Fix #161234: Cycles caustics blurred out after ray differential widening

The ray differential widening for the texture cache blurred out caustics
both from procedural textures and image textures. The widening helps
reduce texture cache memory, but is also fundamentally biased.

The solution now is to tie this to the Filter Glossy setting that must
already be lowered from the default to make caustics work. When left at
the default the texture cache will be most efficient, when set to small
value or zero there will be little or no widening.

So this setting now becomes a more general bias control for shading
after a rough glossy or diffuse bounce, and the tooltip was updated.

Pull Request: https://projects.blender.org/blender/blender/pulls/162153

intern/cycles/blender/addon/properties.py

@@ -658,8 +658,9 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
 
     blur_glossy: FloatProperty(
         name="Filter Glossy",
-        description="Adaptively blur glossy shaders after blurry bounces, "
-        "to reduce noise at the cost of accuracy",
+        description="Adaptively blur glossy shaders and image textures after blurry bounces, "
+        "to reduce noise and improve texture cache efficiency at the cost of accuracy. Lower "
+        "this value to render caustics",
         min=0.0, max=10.0,
         default=1.0,
     )

intern/cycles/kernel/closure/bsdf.h

@@ -68,14 +68,22 @@ ccl_device_inline float bsdf_get_roughness_pass_squared(const ccl_private Shader
  *
  * To achieve that, the sampled roughness is computed as a MIS weighted
  * average. This makes it so directions with high contribution from sharp
- * BSDFs have a lower roughness, as they will have a high MIS weight. */
-ccl_device_forceinline float bsdf_widen_dD(const float prev_dD, const float avg_roughness_squared)
+ * BSDFs have a lower roughness, as they will have a high MIS weight.
+ *
+ * The amount of widening is scaled by Filter Glossy, which is the setting
+ * to control how much caustics are blurred out. For accurate caustics
+ * image texture lookups and bump ray differentials must be small enough
+ * to capture detail at secondary bounces. */
+ccl_device_forceinline float bsdf_widen_dD(const KernelGlobals kg,
+                                           const float prev_dD,
+                                           const float avg_roughness_squared)
 {
   if (!(avg_roughness_squared > 0.0f)) {
     return prev_dD;
   }
 
-  return max(prev_dD, sqrtf(avg_roughness_squared));
+  const float scale = kernel_data.integrator.differential_widen_scale;
+  return max(prev_dD, scale * sqrtf(avg_roughness_squared));
 }
 
 /* An additional term to smooth illumination on grazing angles when using bump mapping

intern/cycles/kernel/closure/volume.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include "kernel/globals.h"
 #include "kernel/types.h"
 
 #include "kernel/closure/volume_draine.h"
@@ -74,10 +75,11 @@ ccl_device int volume_phase_sample(const ccl_private ShaderData *sd,
 
 /* Widen the compact ray differential dD after a phase function scatter to
  * match the lobe's angular spread. See bsdf_widen_dD for details. */
-ccl_device_forceinline float volume_phase_widen_dD(const float prev_dD,
+ccl_device_forceinline float volume_phase_widen_dD(const KernelGlobals kg,
+                                                   const float prev_dD,
                                                    const float sampled_roughness)
 {
-  return max(prev_dD, sampled_roughness);
+  return max(prev_dD, kernel_data.integrator.differential_widen_scale * sampled_roughness);
 }
 
 ccl_device bool volume_phase_equal(const ccl_private ShaderClosure *c1,

intern/cycles/kernel/data_template.h

@@ -195,6 +195,7 @@ KERNEL_STRUCT_MEMBER(integrator, int, transparent_shadows)
 KERNEL_STRUCT_MEMBER(integrator, int, caustics_reflective)
 KERNEL_STRUCT_MEMBER(integrator, int, caustics_refractive)
 KERNEL_STRUCT_MEMBER(integrator, float, filter_glossy)
+KERNEL_STRUCT_MEMBER(integrator, float, differential_widen_scale)
 /* Seed. */
 KERNEL_STRUCT_MEMBER_DONT_SPECIALIZE
 KERNEL_STRUCT_MEMBER(integrator, int, seed)

intern/cycles/kernel/integrator/shade_surface.h

@@ -433,7 +433,7 @@ ccl_device
 #ifdef __RAY_DIFFERENTIALS__
     /* Widen ray differences, with same logic as forward sampling to ensure
      * both MIS strategies converge to the same result. */
-    ray.dD = bsdf_widen_dD(INTEGRATOR_STATE(state, ray, dD), avg_roughness_squared);
+    ray.dD = bsdf_widen_dD(kg, INTEGRATOR_STATE(state, ray, dD), avg_roughness_squared);
 #endif
   }
 
@@ -586,7 +586,8 @@ ccl_device_forceinline int integrate_surface_bsdf_bssrdf_bounce(
 
     /* Widen ray differences, with same logic as NEE sampling to ensure
      * both MIS strategies converge to the same result. */
-    const float dD = bsdf_widen_dD(INTEGRATOR_STATE(state, ray, dD), bsdf_avg_roughness_squared);
+    const float dD = bsdf_widen_dD(
+        kg, INTEGRATOR_STATE(state, ray, dD), bsdf_avg_roughness_squared);
     INTEGRATOR_STATE_WRITE(state, ray, dD) = dD;
 #endif
   }

intern/cycles/kernel/integrator/shade_volume.h

@@ -2652,8 +2652,8 @@ ccl_device_forceinline bool integrate_volume_phase_scatter(
   INTEGRATOR_STATE_WRITE(state, ray, tmax) = FLT_MAX;
 #  ifdef __RAY_DIFFERENTIALS__
   INTEGRATOR_STATE_WRITE(state, ray, dP) = differential_make_compact(sd->dP);
-  INTEGRATOR_STATE_WRITE(state, ray, dD) = volume_phase_widen_dD(INTEGRATOR_STATE(state, ray, dD),
-                                                                 sampled_roughness);
+  INTEGRATOR_STATE_WRITE(state, ray, dD) = volume_phase_widen_dD(
+      kg, INTEGRATOR_STATE(state, ray, dD), sampled_roughness);
 #  endif
   // Save memory by storing last hit prim and object in isect
   INTEGRATOR_STATE_WRITE(state, isect, prim) = sd->prim;

intern/cycles/scene/integrator.cpp

@@ -267,6 +267,7 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
   kintegrator->caustics_reflective = caustics_reflective;
   kintegrator->caustics_refractive = caustics_refractive;
   kintegrator->filter_glossy = (filter_glossy == 0.0f) ? FLT_MAX : 1.0f / filter_glossy;
+  kintegrator->differential_widen_scale = min(1.0f, filter_glossy);
 
   kintegrator->filter_closures = 0;
   if (!use_direct_light) {

tests/files/render/bsdf/cycles_renders/ray_portal.png

@@ -1,3 +1,3 @@
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-size 19943
+oid sha256:641d7d513fcc4cf14920a3ff0f15b218b2bb1d6c9518ab925aa65e2e34653055
+size 19911

tests/files/render/displacement/cycles_renders/bump_glass.png

@@ -1,3 +1,3 @@
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+size 21473

tests/files/render/displacement/cycles_renders/bump_glossy.png

@@ -1,3 +1,3 @@
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-size 19280
+oid sha256:b6584cddd82dc2df6f48dcfaa320928563c7e332c52abbe4228af58ef58415ba
+size 19326

tests/files/render/displacement/cycles_renders/bump_normal_texture.png

@@ -1,3 +1,3 @@
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-size 44582
+oid sha256:e52195975bc852249169e0b39724136d1f72c3a56671e15d8ec4df5aa87b3625
+size 44573

tests/files/render/image_mipmap/cycles_renders/image_mipmap_filter_glossy.png

@@ -0,0 +1,3 @@
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+oid sha256:bb0c4ea6ffb9ab361dbf4736e7b48f8844840bc03f948acb9a58cc63a113dd01
+size 38382

tests/files/render/image_mipmap/image_mipmap_filter_glossy.blend

@@ -0,0 +1,3 @@
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+size 123809