Adaptive Sampling for Cycles.

This feature takes some inspiration from
"RenderMan: An Advanced Path Tracing Architecture for Movie Rendering" and
"A Hierarchical Automatic Stopping Condition for Monte Carlo Global Illumination"

The basic principle is as follows:
While samples are being added to a pixel, the adaptive sampler writes half
of the samples to a separate buffer. This gives it two separate estimates
of the same pixel, and by comparing their difference it estimates convergence.
Once convergence drops below a given threshold, the pixel is considered done.

When a pixel has not converged yet and needs more samples than the minimum,
its immediate neighbors are also set to take more samples. This is done in order
to more reliably detect sharp features such as caustics. A 3x3 box filter that
is run periodically over the tile buffer is used for that purpose.

After a tile has finished rendering, the values of all passes are scaled as if
they were rendered with the full number of samples. This way, any code operating
on these buffers, for example the denoiser, does not need to be changed for
per-pixel sample counts.

Reviewed By: brecht, #cycles

Differential Revision: https://developer.blender.org/D4686

6 files changed, 70 insertions, 5 deletions

diff --git a/intern/cycles/blender/addon/engine.py b/intern/cycles/blender/addon/engine.py
index 7917edf8c88..afa573c8dc5 100644
--- a/intern/cycles/blender/addon/engine.py
+++ b/intern/cycles/blender/addon/engine.py
@@ -255,6 +255,7 @@ def list_render_passes(srl):
     if crl.pass_debug_bvh_traversed_instances: yield ("Debug BVH Traversed Instances", "X",   'VALUE')
     if crl.pass_debug_bvh_intersections:       yield ("Debug BVH Intersections",       "X",   'VALUE')
     if crl.pass_debug_ray_bounces:             yield ("Debug Ray Bounces",             "X",   'VALUE')
+    if crl.pass_debug_sample_count:            yield ("Debug Sample Count",            "X",   'VALUE')
     if crl.use_pass_volume_direct:             yield ("VolumeDir",                     "RGB", 'COLOR')
     if crl.use_pass_volume_indirect:           yield ("VolumeInd",                     "RGB", 'COLOR')
 
diff --git a/intern/cycles/blender/addon/properties.py b/intern/cycles/blender/addon/properties.py
index 77dc29e11e8..f2c40f509e8 100644
--- a/intern/cycles/blender/addon/properties.py
+++ b/intern/cycles/blender/addon/properties.py
@@ -350,6 +350,24 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
         default=0.01,
     )
 
+    use_adaptive_sampling: BoolProperty(
+        name="Use adaptive sampling",
+        description="Automatically determine the number of samples per pixel based on a variance estimation",
+        default=False,
+    )
+    adaptive_threshold: FloatProperty(
+        name="Adaptive Sampling Threshold",
+        description="Zero for automatic setting based on AA samples",
+        min=0.0, max=1.0,
+        default=0.0,
+    )
+    adaptive_min_samples: IntProperty(
+        name="Adaptive Min Samples",
+        description="Minimum AA samples for adaptive sampling. Zero for automatic setting based on AA samples",
+        min=0, max=4096,
+        default=0,
+    )
+
     min_light_bounces: IntProperty(
             name="Min Light Bounces",
             description="Minimum number of light bounces. Setting this higher reduces noise in the first bounces, "
@@ -1298,7 +1316,12 @@ class CyclesRenderLayerSettings(bpy.types.PropertyGroup):
         default=False,
         update=update_render_passes,
     )
-
+    pass_debug_sample_count: BoolProperty(
+        name="Debug Sample Count",
+        description="Number of samples/camera rays per pixel",
+        default=False,
+        update=update_render_passes,
+    )
     use_pass_volume_direct: BoolProperty(
         name="Volume Direct",
         description="Deliver direct volumetric scattering pass",
diff --git a/intern/cycles/blender/addon/ui.py b/intern/cycles/blender/addon/ui.py
index ed9e3a4c9cf..d04418fc957 100644
--- a/intern/cycles/blender/addon/ui.py
+++ b/intern/cycles/blender/addon/ui.py
@@ -190,6 +190,7 @@ class CYCLES_RENDER_PT_sampling(CyclesButtonsPanel, Panel):
             col.prop(cscene, "aa_samples", text="Render")
             col.prop(cscene, "preview_aa_samples", text="Viewport")
 
+        col.prop(cscene, "use_adaptive_sampling", text="Adaptive Sampling")
         # Viewport denoising is currently only supported with OptiX
         if show_optix_denoising(context):
             col = layout.column()
@@ -247,7 +248,13 @@ class CYCLES_RENDER_PT_sampling_advanced(CyclesButtonsPanel, Panel):
         row.prop(cscene, "seed")
         row.prop(cscene, "use_animated_seed", text="", icon='TIME')
 
-        layout.prop(cscene, "sampling_pattern", text="Pattern")
+        col = layout.column(align=True)
+        col.active = not(cscene.use_adaptive_sampling)
+        col.prop(cscene, "sampling_pattern", text="Pattern")
+        col = layout.column(align=True)
+        col.active = cscene.use_adaptive_sampling
+        col.prop(cscene, "adaptive_min_samples", text="Adaptive Min Samples")
+        col.prop(cscene, "adaptive_threshold", text="Adaptive Threshold")
 
         layout.prop(cscene, "use_square_samples")
 
@@ -813,6 +820,8 @@ class CYCLES_RENDER_PT_passes_data(CyclesButtonsPanel, Panel):
         col.prop(cycles_view_layer, "denoising_store_passes", text="Denoising Data")
         col = flow.column()
         col.prop(cycles_view_layer, "pass_debug_render_time", text="Render Time")
+        col = flow.column()
+        col.prop(cycles_view_layer, "pass_debug_sample_count", text="Sample Count")
 
         layout.separator()
 
diff --git a/intern/cycles/blender/blender_session.cpp b/intern/cycles/blender/blender_session.cpp
index 5cfb1200c7c..ac307743e48 100644
--- a/intern/cycles/blender/blender_session.cpp
+++ b/intern/cycles/blender/blender_session.cpp
@@ -470,7 +470,8 @@ void BlenderSession::render(BL::Depsgraph &b_depsgraph_)
   b_rlay_name = b_view_layer.name();
 
   /* add passes */
-  vector<Pass> passes = sync->sync_render_passes(b_rlay, b_view_layer);
+  vector<Pass> passes = sync->sync_render_passes(
+      b_rlay, b_view_layer, session_params.adaptive_sampling);
   buffer_params.passes = passes;
 
   PointerRNA crl = RNA_pointer_get(&b_view_layer.ptr, "cycles");
diff --git a/intern/cycles/blender/blender_sync.cpp b/intern/cycles/blender/blender_sync.cpp
index 50442c6ebdc..8f00f9ccda0 100644
--- a/intern/cycles/blender/blender_sync.cpp
+++ b/intern/cycles/blender/blender_sync.cpp
@@ -296,6 +296,16 @@ void BlenderSync::sync_integrator()
   integrator->sample_all_lights_indirect = get_boolean(cscene, "sample_all_lights_indirect");
   integrator->light_sampling_threshold = get_float(cscene, "light_sampling_threshold");
 
+  if (RNA_boolean_get(&cscene, "use_adaptive_sampling")) {
+    integrator->sampling_pattern = SAMPLING_PATTERN_PMJ;
+    integrator->adaptive_min_samples = get_int(cscene, "adaptive_min_samples");
+    integrator->adaptive_threshold = get_float(cscene, "adaptive_threshold");
+  }
+  else {
+    integrator->adaptive_min_samples = INT_MAX;
+    integrator->adaptive_threshold = 0.0f;
+  }
+
   int diffuse_samples = get_int(cscene, "diffuse_samples");
   int glossy_samples = get_int(cscene, "glossy_samples");
   int transmission_samples = get_int(cscene, "transmission_samples");
@@ -312,6 +322,8 @@ void BlenderSync::sync_integrator()
     integrator->mesh_light_samples = mesh_light_samples * mesh_light_samples;
     integrator->subsurface_samples = subsurface_samples * subsurface_samples;
     integrator->volume_samples = volume_samples * volume_samples;
+    integrator->adaptive_min_samples = min(
+        integrator->adaptive_min_samples * integrator->adaptive_min_samples, INT_MAX);
   }
   else {
     integrator->diffuse_samples = diffuse_samples;
@@ -484,6 +496,8 @@ PassType BlenderSync::get_pass_type(BL::RenderPass &b_pass)
   MAP_PASS("Debug Ray Bounces", PASS_RAY_BOUNCES);
 #endif
   MAP_PASS("Debug Render Time", PASS_RENDER_TIME);
+  MAP_PASS("AdaptiveAuxBuffer", PASS_ADAPTIVE_AUX_BUFFER);
+  MAP_PASS("Debug Sample Count", PASS_SAMPLE_COUNT);
   if (string_startswith(name, cryptomatte_prefix)) {
     return PASS_CRYPTOMATTE;
   }
@@ -519,7 +533,9 @@ int BlenderSync::get_denoising_pass(BL::RenderPass &b_pass)
   return -1;
 }
 
-vector<Pass> BlenderSync::sync_render_passes(BL::RenderLayer &b_rlay, BL::ViewLayer &b_view_layer)
+vector<Pass> BlenderSync::sync_render_passes(BL::RenderLayer &b_rlay,
+                                             BL::ViewLayer &b_view_layer,
+                                             bool adaptive_sampling)
 {
   vector<Pass> passes;
 
@@ -595,6 +611,10 @@ vector<Pass> BlenderSync::sync_render_passes(BL::RenderLayer &b_rlay, BL::ViewLa
     b_engine.add_pass("Debug Render Time", 1, "X", b_view_layer.name().c_str());
     Pass::add(PASS_RENDER_TIME, passes, "Debug Render Time");
   }
+  if (get_boolean(crp, "pass_debug_sample_count")) {
+    b_engine.add_pass("Debug Sample Count", 1, "X", b_view_layer.name().c_str());
+    Pass::add(PASS_SAMPLE_COUNT, passes, "Debug Sample Count");
+  }
   if (get_boolean(crp, "use_pass_volume_direct")) {
     b_engine.add_pass("VolumeDir", 3, "RGB", b_view_layer.name().c_str());
     Pass::add(PASS_VOLUME_DIRECT, passes, "VolumeDir");
@@ -641,6 +661,13 @@ vector<Pass> BlenderSync::sync_render_passes(BL::RenderLayer &b_rlay, BL::ViewLa
                                                         CRYPT_ACCURATE);
   }
 
+  if (adaptive_sampling) {
+    Pass::add(PASS_ADAPTIVE_AUX_BUFFER, passes);
+    if (!get_boolean(crp, "pass_debug_sample_count")) {
+      Pass::add(PASS_SAMPLE_COUNT, passes);
+    }
+  }
+
   RNA_BEGIN (&crp, b_aov, "aovs") {
     bool is_color = (get_enum(b_aov, "type") == 1);
     string name = get_string(b_aov, "name");
@@ -880,6 +907,8 @@ SessionParams BlenderSync::get_session_params(BL::RenderEngine &b_engine,
   params.use_profiling = params.device.has_profiling && !b_engine.is_preview() && background &&
                          BlenderSession::print_render_stats;
 
+  params.adaptive_sampling = RNA_boolean_get(&cscene, "use_adaptive_sampling");
+
   return params;
 }
 
diff --git a/intern/cycles/blender/blender_sync.h b/intern/cycles/blender/blender_sync.h
index 219a3615835..0a7174e407b 100644
--- a/intern/cycles/blender/blender_sync.h
+++ b/intern/cycles/blender/blender_sync.h
@@ -71,7 +71,9 @@ class BlenderSync {
                  int height,
                  void **python_thread_state);
   void sync_view_layer(BL::SpaceView3D &b_v3d, BL::ViewLayer &b_view_layer);
-  vector<Pass> sync_render_passes(BL::RenderLayer &b_render_layer, BL::ViewLayer &b_view_layer);
+  vector<Pass> sync_render_passes(BL::RenderLayer &b_render_layer,
+                                  BL::ViewLayer &b_view_layer,
+                                  bool adaptive_sampling);
   void sync_integrator();
   void sync_camera(BL::RenderSettings &b_render,
                    BL::Object &b_override,