1 files changed, 45 insertions, 20 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl b/source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl
index fdbb70e917d..05496ad4ab0 100644
--- a/source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl
+++ b/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(safe_sqrt(1.0 - N1_z2), safe_sqrt(N1_z2));
+    vec2 N2 = vec2(safe_sqrt(1.0 - N2_z2), safe_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 if (valid1 || valid2) {
+    float Nz2 = valid1 ? N1_z2 : N2_z2;
+    N_new = vec2(safe_sqrt(1.0 - Nz2), safe_sqrt(Nz2));
   }
   else {
-    /* Bad incident. */
-    R = -I;
-    NgR = dot(Ng, R);
-    threshold = 0.01;
+    return Ng;
   }
-  /* Lift the reflection above the threshold. */
-  R = R + Ng * (threshold - NgR);
-  /* Find a bisector. */
-  return safe_normalize(I * length(R) + R * length(I));
+  return N_new.x * X + N_new.y * Ng;
 }
 
 /* ----------- Cone angle Approximation --------- */