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Adding Parent-Child Relationships in Untold Engine

The Untold Engine includes a Scene Graph data structure, designed to manage hierarchical transformations efficiently. This enables parent-child relationships between entities, where a child's transformation (position, rotation, scale) is relative to its parent. For example, a car's wheels (children) move and rotate relative to the car body (parent).

Why Use Parent-Child Relationships?

Parent-child relationships are useful when you want multiple entities to move or transform together. When a parent entity changes its position, rotation, or scale, its child entities inherit those changes automatically. This is ideal for scenarios like:

  • A car (parent) and its wheels (children)
  • A robot (parent) with movable arms and legs (children)
  • A group of objects that should remain in a fixed configuration relative to each other

Assigning Parent-Child Relationships

To assign a parent to an entity, use the setParent function. This function establishes a hierarchical relationship between the specified entities.

// Create child and parent entities
let childEntity = createEntity()
let parentEntity = createEntity()

// Set parent-child relationship
setParent(childId: childEntity, parentId: parentEntity)

What Happens Behind the Scenes?

  1. Transformation Inheritance:
  2. Once the relationship is established, any transformation applied to the parent entity (e.g., movement, rotation) will automatically affect the child entity.

  3. The child’s transformation is expressed relative to the parent.

  4. Independent Local Transformations:

  5. While the child inherits the parent's transformations, it can also have its own independent local transformations, such as offset positions or rotations relative to the parent.