Geometry Nodes: Allow reading converted attribute directly from spline

Often it would be beneficial to avoid the virtual array implementation
in `geometry_component_curve.cc` that flattens an attribute for every
spline and instead read an attribute separately for every input spline.
This commit implements functions to do that.

The downside is some code duplication-- we now have two places handling
this conversion. However, we can head in this general direction for the
attribute API anyway and support accessing attributes in smaller
contiguous chunks where necessary.

No functional changes in this commit.

Differential Revision: https://developer.blender.org/D11456

1 files changed, 28 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/attribute_access.cc b/source/blender/blenkernel/intern/attribute_access.cc
index d36e9ed3e86..1495eb23254 100644
--- a/source/blender/blenkernel/intern/attribute_access.cc
+++ b/source/blender/blenkernel/intern/attribute_access.cc
@@ -46,6 +46,8 @@ using blender::StringRef;
 using blender::StringRefNull;
 using blender::fn::GMutableSpan;
 using blender::fn::GSpan;
+using blender::fn::GVArray_For_GSpan;
+using blender::fn::GVArray_For_SingleValue;
 
 namespace blender::bke {
 
@@ -628,6 +630,32 @@ std::optional<GSpan> CustomDataAttributes::get_for_read(const StringRef name) co
   return {};
 }
 
+/**
+ * Return a virtual array for a stored attribute, or a single value virtual array with the default
+ * value if the attribute doesn't exist. If no default value is provided, the default value for the
+ * type will be used.
+ */
+GVArrayPtr CustomDataAttributes::get_for_read(const StringRef name,
+                                              const CustomDataType data_type,
+                                              const void *default_value) const
+{
+  const CPPType *type = blender::bke::custom_data_type_to_cpp_type(data_type);
+
+  std::optional<GSpan> attribute = this->get_for_read(name);
+  if (!attribute) {
+    const int domain_size = this->size_;
+    return std::make_unique<GVArray_For_SingleValue>(
+        *type, domain_size, (default_value == nullptr) ? type->default_value() : default_value);
+  }
+
+  if (attribute->type() == *type) {
+    return std::make_unique<GVArray_For_GSpan>(*attribute);
+  }
+  const blender::nodes::DataTypeConversions &conversions =
+      blender::nodes::get_implicit_type_conversions();
+  return conversions.try_convert(std::make_unique<GVArray_For_GSpan>(*attribute), *type);
+}
+
 std::optional<GMutableSpan> CustomDataAttributes::get_for_write(const StringRef name)
 {
   BLI_assert(size_ != 0);