Cycles: add Path Guiding on CPU through Intel OpenPGL

This adds path guiding features into Cycles by integrating Intel's Open Path
Guiding Library. It can be enabled in the Sampling > Path Guiding panel in the
render properties.

This feature helps reduce noise in scenes where finding a path to light is
difficult for regular path tracing.

The current implementation supports guiding directional sampling decisions on
surfaces, when the material contains a least one diffuse component, and in
volumes with isotropic and anisotropic Henyey-Greenstein phase functions.

On surfaces, the guided sampling decision is proportional to the product of
the incident radiance and the normal-oriented cosine lobe and in volumes it
is proportional to the product of the incident radiance and the phase function.

The incident radiance field of a scene is learned and updated during rendering
after each per-frame rendering iteration/progression.

At the moment, path guiding is only supported by the CPU backend. Support for
GPU backends will be added in future versions of OpenPGL.

Ref T92571

Differential Revision: https://developer.blender.org/D15286

1 files changed, 3 insertions, 0 deletions

diff --git a/intern/cycles/kernel/integrator/shade_background.h b/intern/cycles/kernel/integrator/shade_background.h
index 30ce0999258..8fc5689683a 100644
--- a/intern/cycles/kernel/integrator/shade_background.h
+++ b/intern/cycles/kernel/integrator/shade_background.h
@@ -5,6 +5,7 @@
 
 #include "kernel/film/light_passes.h"
 
+#include "kernel/integrator/guiding.h"
 #include "kernel/integrator/surface_shader.h"
 
 #include "kernel/light/light.h"
@@ -124,6 +125,7 @@ ccl_device_inline void integrate_background(KernelGlobals kg,
       mis_weight = light_sample_mis_weight_forward(kg, mis_ray_pdf, pdf);
     }
 
+    guiding_record_background(kg, state, L, mis_weight);
     L *= mis_weight;
   }
 
@@ -185,6 +187,7 @@ ccl_device_inline void integrate_distant_lights(KernelGlobals kg,
       }
 
       /* Write to render buffer. */
+      guiding_record_background(kg, state, light_eval, mis_weight);
       film_write_surface_emission(
           kg, state, light_eval, mis_weight, render_buffer, kernel_data.background.lightgroup);
     }