Cycles: Add option to split curve motion primitives by time steps

The idea is to create several smaller BVH nodes for each of the motion
curve primitives. This acts as a forced spatial split for the single
primitive.

This gives up render time speedup of motion blurred hair in the cost
of extra memory usage. The numbers goes as:

BVH Steps     Render time (sec)       Memory usage (MB)
    0               258                    191
    1               123                    278
    2                69                    453
    3                43                    627

Scene used for the tests is the agent's hair from one of the barber
shop scenes.

Currently it's only limited to scenes without spatial split enabled,
since the spatial split builder requires some changes to work properly
with motion steps coordinates.

3 files changed, 11 insertions, 0 deletions

diff --git a/intern/cycles/blender/addon/properties.py b/intern/cycles/blender/addon/properties.py
index 3616b13e751..802b9b76c5d 100644
--- a/intern/cycles/blender/addon/properties.py
+++ b/intern/cycles/blender/addon/properties.py
@@ -528,6 +528,12 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
                 description="Use special type BVH optimized for hair (uses more ram but renders faster)",
                 default=True,
                 )
+        cls.debug_bvh_time_steps = IntProperty(
+                name="BVH Time Steps",
+                description="Split BVH primitives by this number of time steps to speed up render time in cost of memory",
+                default=0,
+                min=0, max=16,
+                )
         cls.tile_order = EnumProperty(
                 name="Tile Order",
                 description="Tile order for rendering",
diff --git a/intern/cycles/blender/addon/ui.py b/intern/cycles/blender/addon/ui.py
index 53bd208c0e7..d26ab73fac3 100644
--- a/intern/cycles/blender/addon/ui.py
+++ b/intern/cycles/blender/addon/ui.py
@@ -432,6 +432,10 @@ class CyclesRender_PT_performance(CyclesButtonsPanel, Panel):
         col.prop(cscene, "debug_use_spatial_splits")
         col.prop(cscene, "debug_use_hair_bvh")
 
+        row = col.row()
+        row.active = not cscene.debug_use_spatial_splits
+        row.prop(cscene, "debug_bvh_time_steps")
+
 
 class CyclesRender_PT_layer_options(CyclesButtonsPanel, Panel):
     bl_label = "Layer"
diff --git a/intern/cycles/blender/blender_sync.cpp b/intern/cycles/blender/blender_sync.cpp
index 38b2ce19e8a..a9b7d7d3583 100644
--- a/intern/cycles/blender/blender_sync.cpp
+++ b/intern/cycles/blender/blender_sync.cpp
@@ -498,6 +498,7 @@ SceneParams BlenderSync::get_scene_params(BL::Scene& b_scene,
 
 	params.use_bvh_spatial_split = RNA_boolean_get(&cscene, "debug_use_spatial_splits");
 	params.use_bvh_unaligned_nodes = RNA_boolean_get(&cscene, "debug_use_hair_bvh");
+	params.num_bvh_motion_curve_steps = RNA_int_get(&cscene, "debug_bvh_time_steps");
 
 	if(background && params.shadingsystem != SHADINGSYSTEM_OSL)
 		params.persistent_data = r.use_persistent_data();