Geometry Nodes: Only create instance IDs when they exist

Instance IDs serve no purpose for rendering when they aren't stable from
one frame to the next, and if the index is used in the end anyway, there
is no point in storing a vector of IDs and copying it around.

This commit exposes the `id` attribute on the instances component,
makes it optional-- only generated by default with the distribute points
on faces node.

Since the string to curves node only added the index as each instance's
ID, I removed it. This means that it would be necessary to add the ID
data manually if the initial index actually helps (when deleting only
certain characters, for example).

Differential Revision: https://developer.blender.org/D12980

1 files changed, 108 insertions, 9 deletions

diff --git a/source/blender/blenkernel/intern/geometry_component_instances.cc b/source/blender/blenkernel/intern/geometry_component_instances.cc
index 4204d62e1a7..d02121b44a6 100644
--- a/source/blender/blenkernel/intern/geometry_component_instances.cc
+++ b/source/blender/blenkernel/intern/geometry_component_instances.cc
@@ -60,7 +60,9 @@ void InstancesComponent::reserve(int min_capacity)
 {
   instance_reference_handles_.reserve(min_capacity);
   instance_transforms_.reserve(min_capacity);
-  instance_ids_.reserve(min_capacity);
+  if (!instance_ids_.is_empty()) {
+    this->instance_ids_ensure();
+  }
 }
 
 /**
@@ -73,7 +75,9 @@ void InstancesComponent::resize(int capacity)
 {
   instance_reference_handles_.resize(capacity);
   instance_transforms_.resize(capacity);
-  instance_ids_.resize(capacity);
+  if (!instance_ids_.is_empty()) {
+    this->instance_ids_ensure();
+  }
 }
 
 void InstancesComponent::clear()
@@ -85,15 +89,15 @@ void InstancesComponent::clear()
   references_.clear();
 }
 
-void InstancesComponent::add_instance(const int instance_handle,
-                                      const float4x4 &transform,
-                                      const int id)
+void InstancesComponent::add_instance(const int instance_handle, const float4x4 &transform)
 {
   BLI_assert(instance_handle >= 0);
   BLI_assert(instance_handle < references_.size());
   instance_reference_handles_.append(instance_handle);
   instance_transforms_.append(transform);
-  instance_ids_.append(id);
+  if (!instance_ids_.is_empty()) {
+    this->instance_ids_ensure();
+  }
 }
 
 blender::Span<int> InstancesComponent::instance_reference_handles() const
@@ -125,6 +129,22 @@ blender::Span<int> InstancesComponent::instance_ids() const
 }
 
 /**
+ * Make sure the ID storage size matches the number of instances. By directly resizing the
+ * component's vectors internally, it is possible to be in a situation where the IDs are not
+ * empty but they do not have the correct size; this function resolves that.
+ */
+blender::MutableSpan<int> InstancesComponent::instance_ids_ensure()
+{
+  instance_ids_.append_n_times(0, this->instances_amount() - instance_ids_.size());
+  return instance_ids_;
+}
+
+void InstancesComponent::instance_ids_clear()
+{
+  instance_ids_.clear_and_make_inline();
+}
+
+/**
  * With write access to the instances component, the data in the instanced geometry sets can be
  * changed. This is a function on the component rather than each reference to ensure `const`
  * correctness for that reason.
@@ -327,8 +347,16 @@ static blender::Array<int> generate_unique_instance_ids(Span<int> original_ids)
 blender::Span<int> InstancesComponent::almost_unique_ids() const
 {
   std::lock_guard lock(almost_unique_ids_mutex_);
-  if (almost_unique_ids_.size() != instance_ids_.size()) {
-    almost_unique_ids_ = generate_unique_instance_ids(instance_ids_);
+  if (instance_ids().is_empty()) {
+    almost_unique_ids_.reinitialize(this->instances_amount());
+    for (const int i : almost_unique_ids_.index_range()) {
+      almost_unique_ids_[i] = i;
+    }
+  }
+  else {
+    if (almost_unique_ids_.size() != instance_ids_.size()) {
+      almost_unique_ids_ = generate_unique_instance_ids(instance_ids_);
+    }
   }
   return almost_unique_ids_;
 }
@@ -398,11 +426,82 @@ class InstancePositionAttributeProvider final : public BuiltinAttributeProvider
   }
 };
 
+class InstanceIDAttributeProvider final : public BuiltinAttributeProvider {
+ public:
+  InstanceIDAttributeProvider()
+      : BuiltinAttributeProvider(
+            "id", ATTR_DOMAIN_POINT, CD_PROP_INT32, Creatable, Writable, Deletable)
+  {
+  }
+
+  GVArrayPtr try_get_for_read(const GeometryComponent &component) const final
+  {
+    const InstancesComponent &instances = static_cast<const InstancesComponent &>(component);
+    if (instances.instance_ids().is_empty()) {
+      return {};
+    }
+    return std::make_unique<fn::GVArray_For_Span<int>>(instances.instance_ids());
+  }
+
+  GVMutableArrayPtr try_get_for_write(GeometryComponent &component) const final
+  {
+    InstancesComponent &instances = static_cast<InstancesComponent &>(component);
+    if (instances.instance_ids().is_empty()) {
+      return {};
+    }
+    return std::make_unique<fn::GVMutableArray_For_MutableSpan<int>>(instances.instance_ids());
+  }
+
+  bool try_delete(GeometryComponent &component) const final
+  {
+    InstancesComponent &instances = static_cast<InstancesComponent &>(component);
+    if (instances.instance_ids().is_empty()) {
+      return false;
+    }
+    instances.instance_ids_clear();
+    return true;
+  }
+
+  bool try_create(GeometryComponent &component, const AttributeInit &initializer) const final
+  {
+    InstancesComponent &instances = static_cast<InstancesComponent &>(component);
+    if (instances.instances_amount() == 0) {
+      return false;
+    }
+    MutableSpan<int> ids = instances.instance_ids_ensure();
+    switch (initializer.type) {
+      case AttributeInit::Type::Default: {
+        ids.fill(0);
+        break;
+      }
+      case AttributeInit::Type::VArray: {
+        const GVArray *varray = static_cast<const AttributeInitVArray &>(initializer).varray;
+        varray->materialize_to_uninitialized(IndexRange(varray->size()), ids.data());
+        break;
+      }
+      case AttributeInit::Type::MoveArray: {
+        void *source_data = static_cast<const AttributeInitMove &>(initializer).data;
+        ids.copy_from({static_cast<int *>(source_data), instances.instances_amount()});
+        MEM_freeN(source_data);
+        break;
+      }
+    }
+    return true;
+  }
+
+  bool exists(const GeometryComponent &component) const final
+  {
+    const InstancesComponent &instances = static_cast<const InstancesComponent &>(component);
+    return !instances.instance_ids().is_empty();
+  }
+};
+
 static ComponentAttributeProviders create_attribute_providers_for_instances()
 {
   static InstancePositionAttributeProvider position;
+  static InstanceIDAttributeProvider id;
 
-  return ComponentAttributeProviders({&position}, {});
+  return ComponentAttributeProviders({&position, &id}, {});
 }
 }  // namespace blender::bke