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Entities

  • Artikkeli

An Entity represents a movable object in space and is the fundamental building block of remotely rendered content.

Entity properties

Entities have a transform defined by a position, rotation, and scale. By themselves entities don't have any observable functionality. Instead, behavior is added through components, which are attached to entities. For instance, attaching a CutPlaneComponent creates a cut plane at the position of the entity.

The most important aspect of the entity itself is the hierarchy and the resulting hierarchical transform. For example, when multiple entities are attached as children to a shared parent entity, all of these entities can be moved, rotated, and scaled in unison by changing the transform of the parent entity. Also, the entity's enabled state can be used to turn off visibility and responses to ray casts for a full sub graph in the hierarchy.

An entity is uniquely owned by its parent, meaning that when the parent is destroyed with Entity.Destroy(), so are its children and all connected components. Thus, removing a model from the scene is accomplished by calling Destroy on the root node of a model, returned by RenderingSession.Connection.LoadModelAsync() or its SAS variant RenderingSession.Connection.LoadModelFromSasAsync().

Entities are created when the server loads content or when the user wants to add an object to the scene. For example, if a user wants to add a cut plane to visualize the interior of a mesh, the user can create an entity where the plane should exist and then add the cut plane component to it.

Create an entity

To add a new entity to the scene, for example to pass it as a root object for loading models or attaching components to it, use the following code:

Entity CreateNewEntity(RenderingSession session)
{
    Entity entity = session.Connection.CreateEntity();
    entity.Position = new LocalPosition(1, 2, 3);
    return entity;
}

ApiHandle<Entity> CreateNewEntity(ApiHandle<RenderingSession> session)
{
    ApiHandle<Entity> entity(nullptr);
    if (auto entityRes = session->Connection()->CreateEntity())
    {
        entity = entityRes.value();
        entity->SetPosition(Double3{ 1, 2, 3 });
        return entity;
    }
    return entity;
}

Query functions

There are two types of query functions on entities: synchronous and asynchronous calls. Synchronous queries can only be used for data that is present on the client and doesn't involve much computation. Examples are querying for components, relative object transforms, or parent/child relationships. Asynchronous queries are used for data that only resides on the server or involves extra computation that would be too expensive to run on the client. Examples are spatial bounds queries or meta data queries.

Querying components

To find a component of a specific type, use FindComponentOfType:

CutPlaneComponent cutplane = (CutPlaneComponent)entity.FindComponentOfType(ObjectType.CutPlaneComponent);

// or alternatively:
CutPlaneComponent cutplane = entity.FindComponentOfType<CutPlaneComponent>();

ApiHandle<CutPlaneComponent> cutplane = entity->FindComponentOfType(ObjectType::CutPlaneComponent)->as<CutPlaneComponent>();

// or alternatively:
ApiHandle<CutPlaneComponent> cutplane = entity->FindComponentOfType<CutPlaneComponent>();

Querying transforms

Transform queries are synchronous calls on the object. It's important to note that transforms stored on the API side are local space transforms, relative to the object's parent. Exceptions are root objects, for which local space and world space are identical.

// local space transform of the entity
Double3 translation = entity.Position;
Quaternion rotation = entity.Rotation;
Float3 scale = entity.Scale;

// local space transform of the entity
Double3 translation = entity->GetPosition();
Quaternion rotation = entity->GetRotation();
Float3 scale = entity->GetScale();

In case all tree transform components (position, rotation and scale) need to be retrieved or set simultaneously, it's recommended to use the entity's LocalTransform property:

// local space transform of the entity
Transform localTransform = entity.LocalTransform;
Double3 translation = localTransform.Position;
Quaternion rotation = localTransform.Rotation;
Float3 scale = localTransform.Scale;

// local space transform of the entity
Transform localTransform = entity->GetLocalTransform();
Double3& translation = localTransform.Position;
Quaternion& rotation = localTransform.Rotation;
Float3& scale = localTransform.Scale;

There's also a helper function to retrieve an entity's global (world space) transform:

// global space transform of the entity
Transform globalTransform = entity.GlobalTransform;
Double3 translation = globalTransform.Position;

// global space transform of the entity
Transform globalTransform = entity->GetGlobalTransform();
Double3& translation = globalTransform.Position;

When GlobalTransform is called, the global transform is computed on-the-fly by traversing up the entity hierarchy. This traversal involves significant computation, but compared to doing the same operations on the client side through class Entity, the built-in function is faster. Still, calling GlobalTransform on a larger set of entities might impose a performance bottleneck.

LocalToGlobalMatrix is a variant of GlobalTransform that computes the global transform as a matrix, which is convenient in the context of Unity:

UnityEngine.Matrix4x4 globalMatrix = entity.LocalToGlobalMatrix.toUnity();
UnityEngine.Vector3 localPos = new UnityEngine.Vector3(0, 0, 0);
UnityEngine.Vector3 globalPos = globalMatrix.MultiplyPoint(localPos);

Querying spatial bounds

Bounds queries are asynchronous calls that operate on a full object hierarchy, using one entity as a root. See the dedicated chapter about object bounds.

Querying metadata

Metadata is extra data stored on objects that is ignored by the server. Object metadata is essentially a set of (name, value) pairs, where value can be of numeric, boolean or string type. Metadata can be exported with the model.

Metadata queries are asynchronous calls on a specific entity. The query only returns the metadata of a single entity, not the merged information of a sub graph.

Task<ObjectMetadata> metaDataQuery = entity.QueryMetadataAsync();
ObjectMetadata metaData = await metaDataQuery;
ObjectMetadataEntry entry = metaData.GetMetadataByName("MyInt64Value");
System.Int64 intValue = entry.AsInt64;
// ...

entity->QueryMetadataAsync([](Status status, ApiHandle<ObjectMetadata> metaData) 
{
    if (status == Status::OK)
    {
        ApiHandle<ObjectMetadataEntry> entry = *metaData->GetMetadataByName("MyInt64Value");
        int64_t intValue = *entry->GetAsInt64();

        // ...
    }
});

The query succeeds even if the object doesn't hold any metadata.

API documentation

Next steps