Cycles: Widen ray differentials based on BSDF roughness

This reduces texture cache memory usage, with practically identical
renders. Where it does show some differences is with strong bump maps
and refaction.

Pull Request: https://projects.blender.org/blender/blender/pulls/157244

intern/cycles/kernel/closure/bsdf.h

@@ -57,6 +57,15 @@ ccl_device_inline float bsdf_get_roughness_pass_squared(const ccl_private Shader
   return bsdf_get_specular_roughness_squared(sc);
 }
 
+/* Widen the compact ray differential dD after a non-specular bounce so that
+ * texture mip selection on subsequent hits reflects the BSDF lobe's angular
+ * spread. This significantly save memory, and is needed to make image cache
+ * memory usage scale with render tile size rather than overall resolution. */
+ccl_device_forceinline float bsdf_widen_dD(const float prev_dD, const float2 sampled_roughness)
+{
+  return max(prev_dD, min(sampled_roughness.x, sampled_roughness.y));
+}
+
 /* An additional term to smooth illumination on grazing angles when using bump mapping
  * based on "A Microfacet-Based Shadowing Function to Solve the Bump Terminator Problem"
  * by Alejandro Conty Estevez, Pascal Lecocq, and Clifford Stein. It preserves detail

intern/cycles/kernel/closure/volume.h

@@ -72,6 +72,14 @@ 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,
+                                                   const float sampled_roughness)
+{
+  return max(prev_dD, sampled_roughness);
+}
+
 ccl_device bool volume_phase_equal(const ccl_private ShaderClosure *c1,
                                    const ccl_private ShaderClosure *c2)
 {

intern/cycles/kernel/integrator/shade_surface.h

@@ -563,6 +563,8 @@ ccl_device_forceinline int integrate_surface_bsdf_bssrdf_bounce(
     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) = bsdf_widen_dD(INTEGRATOR_STATE(state, ray, dD),
+                                                           bsdf_sampled_roughness);
 #endif
   }
 

intern/cycles/kernel/integrator/shade_volume.h

@@ -2606,6 +2606,8 @@ 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);
 #  endif
   // Save memory by storing last hit prim and object in isect
   INTEGRATOR_STATE_WRITE(state, isect, prim) = sd->prim;

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

@@ -1,3 +1,3 @@
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+size 19943

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

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+size 21345

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

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+size 19280

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

@@ -1,3 +1,3 @@
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+size 44582

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

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+oid sha256:ece45f07116794f4cb145c053b5f06917063bcac891545466774b3a2c1f6987a
+size 31931

tests/files/render/principled_bsdf/cycles_renders/principled_bsdf_emission.png

@@ -1,3 +1,3 @@
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+oid sha256:426e23bdfa0a8eda5cd9c8d96942e39165028b231af692de824a8562925f85ac
+size 31512

tests/files/render/shader/cycles_renders/normal_mapping_light_leak.png

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