wave5-write-group-custom-transform

WriteGroup Custom Transform

Intent

Bypass the standard Unity transform system for specific entities by registering a custom component in the LocalToWorld write group, allowing a developer-owned system to compute LocalToWorld for those entities.

Use When

• Non-standard transform type needed: 2D, billboard, procedural, skeletal
• Standard transform pipeline must be completely replaced for a subset of entities

Avoid When

• Only post-processing after the standard transform is needed — use a system [UpdateAfter(TransformSystemGroup)] instead
• The entity has a parent — custom system must handle parent hierarchy propagation manually

Senior Pattern

// 1. Custom component registers in the write group:
[WriteGroup(typeof(LocalToWorld))]
public struct LocalTransform2D : IComponentData
{
    public float2 Position;
    public float Rotation;
}

// 2. Baker uses ManualOverride:
public class LocalTransform2DAuthoring : MonoBehaviour { }
public class LocalTransform2DBaker : Baker<LocalTransform2DAuthoring>
{
    public override void Bake(LocalTransform2DAuthoring authoring)
    {
        var entity = GetEntity(TransformUsageFlags.ManualOverride);
        AddComponent(entity, new LocalTransform2D { });
        AddComponent(entity, new LocalToWorld());
    }
}

// 3. Custom system in TransformSystemGroup after ParentSystem:
[UpdateInGroup(typeof(TransformSystemGroup))]
[UpdateAfter(typeof(ParentSystem))]
[BurstCompile]
public partial struct LocalToWorld2DSystem : ISystem
{
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        state.Dependency = new Compute2DJob().ScheduleParallel(state.Dependency);
    }
}

[BurstCompile]
public partial struct Compute2DJob : IJobEntity
{
    [BurstCompile]
    public void Execute(in LocalTransform2D t2d, ref LocalToWorld ltw)
    {
        float cos = math.cos(t2d.Rotation);
        float sin = math.sin(t2d.Rotation);
        ltw.Value = new float4x4(
            new float4(cos, sin, 0, 0),
            new float4(-sin, cos, 0, 0),
            new float4(0, 0, 1, 0),
            new float4(t2d.Position.x, t2d.Position.y, 0, 1)
        );
    }
}