || valid2) {
+    float Nz2 = valid1 ? N1_z2 : N2_z2;
+    N_new_x = sqrt(1.0 - Nz2);
+    N_new_z = sqrt(Nz2);
+  }
   else {
-    R = -I;
-    NgR = dot(Ng, R);
-    threshold = 0.01;
+    return Ng;
   }
 
-  R = R + Ng * (threshold - NgR);
-  return normalize(I * length(R) + R * length(I));
+  return N_new_x * X + N_new_z * Ng;
 }
 
 #endif /* CCL_STDOSL_H */