Transform concepts
You can use the Unity.Transforms namespace to control the position, rotation, and scale of any entity in your project.
The following components are used in the transform system:
There are two systems that update the transform system:
Transform hierarchy
Unity.Transforms is hierarchical, which means that you can transform Entities based on their relationship to each other.
For example, a car body can be the parent of its wheels. The wheels are children of the car body. When the car body moves, the wheels move with it. You can also move and rotate the wheels relative to the car body.
An entity can have multiple children, but only one parent. Children can have their own child entities. These multiple levels of parent-child relationships form a transform hierarchy. The entity at the top of a hierarchy, without a parent, is the root.
To declare a Transform hierarchy, you must do this from the bottom up. This means that you use Parent to declare an Entity's parent, rather than declare its children. If you want to declare a child of an Entity, find the Entities that you want to be children and set their parent to be the target Entity. For more information, see the Using a hierarchy documentation.
The LocalToWorld component
The LocalToWorld matrix represents the transform from local space to world space. This matrix is what the rendering system uses to render the entity's geometry. By default, the LocalToWorldSystem updates this component from the LocalToWorld component. This default update means that you don't need to update LocalToWorld, and the system does that for you. To disable this automatic update use a [WriteGroup(typeof(LocalToWorld))] attribute on a component. With this write group, you have complete control over LocalToWorld. For more information, see the documentation on write groups.
Note
The LocalToWorld component value might be out of date or invalid while the SimulationSystemGroup is running. This is because the transform system only updates the component value when the TransformSystemGroup runs. It might also contain additional offsets applied for graphical smoothing purposes. Therefore, while the LocalToWorld component might be useful as a fast approximation of an entity's world-space transformation when its latency is acceptable, you shouldn't rely on it if you need an accurate, up-to-date world transform for simulation purposes. In those cases, use the ComputeWorldTransformMatrix method.
The LocalTransform component
The LocalTransform component has three properties. They control the position, rotation, and scale of the entity. When this component is present, it controls LocalToWorld:
public struct LocalTransform : IComponentData
{
public float3 Position;
public float Scale;
public quaternion Rotation;
}
If the entity has a Parent component, Position, Rotation, and Scale are relative to that parent. If the entity doesn't have a Parent component, the transform is relative to the origin of the world.
The PostTransformMatrix component
LocalTransform only supports uniform scale. To render geometry with nonuniform scale, you must use a PostTransformMatrix matrix. This is applied after LocalTransform. You can also use this component to introduce shear transforms, or to translate the entity relative to its pivot.
The Parent component
The Parent component defines the hierarchy. You put this on every child that you want to be part of a hierarchy.
The Child component
The Child component buffer holds all children of a parent. ParentSystem manages this buffer and its contents. You only need to manage the Parent component. The system will take care of maintaining the corresponding Child component.
The ParentSystem
The ParentSystem maintains the Child component buffer, based on the Parent component of the children. When you set a Parent component on a child, Unity only updates the parent's Child component when the ParentSystem has run.
The LocalToWorldSystem
LocalToWorldSystem computes and updates the LocalToWorld component, based on the LocalTransform component and the hierarchy. When you set a LocalTransform component on an entity, Unity only updates its LocalToWorld component when the LocalToWorldSystem has run.