osgEarth的Rex引擎原理分析（四十一）osg::State和osg::StateSet的关系

本文链接：https://blog.youkuaiyun.com/hankern/article/details/86402163

本文详细分析了osgEarth中的osg::State和osg::StateSet的关系。osg::State作为全局的OpenGL状态机，负责在不同StateSet之间切换，以满足图形绘制需求。而osg::StateSet则是个体图形节点的状态集合，是OpenGL状态的子集，具有局部性。文章列举了一系列后续将要探讨的osgEarth相关问题，涵盖地理信息引擎、地形引擎、瓦片加载和资源管理等多个方面。

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目标：（三十四）中的101

osg::State是osg封装的opengl状态机，是全局的。osg状态机会在各个StateSet中切换，以完成图形的绘制。

osg/State
/** Encapsulates the current applied OpenGL modes, attributes and vertex arrays settings,
  * implements lazy state updating and provides accessors for querying the current state.
  * The venerable Red Book says that "OpenGL is a state machine", and this class
  * represents the OpenGL state in OSG. Furthermore, \c State also has other
  * important features:
  * - It works as a stack of states (see \c pushStateSet() and
  *   \c popStateSet()). Manipulating this stack of OpenGL states manually is
  *   seldom needed, since OSG does this in the most common situations.
  * - It implements lazy state updating. This means that, if one requests a
  *   state change and that particular state is already in the requested state,
  *   no OpenGL call will be made. This ensures that the OpenGL pipeline is not
  *   stalled by unnecessary state changes.
  * - It allows to query the current OpenGL state without calls to \c glGet*(),
  *   which typically stall the graphics pipeline (see, for instance,
  *   \c captureCurrentState() and \c getModelViewMatrix()).
  */
class OSG_EXPORT State : public Referenced

osg::StateSet是每一个图形节点的状态集，是opengl状态的子集，是局部的。

osg/StateSet
/** Stores a set of modes and attributes which represent a set of OpenGL