OpenSceneGraph源码分析：osgQt模块

济南友泉软件有限公司

osgQt基于QGLWidget实现了在Qt窗口内OSG渲染操作。Qt以其开源、跨平台、方便快捷、现代化的界面风格等优点，已经成为了目前桌面版CAD/CAE/CAM等软件开发的首选组件。因此，非常有必要在OSG的基础之上，研究Qt桌面系统内集成OSG渲染功能的相关技术。

注1：文章内容会不定期更新。

注2：限于笔者研究水平，难免有表述不当之处，欢迎批评指正。

零、系统配置

一、编译、安装

在进行osgQt编译安装之前，需要完成OSG的编译安装，可以参照笔者前面的博客。完成编译安装之后，便可编译安装osgQt。

1.1 下载代码

首先，需要从GitHub上下载osgQt,

1.2 构建项目

打开CMake，按照下表构建项目配置，

1.3 编译安装

打开生成的osgQt.sln，构建ALL_BUILD项目完成编译；构建“INSTALL”项目完成安装，

二、代码分析

从整体上来看，osgQt实际上是仿照osgViewer::run()函数，借助于QTimer、QGLWidget完成了在Qt窗口内渲染OSG场景。

    2.1 HeartBeat

HeartBeat是个单例类，借助于QTimer，触发OSG场景渲染。与osgViewer::run()实现相似，HeartBeat::timeEvent(QTimeEvent*)也实现了渲染帧率控制的功能(相当于控制”渲染频率“的节流阀)。

为了触发OSG场景渲染，首先需要调用HeartBeat::init( osgViewer::ViewerBase *viewer )，这样便可将QTimer绑定到osgViewer::ViewerBase，进而不断的触发场景的渲染。

   2.2 osgQt::GraphicsWindowQt

从运行原理上来看，OSG实际上是对OpenGL状态机采用C  进行了封装，而OpenGL在底层使用图形上下文（Graphics Context）来描述状态机相关信息。使用OpenGL必须要首先指定图形上下文。osgQt::GraphicsWindowQt就是用来提供QGLWidget的图形上下文。

Ref. from OpenGL Wiki =====================================================

An OpenGL context represents many things. A context stores all of the state associated with this instance of OpenGL. It represents the (potentially visible) default framebuffer that rendering commands will draw to when not drawing to a framebuffer object. Think of a context as an object that holds all of OpenGL; when a context is destroyed, OpenGL is destroyed.

Contexts are localized within a particular process of execution (an application, more or less) on an operating system. A process can create multiple OpenGL contexts. Each context can represent a separate viewable surface, like a window in an application.

Each context has its own set of OpenGL Objects, which are independent of those from other contexts. A context's objects can be shared with other contexts. Most OpenGL objects are sharable, including Sync Objects and GLSL Objects. Container Objects are not sharable, nor are Query Objects.

Any object sharing must be made explicitly, either as the context is created or before a newly created context creates any objects. However, contexts do not have to share objects; they can remain completely separate from one another.

In order for any OpenGL commands to work, a context must be current; all OpenGL commands affect the state of whichever context is current. The current context is a thread-local variable, so a single process can have several threads, each of which has its own current context. However, a single context cannot be current in multiple threads at the same time.

=====================================================Ref. from OpenGL Wiki

  2.3 osg::GLWidget

osg::GLWidget在QWidget渲染功能基础之上，将Qt键盘、鼠标等事件转化为OSG事件，并触发相关事件函数的调用。

附录A：常见问题

Q1. 在Release配置下，编译osgQt可能会报以下问题，

A1. 在osgQt项目属性中，去掉"debug.lib"、“optimized.lib"等库引用，即可顺利完成编译。

网络资料

osgQt

https://github.com/openscenegraph/osgQt

OpenGL Wiki 

https://www.khronos.org/opengl/wiki/

OpenSceneGraph编译、安装、开发环境    

https://blog.csdn.net/qq_26221775?spm=1010.2135.3001.5343