Cycles: Added shadow terminator offset parameter.

A new user parameter can be used to shift the shadow terminator
towards the light source. With it, one can hide some of the
artifacts that appear on coarse meshes with smooth shading.

Note that this technique is not engery conserving.

This is based on the work by the Appleseed renderer team.

Differential Revision: https://developer.blender.org/D7634

intern/cycles/blender/addon/properties.py

@@ -1205,6 +1205,13 @@ class CyclesObjectSettings(bpy.types.PropertyGroup):
         default=1.0,
     )
 
+    shadow_terminator_offset: FloatProperty(
+        name="Shadow Terminator Offset",
+        description="Push the shadow terminator towards the light to hide artifacts on low poly geometry",
+        min=0.0, max=1.0,
+        default=0.0,
+    )
+
     is_shadow_catcher: BoolProperty(
         name="Shadow Catcher",
         description="Only render shadows on this object, for compositing renders into real footage",

intern/cycles/blender/addon/ui.py

@@ -1209,6 +1209,27 @@ def has_geometry_visibility(ob):
     return ob and ((ob.type in {'MESH', 'CURVE', 'SURFACE', 'FONT', 'META', 'LIGHT'}) or
                     (ob.instance_type == 'COLLECTION' and ob.instance_collection))
 
+class CYCLES_OBJECT_PT_shading(CyclesButtonsPanel, Panel):
+    bl_label = "Shading"
+    bl_context = "object"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    @classmethod
+    def poll(cls, context):
+        return  CyclesButtonsPanel.poll(context) and (context.object)
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+
+        flow = layout.grid_flow(row_major=False, columns=0, even_columns=True, even_rows=False, align=False)
+        layout = self.layout
+        ob = context.object
+        cob = ob.cycles
+
+        if has_geometry_visibility(ob):
+            col = flow.column()
+            col.prop(cob, "shadow_terminator_offset")
 
 class CYCLES_OBJECT_PT_visibility(CyclesButtonsPanel, Panel):
     bl_label = "Visibility"
@@ -2268,6 +2289,7 @@ classes = (
     CYCLES_CAMERA_PT_dof_aperture,
     CYCLES_PT_context_material,
     CYCLES_OBJECT_PT_motion_blur,
+    CYCLES_OBJECT_PT_shading,
     CYCLES_OBJECT_PT_visibility,
     CYCLES_OBJECT_PT_visibility_ray_visibility,
     CYCLES_OBJECT_PT_visibility_culling,

intern/cycles/blender/blender_object.cpp

@@ -238,6 +238,12 @@ Object *BlenderSync::sync_object(BL::Depsgraph &b_depsgraph,
     object_updated = true;
   }
 
+  float shadow_terminator_offset = get_float(cobject, "shadow_terminator_offset");
+  if (shadow_terminator_offset != object->shadow_terminator_offset) {
+    object->shadow_terminator_offset = shadow_terminator_offset;
+    object_updated = true;
+  }
+
   /* sync the asset name for Cryptomatte */
   BL::Object parent = b_ob.parent();
   ustring parent_name;

intern/cycles/kernel/closure/bsdf.h

@@ -97,6 +97,18 @@ ccl_device_inline float bump_shadowing_term(float3 Ng, float3 N, float3 I)
   return -g2 * g + g2 + g;
 }
 
+/* Shadow terminator workaround, taken from Appleseed.
+ * Original code is under the MIT License
+ * Copyright (c) 2019 Francois Beaune, The appleseedhq Organization */
+ccl_device_inline float shift_cos_in(float cos_in, const float frequency_multiplier)
+{
+  cos_in = min(cos_in, 1.0f);
+
+  const float angle = fast_acosf(cos_in);
+  const float val = max(cosf(angle * frequency_multiplier), 0.0f) / cos_in;
+  return val;
+}
+
 ccl_device_inline int bsdf_sample(KernelGlobals *kg,
                                   ShaderData *sd,
                                   const ShaderClosure *sc,
@@ -444,9 +456,16 @@ ccl_device_inline int bsdf_sample(KernelGlobals *kg,
       }
     }
   }
-  else if (label & LABEL_DIFFUSE) {
-    if (!isequal_float3(sc->N, sd->N)) {
-      *eval *= bump_shadowing_term((label & LABEL_TRANSMIT) ? -sd->N : sd->N, sc->N, *omega_in);
+  else {
+    /* Shadow terminator offset. */
+    const float frequency_multiplier = kernel_tex_fetch(__objects, sd->object).shadow_terminator_offset;
+    if (frequency_multiplier > 1.0f) {
+      *eval *= shift_cos_in(dot(*omega_in, sc->N), frequency_multiplier);
+    }
+    if (label & LABEL_DIFFUSE) {
+      if (!isequal_float3(sc->N, sd->N)) {
+        *eval *= bump_shadowing_term((label & LABEL_TRANSMIT) ? -sd->N : sd->N, sc->N, *omega_in);
+      }
     }
   }
 
@@ -561,6 +580,11 @@ ccl_device_inline
         eval *= bump_shadowing_term(sd->N, sc->N, omega_in);
       }
     }
+    /* Shadow terminator offset. */
+    const float frequency_multiplier = kernel_tex_fetch(__objects, sd->object).shadow_terminator_offset;
+    if (frequency_multiplier > 1.0f) {
+      eval *= shift_cos_in(dot(omega_in, sc->N), frequency_multiplier);
+    }
   }
   else {
     switch (sc->type) {

intern/cycles/kernel/kernel_types.h

@@ -1480,6 +1480,9 @@ typedef struct KernelObject {
 
   float cryptomatte_object;
   float cryptomatte_asset;
+
+  float shadow_terminator_offset;
+  float pad1, pad2, pad3;
 } KernelObject;
 static_assert_align(KernelObject, 16);
 

intern/cycles/render/object.cpp

@@ -101,6 +101,7 @@ NODE_DEFINE(Object)
   SOCKET_POINT(dupli_generated, "Dupli Generated", make_float3(0.0f, 0.0f, 0.0f));
   SOCKET_POINT2(dupli_uv, "Dupli UV", make_float2(0.0f, 0.0f));
   SOCKET_TRANSFORM_ARRAY(motion, "Motion", array<Transform>());
+  SOCKET_FLOAT(shadow_terminator_offset, "Terminator Offset", 0.0f);
 
   SOCKET_BOOLEAN(is_shadow_catcher, "Shadow Catcher", false);
 
@@ -534,6 +535,7 @@ void ObjectManager::device_update_object_transform(UpdateObjectTransformState *s
   uint32_t hash_asset = util_murmur_hash3(ob->asset_name.c_str(), ob->asset_name.length(), 0);
   kobject.cryptomatte_object = util_hash_to_float(hash_name);
   kobject.cryptomatte_asset = util_hash_to_float(hash_asset);
+  kobject.shadow_terminator_offset = 1.0f / (1.0f - 0.5f * ob->shadow_terminator_offset);
 
   /* Object flag. */
   if (ob->use_holdout) {

intern/cycles/render/object.h

@@ -59,6 +59,7 @@ class Object : public Node {
   bool hide_on_missing_motion;
   bool use_holdout;
   bool is_shadow_catcher;
+  float shadow_terminator_offset;
 
   float3 dupli_generated;
   float2 dupli_uv;