Cycles: Added Shadow terminator offset.

This copies the approach taken by the Appleseed renderer. A per-object
factor that pushes the shadow terminator in the BSDF towards the light
source.

intern/cycles/blender/addon/properties.py

@@ -1138,6 +1138,13 @@ class CyclesObjectSettings(bpy.types.PropertyGroup):
         default=1.0,
     )
 
+    terminator_offset: FloatProperty(
+        name="Shadow Termiantor 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

@@ -1212,8 +1212,11 @@ def draw(self, context):
         col = flow.column()
         col.prop(ob, "hide_select", text="Selectable", invert_checkbox=True, toggle=False)
 
+        cob = ob.cycles
+        col = flow.column()
+        col.prop(cob, "terminator_offset")
+
         if has_geometry_visibility(ob):
-            cob = ob.cycles
             col = flow.column()
             col.prop(cob, "is_shadow_catcher")
             col = flow.column()

intern/cycles/blender/blender_object.cpp

@@ -430,6 +430,12 @@ Object *BlenderSync::sync_object(BL::Depsgraph &b_depsgraph,
     object_updated = true;
   }
 
+  float terminator_offset = get_float(cobject, "terminator_offset");
+   if (terminator_offset != object->terminator_offset) {
+     object->terminator_offset = 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

@@ -95,6 +95,34 @@ ccl_device_inline float bump_shadowing_term(float3 Ng, float3 N, float3 I)
   return -g2 * g + g2 + g;
 }
 
+/* Another shadow terminator fix, this time taken from Appleseed.
+ * Original code is unter the MIT License
+ * Copyright (c) 2019 Francois Beaune, The appleseedhq Organization */
+ccl_device_inline float shift_cos_in(const float cos_in, const float correction_factor)
+{
+    assert(std::abs(cos_in) <= 1.0f);
+    assert(correction_factor >= 0.0f && correction_factor < 1.0f);
+
+    if (correction_factor == 0.0f)
+        return cos_in;
+
+    const float angle = fast_acosf(cos_in);
+    const float frequency_multiplier = 1.0f / (1.0f - correction_factor);
+    return max(cosf(angle * frequency_multiplier), 0.0f);
+}
+
+ccl_device_inline float shift_cos_in_fast(const float cos_in, const float frequency_multiplier)
+{
+    assert(fabsf(cos_in) <= 1.0f);
+    assert(frequency_multiplier >= 1.0f);
+
+    if (frequency_multiplier == 1.0f)
+        return cos_in;
+
+    const float angle = fast_acosf(cos_in);
+    return max(cosf(angle * frequency_multiplier), 0.0f);
+}
+
 ccl_device_inline int bsdf_sample(KernelGlobals *kg,
                                   ShaderData *sd,
                                   const ShaderClosure *sc,
@@ -450,6 +478,12 @@ ccl_device_inline int bsdf_sample(KernelGlobals *kg,
     if (!isequal_float3(sc->N, sd->N)) {
       *eval *= bump_shadowing_term((label & LABEL_TRANSMIT) ? -sd->N : sd->N, sc->N, *omega_in);
     }
+    if (!(label & LABEL_TRANSMIT) && !isequal_float3(sd->Ng, sd->N)) {
+      const float frequency_multiplier = kernel_tex_fetch(__objects, sd->object).terminator_offset;
+      if(frequency_multiplier > 0.0f) {
+        *eval *= shift_cos_in_fast(dot(*omega_in, sd->N), frequency_multiplier);
+      }
+    }
   }
 
   return label;
@@ -566,6 +600,12 @@ ccl_device_inline
       if (!isequal_float3(sc->N, sd->N)) {
         eval *= bump_shadowing_term(sd->N, sc->N, omega_in);
       }
+      if (!isequal_float3(sd->Ng, sd->N)) {
+        const float frequency_multiplier = kernel_tex_fetch(__objects, sd->object).terminator_offset;
+        if(frequency_multiplier > 0.0f) {
+          eval *= shift_cos_in_fast(dot(omega_in, sd->N), frequency_multiplier);
+        }
+      }
     }
   }
   else {
@@ -657,6 +697,12 @@ ccl_device_inline
       if (!isequal_float3(sc->N, sd->N)) {
         eval *= bump_shadowing_term(-sd->N, sc->N, omega_in);
       }
+      if (isequal_float3(sd->Ng, sd->N)) {
+        const float frequency_multiplier = kernel_tex_fetch(__objects, sd->object).terminator_offset;
+        if(frequency_multiplier > 0.0f) {
+          eval *= shift_cos_in_fast(dot(omega_in, -sd->N), frequency_multiplier);
+        }
+      }
     }
   }
 

intern/cycles/kernel/kernel_types.h

@@ -1459,6 +1459,9 @@ typedef struct KernelObject {
 
   float cryptomatte_object;
   float cryptomatte_asset;
+
+  float terminator_offset;
+  float pad1, pad2, pad3;
 } KernelObject;
 static_assert_align(KernelObject, 16);
 

intern/cycles/render/object.cpp

@@ -98,6 +98,7 @@ 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(terminator_offset, "Terminator Offset", 0.0f);
 
   SOCKET_BOOLEAN(is_shadow_catcher, "Shadow Catcher", false);
 
@@ -497,6 +498,7 @@ 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.terminator_offset = 1.0f / (1.0f - ob->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 terminator_offset;
 
   float3 dupli_generated;
   float2 dupli_uv;