Hierarchy and scene graph support

[jakeklassen]

Context

The shmup demo uses a simple parent-child pattern for hierarchy, but it doesn't handle deep hierarchies, processing order, rotation/scale inheritance, or attach/detach operations. Inspired by Wicked Engine's ECS article.

Current State

// Simple parent reference + manual position update
parent?: SetRequired<Entity, "transform">;
localTransform?: Transform;

entity.transform.position.x =
  entity.parent.transform.position.x + entity.localTransform.position.x;

Potential Approaches

1. Depth-based Sorting (Pattern-based)

Add hierarchyDepth to entities and sort before processing. Simple, fits objECS philosophy, but O(n log n) sorting per frame.

2. SceneGraph Class (Structured)

Dedicated hierarchy manager alongside the ECS with attach/detach/traverse (topological order). Could be published as objecs-hierarchy package.

3. Matrix-based Transforms (Full Scene Graph)

For proper rotation/scale inheritance using matrix math. Requires a small math library.

Challenges

Wicked Engine	objECS
Integer entity IDs	Object references
Ordered arrays per component	Unordered collections
Component managers with sorting	Simple archetypes

Open Questions

• Should SceneGraph automatically sync with World entity deletions?
• Should transforms be split into localTransform and worldTransform always?
• How to handle dirty flags / caching for performance?
• Should hierarchy be query-able via archetypes? (e.g., world.archetype("parent"))
• Archetypes may need to support sorting for this to work well

References

• Wicked Engine ECS
• Unity's Transform hierarchy
• Bevy's Parent/Children components