Revert "EEVEE: Ensure Reflection: Use new implementation"

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 5c4d24e1fd.

Ref T88368, D10084

source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl

@@ -138,30 +138,55 @@ void accumulate_light(vec3 light, float fac, inout vec4 accum)
 /* Same thing as Cycles without the comments to make it shorter. */
 vec3 ensure_valid_reflection(vec3 Ng, vec3 I, vec3 N)
 {
-  vec3 R;
-  float NI = dot(N, I);
-  float NgR, threshold;
-  /* Check if the incident ray is coming from behind normal N. */
-  if (NI > 0.0) {
-    /* Normal reflection. */
-    R = (2.0 * NI) * N - I;
-    NgR = dot(Ng, R);
-    /* Reflection rays may always be at least as shallow as the incoming ray. */
-    threshold = min(0.9 * dot(Ng, I), 0.01);
-    if (NgR >= threshold) {
-      return N;
+  vec3 R = -reflect(I, N);
+
+  /* Reflection rays may always be at least as shallow as the incoming ray. */
+  float threshold = min(0.9 * dot(Ng, I), 0.025);
+  if (dot(Ng, R) >= threshold) {
+    return N;
+  }
+
+  float NdotNg = dot(N, Ng);
+  vec3 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);
+  bool valid1 = (N1_z2 > 1e-5) && (N1_z2 <= (1.0 + 1e-5));
+  bool valid2 = (N2_z2 > 1e-5) && (N2_z2 <= (1.0 + 1e-5));
+
+  vec2 N_new;
+  if (valid1 && valid2) {
+    /* If both are possible, do the expensive reflection-based check. */
+    vec2 N1 = vec2(sqrt(1.0 - N1_z2), sqrt(N1_z2));
+    vec2 N2 = vec2(sqrt(1.0 - N2_z2), sqrt(N2_z2));
+
+    float R1 = 2.0 * (N1.x * Ix + N1.y * Iz) * N1.y - Iz;
+    float R2 = 2.0 * (N2.x * Ix + N2.y * Iz) * N2.y - Iz;
+
+    valid1 = (R1 >= 1e-5);
+    valid2 = (R2 >= 1e-5);
+    if (valid1 && valid2) {
+      N_new = (R1 < R2) ? N1 : N2;
+    }
+    else {
+      N_new = (R1 > R2) ? N1 : N2;
     }
   }
-  else {
-    /* Bad incident. */
-    R = -I;
-    NgR = dot(Ng, R);
-    threshold = 0.01;
+  else if (valid1 || valid2) {
+    float Nz2 = valid1 ? N1_z2 : N2_z2;
+    N_new = vec2(sqrt(1.0 - Nz2), sqrt(Nz2));
   }
-  /* Lift the reflection above the threshold. */
-  R = R + Ng * (threshold - NgR);
-  /* Find a bisector. */
-  return safe_normalize(I * length(R) + R * length(I));
+  else {
+    return Ng;
+  }
+  return N_new.x * X + N_new.y * Ng;
 }
 
 /* ----------- Cone angle Approximation --------- */