EEVEE-Next: Add pdf to LOD to reduce noise from HDRI

Sphere lightprobe have pre-filtered LODs.
Sample lower LODs when PDF is high to reduce variance.

source/blender/draw/engines/eevee_next/shaders/eevee_lightprobe_eval_lib.glsl

@@ -79,15 +79,16 @@ vec3 lightprobe_spherical_sample_normalized_with_parallax(LightProbeSample samp,
   return normalization_factor * lightprobe_spheres_sample(L, lod, probe.atlas_coord).rgb;
 }
 
-float pdf_to_lod(float pdf)
+float pdf_to_lod(float inv_pdf)
 {
-  return 0.0; /* TODO */
+  float blur_pdf = saturate((2.0 * M_PI) * inv_pdf);
+  return blur_pdf * 0.0;
 }
 
-vec3 lightprobe_eval_direction(LightProbeSample samp, vec3 P, vec3 L, float pdf)
+vec3 lightprobe_eval_direction(LightProbeSample samp, vec3 P, vec3 L, float inv_pdf)
 {
   vec3 radiance_sh = lightprobe_spherical_sample_normalized_with_parallax(
-      samp, P, L, pdf_to_lod(pdf));
+      samp, P, L, pdf_to_lod(inv_pdf));
   return radiance_sh;
 }
 

source/blender/draw/engines/eevee_next/shaders/eevee_ray_trace_fallback_comp.glsl

@@ -70,8 +70,7 @@ void main()
   float clamp_indirect = uniform_buf.clamp.surface_indirect;
   samp.volume_irradiance = spherical_harmonics_clamp(samp.volume_irradiance, clamp_indirect);
 
-  vec3 radiance = lightprobe_eval_direction(
-      samp, ray.origin, ray.direction, safe_rcp(ray_pdf_inv));
+  vec3 radiance = lightprobe_eval_direction(samp, ray.origin, ray.direction, ray_pdf_inv);
   /* Set point really far for correct reprojection of background. */
   float hit_time = 1000.0;
 

source/blender/draw/engines/eevee_next/shaders/eevee_ray_trace_planar_comp.glsl

@@ -101,7 +101,7 @@ void main()
     vec3 Ng = ray.direction;
     /* Fallback to nearest light-probe. */
     LightProbeSample samp = lightprobe_load(P, Ng, V);
-    radiance = lightprobe_eval_direction(samp, P, ray.direction, safe_rcp(ray_pdf_inv));
+    radiance = lightprobe_eval_direction(samp, P, ray.direction, ray_pdf_inv);
     /* Set point really far for correct reprojection of background. */
     hit.time = 10000.0;
   }

source/blender/draw/engines/eevee_next/shaders/eevee_ray_trace_screen_comp.glsl

@@ -141,7 +141,7 @@ void main()
     float clamp_indirect = uniform_buf.clamp.surface_indirect;
     samp.volume_irradiance = spherical_harmonics_clamp(samp.volume_irradiance, clamp_indirect);
 
-    radiance = lightprobe_eval_direction(samp, ray.origin, ray.direction, safe_rcp(ray_pdf_inv));
+    radiance = lightprobe_eval_direction(samp, ray.origin, ray.direction, ray_pdf_inv);
     /* Set point really far for correct reprojection of background. */
     hit.time = 10000.0;
   }