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

14 files changed, 111 insertions, 25 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d2e2d02dcde..adf71b6ae0f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -411,6 +411,7 @@ option(WITH_CYCLES                   "Enable Cycles Render Engine" ON)
 option(WITH_CYCLES_OSL               "Build Cycles with OpenShadingLanguage support" ON)
 option(WITH_CYCLES_EMBREE            "Build Cycles with Embree support" ON)
 option(WITH_CYCLES_LOGGING           "Build Cycles with logging support" ON)
+option(WITH_CYCLES_DEBUG             "Build Cycles with options useful for debugging (e.g., MIS)" OFF)
 
 option(WITH_CYCLES_STANDALONE        "Build Cycles standalone application" OFF)
 option(WITH_CYCLES_STANDALONE_GUI    "Build Cycles standalone with GUI" OFF)
diff --git a/intern/cycles/CMakeLists.txt b/intern/cycles/CMakeLists.txt
index 6ba80bf9e18..8854170c642 100644
--- a/intern/cycles/CMakeLists.txt
+++ b/intern/cycles/CMakeLists.txt
@@ -226,6 +226,9 @@ add_definitions(
   -DCCL_NAMESPACE_END=}
 )
 
+if(WITH_CYCLES_DEBUG)
+  add_definitions(-DWITH_CYCLES_DEBUG)
+endif()
 if(WITH_CYCLES_STANDALONE_GUI)
   add_definitions(-DWITH_CYCLES_STANDALONE_GUI)
 endif()
diff --git a/intern/cycles/blender/addon/properties.py b/intern/cycles/blender/addon/properties.py
index 986d5d972c1..1e267ccdf4a 100644
--- a/intern/cycles/blender/addon/properties.py
+++ b/intern/cycles/blender/addon/properties.py
@@ -218,6 +218,12 @@ enum_denoising_prefilter = (
     ('ACCURATE', "Accurate", "Prefilter noisy guiding passes before denoising color. Improves quality when guiding passes are noisy using extra processing time", 3),
 )
 
+enum_direct_light_sampling_type = (
+    ('MULTIPLE_IMPORTANCE_SAMPLING', "Multiple Importance Sampling", "Multiple importance sampling is used to combine direct light contributions from next-event estimation and forward path tracing", 0),
+    ('FORWARD_PATH_TRACING', "Forward Path Tracing", "Direct light contributions are only sampled using forward path tracing", 1),
+    ('NEXT_EVENT_ESTIMATION', "Next-Event Estimation", "Direct light contributions are only sampled using next-event estimation", 2),
+)
+
 def update_render_passes(self, context):
     scene = context.scene
     view_layer = context.view_layer
@@ -422,6 +428,13 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
         default=0,
     )
 
+    direct_light_sampling_type: EnumProperty(
+        name="Direct Light Sampling Type",
+        description="The type of strategy used for sampling direct light contributions",
+        items=enum_direct_light_sampling_type,
+        default='MULTIPLE_IMPORTANCE_SAMPLING',
+    )
+
     min_light_bounces: IntProperty(
         name="Min Light Bounces",
         description="Minimum number of light bounces. Setting this higher reduces noise in the first bounces, "
diff --git a/intern/cycles/blender/sync.cpp b/intern/cycles/blender/sync.cpp
index 76118fdc6b7..7e40b88cc1a 100644
--- a/intern/cycles/blender/sync.cpp
+++ b/intern/cycles/blender/sync.cpp
@@ -392,6 +392,12 @@ void BlenderSync::sync_integrator(BL::ViewLayer &b_view_layer, bool background)
     integrator->set_ao_bounces(0);
   }
 
+#ifdef WITH_CYCLES_DEBUG
+  DirectLightSamplingType direct_light_sampling_type = (DirectLightSamplingType)get_enum(
+      cscene, "direct_light_sampling_type", DIRECT_LIGHT_SAMPLING_NUM, DIRECT_LIGHT_SAMPLING_MIS);
+  integrator->set_direct_light_sampling_type(direct_light_sampling_type);
+#endif
+
   const DenoiseParams denoise_params = get_denoise_params(b_scene, b_view_layer, background);
   integrator->set_use_denoise(denoise_params.use);
 
diff --git a/intern/cycles/device/cuda/device_impl.cpp b/intern/cycles/device/cuda/device_impl.cpp
index 5ab73a6b9ef..f7b3c5ad77f 100644
--- a/intern/cycles/device/cuda/device_impl.cpp
+++ b/intern/cycles/device/cuda/device_impl.cpp
@@ -236,6 +236,10 @@ string CUDADevice::compile_kernel_get_common_cflags(const uint kernel_features)
   cflags += " -DWITH_NANOVDB";
 #  endif
 
+#  ifdef WITH_CYCLES_DEBUG
+  cflags += " -DWITH_CYCLES_DEBUG";
+#  endif
+
   return cflags;
 }
 
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);
 
diff --git a/intern/cycles/scene/integrator.cpp b/intern/cycles/scene/integrator.cpp
index 737db8b98d5..9216a8ae615 100644
--- a/intern/cycles/scene/integrator.cpp
+++ b/intern/cycles/scene/integrator.cpp
@@ -52,6 +52,18 @@ NODE_DEFINE(Integrator)
   SOCKET_INT(transparent_min_bounce, "Transparent Min Bounce", 0);
   SOCKET_INT(transparent_max_bounce, "Transparent Max Bounce", 7);
 
+#ifdef WITH_CYCLES_DEBUG
+  static NodeEnum direct_light_sampling_type_enum;
+  direct_light_sampling_type_enum.insert("multiple_importance_sampling",
+                                         DIRECT_LIGHT_SAMPLING_MIS);
+  direct_light_sampling_type_enum.insert("forward_path_tracing", DIRECT_LIGHT_SAMPLING_FORWARD);
+  direct_light_sampling_type_enum.insert("next_event_estimation", DIRECT_LIGHT_SAMPLING_NEE);
+  SOCKET_ENUM(direct_light_sampling_type,
+              "Direct Light Sampling Type",
+              direct_light_sampling_type_enum,
+              DIRECT_LIGHT_SAMPLING_MIS);
+#endif
+
   SOCKET_INT(ao_bounces, "AO Bounces", 0);
   SOCKET_FLOAT(ao_factor, "AO Factor", 0.0f);
   SOCKET_FLOAT(ao_distance, "AO Distance", FLT_MAX);
@@ -171,6 +183,12 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
   kintegrator->ao_bounces_factor = ao_factor;
   kintegrator->ao_additive_factor = ao_additive_factor;
 
+#ifdef WITH_CYCLES_DEBUG
+  kintegrator->direct_light_sampling_type = direct_light_sampling_type;
+#else
+  kintegrator->direct_light_sampling_type = DIRECT_LIGHT_SAMPLING_MIS;
+#endif
+
   /* Transparent Shadows
    * We only need to enable transparent shadows, if we actually have
    * transparent shaders in the scene. Otherwise we can disable it
diff --git a/intern/cycles/scene/integrator.h b/intern/cycles/scene/integrator.h
index 464d96ca01b..52f1b296a20 100644
--- a/intern/cycles/scene/integrator.h
+++ b/intern/cycles/scene/integrator.h
@@ -41,6 +41,10 @@ class Integrator : public Node {
   NODE_SOCKET_API(int, max_transmission_bounce)
   NODE_SOCKET_API(int, max_volume_bounce)
 
+#ifdef WITH_CYCLES_DEBUG
+  NODE_SOCKET_API(DirectLightSamplingType, direct_light_sampling_type)
+#endif
+
   NODE_SOCKET_API(int, transparent_min_bounce)
   NODE_SOCKET_API(int, transparent_max_bounce)