1. 分为以下几类:
(1) data classes: 用来访问scene中的geometry 和 shaders
（2）scene override： 主要是用户自定义的drawing 、shading和effects
（3）Rendering overrides： custom rendering, multi-pass effects, and filtering.
（4）Frameword classes：the objects that provide services and device state information to plug-ins.
2. data classes： 描述可以被渲染的几何体，被shapes用来描绘。。。。绕晕了。
shape和shade和handshake：
（1）The shape is queried for the list of render items it needs to render(including multiple material sub-geometries, wireframe selection, component display, etc).
（2）The shader is queried for its geometry requirements .