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

1 files changed, 13 insertions, 0 deletions

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, "