In retained mode, Graphics3D renders a scene graph,
which may range from one node containing a simple mesh to very complex scenes
that include multiple meshes, textures, animations, lights, etc. Using scene
graphs makes it easier to break a scene into logical parts and create groups
for different parts of an application. Having a scene graph built into the
API also eases the integration with models built using 3-D modeling tools.
A complete scene graph must include the World,
which is the top of the hierarchy, and a Camera. Normally
it will also include a Background, Lights
if necessary, and Meshes of the scene. Animations can
also be included in the scene graph associated with different Nodes.
They can be advanced individually or all at once by calling the animate method
in the top World object of the scene graph. It is also
possible to build a partial scene graph using Group and SkinnedMesh nodes.
The class Object3D is the parent of all the classes
that can be part of the 3-D world. An important property of Object3D is
the userID, which can be used to find individual objects
in a complex graph. Third-party modeling tools can set this userID so
that the designer and programmer can easily work in the same object. Object3D also
includes support for animations, with methods to add and remove AnimationTracks
and to advance the animations.
The scene graph itself is composed by Node objects,
which include the classes Camera, Mesh (including SkinnedMesh and MorphingMesh), Sprite3D, and Light. A Node is
the type of object commonly manipulated in an application. The rendering and
picking of nodes can be enabled or disabled individually. Nodes
can also be manipulated with the usual translation, rotation, and scaling
transformations. Besides the userID, Nodes
can have a scopeID, which can be used to group objects
across different parts of a scene graph for lighting and picking purposes.
A Mesh is a 3-D geometrical object displayed in
a scene. A mesh normally includes the positions of the vertices, colors, and
normals, as well as texture coordinates. It also includes information about
the edges between vertices. The rendering of Meshes is
regulated by their Appearance. This comprises several
properties such as material attributes, textures, compositing mode, etc. A Mesh is
lit only when its Appearance contains a Material definition.
Developers can set this to null to enabled and disable lighting for a particular
object.
Nodes are put together in Groups. Groups
include the ability to perform a picking operation among
the nodes in the group. The World is also a Group,
with the extra properties as indicated above.
A good way to see a scene graph is by making a diagram of it. The figure in section Implementation of the Scene Graph shows how the scene graph looks in the Maze3D game.