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_light.h b/intern/cycles/kernel/integrator/shade_light.h
index f2d65eddfbb..e0b0500dc78 100644
--- a/intern/cycles/kernel/integrator/shade_light.h
+++ b/intern/cycles/kernel/integrator/shade_light.h
@@ -18,6 +18,8 @@ ccl_device_inline void integrate_light(KernelGlobals kg,
   Intersection isect ccl_optional_struct_init;
   integrator_state_read_isect(kg, state, &isect);
 
+  guiding_record_light_surface_segment(kg, state, &isect);
+
   float3 ray_P = INTEGRATOR_STATE(state, ray, P);
   const float3 ray_D = INTEGRATOR_STATE(state, ray, D);
   const float ray_time = INTEGRATOR_STATE(state, ray, time);
@@ -66,6 +68,7 @@ ccl_device_inline void integrate_light(KernelGlobals kg,
   }
 
   /* Write to render buffer. */
+  guiding_record_surface_emission(kg, state, light_eval, mis_weight);
   film_write_surface_emission(kg, state, light_eval, mis_weight, render_buffer, ls.group);
 }