Attributes and Default Values

Problem Description

Generic algorithms like join_geometries and realize_instances create attribute data when merging geometries where one part has an attribute and the other doesn't. Example: copying a default Grease Pencil stroke with no explicit "hardness" attribute into another curves geometry. The "hardness" gets initialized to 0 but should be 1.

https://projects.blender.org/blender/blender/issues/120297

Workaround

Least intrusive solution for now: check attributes with known non-zero default values and initialize them before the join. This avoid the initialization to zero as the default attribute value. Problem: This recreates much of the recursion logic for realizing instances, defeating the purpose of a generic function and adding lots of opportunity for error.

General Solution

Define attribute defaults as an optional part of the attribute declaration.

• AttributeMetaData gets a GPointer default_value. Null for most attributes, meaning zero initialization.
• Only existing attributes have meta-data, so for creating common GP attributes like "hardness" the current utility functions to create attributes, like "opacities_for_write", are still needed.
  This is good! Makes the system usable for custom attributes as well: creating an attribute also defines its default value. Merging custom attributes works the same as "special" attributes like hardness.
• Meta-data is typically created on-the-fly by attribute providers, based on a fixed provider type (for built-in attributes) or the layer type in CustomData. Default values don't have storage there yet.
  Doesn't have to be added for every customized provider, behavior stays the same as current if no default value is provided.