Cycles: add build option to enable a debugging feature for MIS

This patch adds a CMake option "WITH_CYCLES_DEBUG" which builds cycles with
a feature that allows debugging/selecting the direct-light sampling strategy.
The same option may later be used to add other debugging features that could
affect performance in release builds.

The three options are:
* Forward path tracing (e.g., via BSDF or phase function)
* Next-event estimation
* Multiple importance sampling combination of the previous two methods

Such a feature is useful for debugging light different sampling, evaluation,
and pdf methods (e.g., for light sources and BSDFs).

Differential Revision: https://developer.blender.org/D13152

7 files changed, 62 insertions, 25 deletions

diff --git a/intern/cycles/kernel/CMakeLists.txt b/intern/cycles/kernel/CMakeLists.txt
index 39cb886b16e..baa0e9a73d8 100644
--- a/intern/cycles/kernel/CMakeLists.txt
+++ b/intern/cycles/kernel/CMakeLists.txt
@@ -412,17 +412,16 @@ if(WITH_CYCLES_CUDA_BINARIES)
       --use_fast_math
       -o ${CMAKE_CURRENT_BINARY_DIR}/${cuda_file})
 
-    if(${experimental})
-      set(cuda_flags ${cuda_flags} -D __KERNEL_EXPERIMENTAL__)
-      set(name ${name}_experimental)
-    endif()
-
     if(WITH_NANOVDB)
       set(cuda_flags ${cuda_flags}
         -D WITH_NANOVDB
         -I "${NANOVDB_INCLUDE_DIR}")
     endif()
 
+    if(WITH_CYCLES_DEBUG)
+      set(cuda_flags ${cuda_flags} -D WITH_CYCLES_DEBUG)
+    endif()
+
     if(WITH_CYCLES_CUBIN_COMPILER)
       string(SUBSTRING ${arch} 3 -1 CUDA_ARCH)
 
@@ -571,13 +570,8 @@ if(WITH_CYCLES_HIP_BINARIES AND WITH_CYCLES_DEVICE_HIP)
       -ffast-math
       -o ${CMAKE_CURRENT_BINARY_DIR}/${hip_file})
 
-    if(${experimental})
-      set(hip_flags ${hip_flags} -D __KERNEL_EXPERIMENTAL__)
-      set(name ${name}_experimental)
-    endif()
-
     if(WITH_CYCLES_DEBUG)
-      set(hip_flags ${hip_flags} -D __KERNEL_DEBUG__)
+      set(hip_flags ${hip_flags} -D WITH_CYCLES_DEBUG)
     endif()
 
     add_custom_command(
@@ -618,6 +612,10 @@ if(WITH_CYCLES_DEVICE_OPTIX AND WITH_CYCLES_CUDA_BINARIES)
         -I "${NANOVDB_INCLUDE_DIR}")
     endif()
 
+    if(WITH_CYCLES_DEBUG)
+      set(cuda_flags ${cuda_flags} -D WITH_CYCLES_DEBUG)
+    endif()
+
     if(WITH_CYCLES_CUBIN_COMPILER)
       # Needed to find libnvrtc-builtins.so. Can't do it from inside
       # cycles_cubin_cc since the env variable is read before main()
diff --git a/intern/cycles/kernel/integrator/shade_background.h b/intern/cycles/kernel/integrator/shade_background.h
index 31452de1ca4..a8ebbe908ae 100644
--- a/intern/cycles/kernel/integrator/shade_background.h
+++ b/intern/cycles/kernel/integrator/shade_background.h
@@ -20,7 +20,6 @@
 #include "kernel/integrator/shader_eval.h"
 #include "kernel/light/light.h"
 #include "kernel/light/sample.h"
-#include "kernel/sample/mis.h"
 
 CCL_NAMESPACE_BEGIN
 
@@ -81,8 +80,7 @@ ccl_device float3 integrator_eval_background_shader(KernelGlobals kg,
     /* multiple importance sampling, get background light pdf for ray
      * direction, and compute weight with respect to BSDF pdf */
     const float pdf = background_light_pdf(kg, ray_P - ray_D * mis_ray_t, ray_D);
-    const float mis_weight = power_heuristic(mis_ray_pdf, pdf);
-
+    const float mis_weight = light_sample_mis_weight_forward(kg, mis_ray_pdf, pdf);
     L *= mis_weight;
   }
 #  endif
@@ -169,7 +167,7 @@ ccl_device_inline void integrate_distant_lights(KernelGlobals kg,
         /* multiple importance sampling, get regular light pdf,
          * and compute weight with respect to BSDF pdf */
         const float mis_ray_pdf = INTEGRATOR_STATE(state, path, mis_ray_pdf);
-        const float mis_weight = power_heuristic(mis_ray_pdf, ls.pdf);
+        const float mis_weight = light_sample_mis_weight_forward(kg, mis_ray_pdf, ls.pdf);
         light_eval *= mis_weight;
       }
 
diff --git a/intern/cycles/kernel/integrator/shade_light.h b/intern/cycles/kernel/integrator/shade_light.h
index 5abe9e98abc..97ca430752c 100644
--- a/intern/cycles/kernel/integrator/shade_light.h
+++ b/intern/cycles/kernel/integrator/shade_light.h
@@ -84,7 +84,7 @@ ccl_device_inline void integrate_light(KernelGlobals kg,
     /* multiple importance sampling, get regular light pdf,
      * and compute weight with respect to BSDF pdf */
     const float mis_ray_pdf = INTEGRATOR_STATE(state, path, mis_ray_pdf);
-    const float mis_weight = power_heuristic(mis_ray_pdf, ls.pdf);
+    const float mis_weight = light_sample_mis_weight_forward(kg, mis_ray_pdf, ls.pdf);
     light_eval *= mis_weight;
   }
 
diff --git a/intern/cycles/kernel/integrator/shade_surface.h b/intern/cycles/kernel/integrator/shade_surface.h
index 2c478784bc9..c9c586f5ae4 100644
--- a/intern/cycles/kernel/integrator/shade_surface.h
+++ b/intern/cycles/kernel/integrator/shade_surface.h
@@ -27,8 +27,6 @@
 #include "kernel/light/light.h"
 #include "kernel/light/sample.h"
 
-#include "kernel/sample/mis.h"
-
 CCL_NAMESPACE_BEGIN
 
 ccl_device_forceinline void integrate_surface_shader_setup(KernelGlobals kg,
@@ -95,8 +93,7 @@ ccl_device_forceinline void integrate_surface_emission(KernelGlobals kg,
     /* Multiple importance sampling, get triangle light pdf,
      * and compute weight with respect to BSDF pdf. */
     float pdf = triangle_light_pdf(kg, sd, t);
-    float mis_weight = power_heuristic(bsdf_pdf, pdf);
-
+    float mis_weight = light_sample_mis_weight_forward(kg, bsdf_pdf, pdf);
     L *= mis_weight;
   }
 
@@ -155,7 +152,7 @@ ccl_device_forceinline void integrate_surface_direct_light(KernelGlobals kg,
   bsdf_eval_mul3(&bsdf_eval, light_eval / ls.pdf);
 
   if (ls.shader & SHADER_USE_MIS) {
-    const float mis_weight = power_heuristic(ls.pdf, bsdf_pdf);
+    const float mis_weight = light_sample_mis_weight_nee(kg, ls.pdf, bsdf_pdf);
     bsdf_eval_mul(&bsdf_eval, mis_weight);
   }
 
diff --git a/intern/cycles/kernel/integrator/shade_volume.h b/intern/cycles/kernel/integrator/shade_volume.h
index 141433c37a8..eff1042bd59 100644
--- a/intern/cycles/kernel/integrator/shade_volume.h
+++ b/intern/cycles/kernel/integrator/shade_volume.h
@@ -27,8 +27,6 @@
 #include "kernel/light/light.h"
 #include "kernel/light/sample.h"
 
-#include "kernel/sample/mis.h"
-
 CCL_NAMESPACE_BEGIN
 
 #ifdef __VOLUME__
@@ -759,7 +757,7 @@ ccl_device_forceinline void integrate_volume_direct_light(
   const float phase_pdf = shader_volume_phase_eval(kg, sd, phases, ls->D, &phase_eval);
 
   if (ls->shader & SHADER_USE_MIS) {
-    float mis_weight = power_heuristic(ls->pdf, phase_pdf);
+    float mis_weight = light_sample_mis_weight_nee(kg, ls->pdf, phase_pdf);
     bsdf_eval_mul(&phase_eval, mis_weight);
   }
 
diff --git a/intern/cycles/kernel/light/sample.h b/intern/cycles/kernel/light/sample.h
index 6b643a95250..ff5d43ed8cd 100644
--- a/intern/cycles/kernel/light/sample.h
+++ b/intern/cycles/kernel/light/sample.h
@@ -22,6 +22,7 @@
 #include "kernel/light/light.h"
 
 #include "kernel/sample/mapping.h"
+#include "kernel/sample/mis.h"
 
 CCL_NAMESPACE_BEGIN
 
@@ -268,4 +269,36 @@ ccl_device_inline void light_sample_to_volume_shadow_ray(
   shadow_ray_setup(sd, ls, P, ray);
 }
 
+ccl_device_inline float light_sample_mis_weight_forward(KernelGlobals kg,
+                                                        const float forward_pdf,
+                                                        const float nee_pdf)
+{
+#ifdef WITH_CYCLES_DEBUG
+  if (kernel_data.integrator.direct_light_sampling_type == DIRECT_LIGHT_SAMPLING_FORWARD) {
+    return 1.0f;
+  }
+  else if (kernel_data.integrator.direct_light_sampling_type == DIRECT_LIGHT_SAMPLING_NEE) {
+    return 0.0f;
+  }
+  else
+#endif
+    return power_heuristic(forward_pdf, nee_pdf);
+}
+
+ccl_device_inline float light_sample_mis_weight_nee(KernelGlobals kg,
+                                                    const float nee_pdf,
+                                                    const float forward_pdf)
+{
+#ifdef WITH_CYCLES_DEBUG
+  if (kernel_data.integrator.direct_light_sampling_type == DIRECT_LIGHT_SAMPLING_FORWARD) {
+    return 0.0f;
+  }
+  else if (kernel_data.integrator.direct_light_sampling_type == DIRECT_LIGHT_SAMPLING_NEE) {
+    return 1.0f;
+  }
+  else
+#endif
+    return power_heuristic(nee_pdf, forward_pdf);
+}
+
 CCL_NAMESPACE_END
diff --git a/intern/cycles/kernel/types.h b/intern/cycles/kernel/types.h
index 74b34524786..e66c3fe49df 100644
--- a/intern/cycles/kernel/types.h
+++ b/intern/cycles/kernel/types.h
@@ -481,6 +481,16 @@ enum PanoramaType {
   PANORAMA_NUM_TYPES,
 };
 
+/* Direct Light Sampling */
+
+enum DirectLightSamplingType {
+  DIRECT_LIGHT_SAMPLING_MIS = 0,
+  DIRECT_LIGHT_SAMPLING_FORWARD = 1,
+  DIRECT_LIGHT_SAMPLING_NEE = 2,
+
+  DIRECT_LIGHT_SAMPLING_NUM,
+};
+
 /* Differential */
 
 typedef struct differential3 {
@@ -1193,8 +1203,11 @@ typedef struct KernelIntegrator {
   /* Closure filter. */
   int filter_closures;
 
+  /* MIS debuging */
+  int direct_light_sampling_type;
+
   /* padding */
-  int pad1, pad2, pad3;
+  int pad1, pad2;
 } KernelIntegrator;
 static_assert_align(KernelIntegrator, 16);