A Group is collection of parts that snap together to make a single unit, such as:

• A single object (such as a car) built using multiple shapes (wheels, chassis, etc).
• A geographical region (e.g., a city block) whose various objects are only rendered if they are close enough or in line-of-sight to the camera
• A world's scene, level or zone, a large group which may contain other groups (such as the terrain, sky, and various regions) as its parts.
• Several objects whose behavior is closely linked (e.g., a flock of birds)

A group's 'parts' are rendered together and share the group's environment properties:

• Coordinate positioning, mapping this group's vertex coordinates to those of its parent. 'view' describes the technique to use and establishes the properties required to calculate the transformation, such as 'location' position, 'lookat' or 'rotation' orientation, and 'scale' factor. The group's parts are positioned relative to the group's coordinate system.
• Rendering instructions.
.render? is used to determine if the group is rendered and, if so, what shader uniforms apply to all parts (e.g., lighting or a default shader).
• Boundary shape that encloses all parts, used for culling and collision detection.

Properties (see Region)

boundary

TBD - the bounding box, sphere or cylinder for clipping and other purposes

orientation

A Quat specifying the rotation used to orient the shape in its parent's coordinate system. If not specified, no rotation takes place.

origin

The location of this shape's 0,0,0 position in the shape's parent's coordinate system. This can be used to specify where to render this shape within its parent group's space. If not specified, +Xyz(0,0,0) is assumed.

parts

Ordered list of all the group's positioned parts (groups, shapes, etc.).

render?

The method that is performed before rendering the group. The method should return 'false' if the group is not to be rendered. This method will have the render context as 'self', enabling the group to define rendering attributes that will apply to all its parts.

scale

An Xyz vector specifying how much to re-size the shape's vertices when rendering in the parent group, as defined by the 'view' method. The default is no scaling: +Xyz(1.,1.,1.)