[Feature] Scene graph — parent-child hierarchy with propagated world transforms

[JeanPhilippeKernel]

Type

• Feature

Overview

The ECS currently has no parent-child hierarchy between entities. Every entity is a root — the Outliner's flat list is a direct consequence. Adding a scene graph unblocks tree-view display in the Outliner, grouped transforms (e.g. a vehicle and its wheels), and instanced prefab attachment.

What's missing

1. ParentComponent and ChildrenComponent

struct ParentComponent   { EntityID Parent = INVALID_ENTITY; };
struct ChildrenComponent { Core::Containers::Array<EntityID> Children; };

Nothing exists today to link entities in a hierarchy.

2. Local vs World transform split

TransformComponent currently stores one transform that is treated as both local and world (valid only because everything is a root). A scene graph needs:

• TransformComponent — local (relative to parent); what the Inspector edits
• WorldTransformComponent — computed by the propagation system; what the renderer reads

struct WorldTransformComponent {
    Core::Maths::Mat4f Matrix = {};  // ParentWorld × TRS(local)
};

3. TransformDirtyComponent or dirty flag

Without a dirty flag, all transforms are recomputed every frame. A bool Dirty = true on TransformComponent (or a zero-size tag component) allows the propagation system to skip unchanged subtrees.

4. Transform propagation WorldTick system

A system that walks the hierarchy in depth order and writes WorldTransformComponent:

Pass 1 — roots (no ParentComponent): WorldMatrix = TRS(local)
Pass 2 — depth-1 children:           WorldMatrix = Parent.WorldMatrix × TRS(local)
Pass N — repeat until no dirty flags remain

Topological sort or multi-pass with dirty flags; multi-pass is simpler to start.

5. Outliner tree view

Once ParentComponent exists, HierarchyViewUIComponent can render a tree (ImGui TreeNode) instead of a flat list. Drag-to-reparent becomes a context-menu or drag-drop operation.

Acceptance criteria

• ParentComponent and ChildrenComponent defined
• WorldTransformComponent defined; TransformComponent explicitly documented as local-only
• TransformPropagationSystem registered in WorldTick; world transforms correct for 3+ levels of nesting
• WorldTransformComponent is what FillRenderableTransforms and the render bridge read (not TransformComponent)
• Outliner renders a tree (indented children, collapse/expand toggle)
• Reparent via Outliner drag-drop or context menu
• All ECS and scheduler tests pass

Depends on

• [Task] Build Outliner UI wired to the ECS Actor system #604 (flat Outliner) — tree view is an incremental addition once the flat list lands

Estimated effort

3–4 days