EEVEE: Update LUT GGX generation shader

This modifies the principled BSDF and the Glass BSDF which now
have better fit to multiscatter GGX.

Code to generate the LUT have been updated and can run at runtime.

The refraction LUT has been changed to have the critical angle always
centered around one pixel so that interpolation can be mitigated.

Offline LUT data will be updated in another commit

This simplify the BTDF retreival removing the manual clean cut at
low roughness. This maximize the precision of the LUT by scalling
the sides by the critical angle.
I also touched the ior > 1.0 approximation to be smoother.

Also incluse some cleanup of bsdf_sampling.glsl

2 files changed, 22 insertions, 10 deletions

diff --git a/source/blender/gpu/shaders/material/gpu_shader_material_glass.glsl b/source/blender/gpu/shaders/material/gpu_shader_material_glass.glsl
index ba02ae6d886..6788b34c938 100644
--- a/source/blender/gpu/shaders/material/gpu_shader_material_glass.glsl
+++ b/source/blender/gpu/shaders/material/gpu_shader_material_glass.glsl
@@ -6,8 +6,8 @@ void node_bsdf_glass(vec4 color,
                      float roughness,
                      float ior,
                      vec3 N,
-                     float use_multiscatter,
-                     float ssr_id,
+                     const float do_multiscatter,
+                     const float ssr_id,
                      out Closure result)
 {
   CLOSURE_VARS_DECLARE_2(Glossy, Refraction);
@@ -23,11 +23,16 @@ void node_bsdf_glass(vec4 color,
 
   result = CLOSURE_DEFAULT;
 
-  float fresnel = F_eta(in_Refraction_1.ior, dot(in_Glossy_0.N, cameraVec));
+  float NV = dot(in_Refraction_1.N, cameraVec);
+
+  float fresnel = (do_multiscatter != 0.0) ?
+                      btdf_lut(NV, in_Refraction_1.roughness, in_Refraction_1.ior).y :
+                      F_eta(in_Refraction_1.ior, NV);
+
+  vec2 split_sum = brdf_lut(NV, in_Glossy_0.roughness);
+  vec3 brdf = (do_multiscatter != 0.0) ? F_brdf_multi_scatter(vec3(1.0), vec3(1.0), split_sum) :
+                                         F_brdf_single_scatter(vec3(1.0), vec3(1.0), split_sum);
 
-  vec2 split_sum = brdf_lut(dot(in_Glossy_0.N, cameraVec), in_Glossy_0.roughness);
-  vec3 brdf = (use_multiscatter != 0.0) ? F_brdf_multi_scatter(vec3(1.0), vec3(1.0), split_sum) :
-                                          F_brdf_single_scatter(vec3(1.0), vec3(1.0), split_sum);
   out_Glossy_0.radiance = closure_mask_ssr_radiance(out_Glossy_0.radiance, ssr_id);
   out_Glossy_0.radiance *= brdf;
   out_Glossy_0.radiance = render_pass_glossy_mask(vec3(1.0), out_Glossy_0.radiance);
@@ -35,6 +40,10 @@ void node_bsdf_glass(vec4 color,
   closure_load_ssr_data(
       out_Glossy_0.radiance, in_Glossy_0.roughness, in_Glossy_0.N, ssr_id, result);
 
+  float btdf = (do_multiscatter != 0.0) ?
+                   1.0 :
+                   btdf_lut(NV, in_Refraction_1.roughness, in_Refraction_1.ior).x;
+  out_Refraction_1.radiance *= btdf;
   out_Refraction_1.radiance = render_pass_glossy_mask(vec3(1.0), out_Refraction_1.radiance);
   out_Refraction_1.radiance *= color.rgb * (1.0 - fresnel);
   /* Simulate 2nd absorption event. */
diff --git a/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl b/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl
index 15958dcf65e..139dcb33222 100644
--- a/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl
+++ b/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl
@@ -70,7 +70,6 @@ void node_bsdf_principled(vec4 base_color,
   in_Refraction_3.N = N; /* Normalized during eval. */
   in_Refraction_3.roughness = do_multiscatter != 0.0 ? roughness : transmission_roughness;
   in_Refraction_3.ior = ior;
-  
 
   CLOSURE_EVAL_FUNCTION_4(node_bsdf_principled, Diffuse, Glossy, Glossy, Refraction);
 
@@ -92,9 +91,9 @@ void node_bsdf_principled(vec4 base_color,
 
   vec3 base_color_tint = tint_from_color(base_color.rgb);
 
-  /* TODO(fclem) This isn't good for rough glass using multiscatter (since the fresnel is applied
-   * on each microfacet in cycles). */
-  float fresnel = F_eta(in_Refraction_3.ior, NV);
+  float fresnel = (do_multiscatter != 0.0) ?
+                      btdf_lut(NV, in_Glossy_1.roughness, in_Refraction_3.ior).y :
+                      F_eta(in_Refraction_3.ior, NV);
 
   {
     /* Glossy reflections.
@@ -159,7 +158,11 @@ void node_bsdf_principled(vec4 base_color,
   }
 
   if (transmission > 1e-5) {
+    float btdf = (do_multiscatter != 0.0) ?
+                     1.0 :
+                     btdf_lut(NV, in_Refraction_3.roughness, in_Refraction_3.ior).x;
     /* TODO(fclem) This could be going to a transmission render pass instead. */
+    out_Refraction_3.radiance *= btdf;
     out_Refraction_3.radiance = render_pass_glossy_mask(vec3(1), out_Refraction_3.radiance);
     out_Refraction_3.radiance *= base_color.rgb;
     /* Simulate 2nd transmission event. */