本文接着osg窗口的创建（一）中关于GraphicsContext的内容继续阅读。
还是先看看GraphicsContext中的继承图
由于是在Windows平台下分析窗口的创建，因此本文选取Win32中的实现来探讨，其他平台的原理类似，只是在细节上稍有不同。
前文说到了 isRealized的实现，我们在看看实现过程：
GraphicsContext的isRealized实现如下：
inline bool isRealized() const
{
return isRealizedImplementation();
} isRealizedImplementation函数在GraphicsContext中是一个纯虚函数，在GraphicsWindow中的实现仅仅打印出一个提示信息（等同于没实现），真正的实现在GraphicsWindowWin32之中，实现如下：
virtual bool GraphicsWindowWin32::isRealizedImplementation() const
{
return _realized;
}继续看frame中的实现：
if (!isRealized())
{
realize();
}1. realize在ViewerBase中是纯虚函数
2. realize的具体实现在osgViewer::Viewer类中实现：
void Viewer::realize()
{
Contexts contexts;
getContexts(contexts);
if (contexts.empty())
{
OSG_INFO<<"Viewer::realize() - No valid contexts found, setting up view across all screens."<<std::endl;
const char* ptr = 0;
if ((ptr = getenv("OSG_CONFIG_FILE")) != 0)
{
readConfiguration(ptr);
}
else
{
int screenNum = -1;
if ((ptr = getenv("OSG_SCREEN")) != 0)
{
if (strlen(ptr)!=0) screenNum = atoi(ptr);
else screenNum = -1;
}
int x = -1, y = -1, width = -1, height = -1;
if ((ptr = getenv("OSG_WINDOW")) != 0)
{
std::istringstream iss(ptr);
iss >> x >> y >> width >> height;
}
if (width>0 && height>0)
{
if (screenNum>=0) setUpViewInWindow(x, y, width, height, screenNum);
else setUpViewInWindow(x,y,width,height);
}
else if (screenNum>=0)
{
setUpViewOnSingleScreen(screenNum);
}
else
{
setUpViewAcrossAllScreens();
}
}
getContexts(contexts);
}
if (contexts.empty())
{
OSG_NOTICE<<"Viewer::realize() - failed to set up any windows"<<std::endl;
_done = true;
return;
}
unsigned int maxTexturePoolSize = osg::DisplaySettings::instance()->getMaxTexturePoolSize();
if (_camera->getDisplaySettings()) maxTexturePoolSize = std::max(maxTexturePoolSize, _camera->getDisplaySettings()->getMaxTexturePoolSize());
if (_displaySettings.valid()) maxTexturePoolSize = std::max(maxTexturePoolSize, _displaySettings->getMaxTexturePoolSize());
unsigned int maxBufferObjectPoolSize = osg::DisplaySettings::instance()->getMaxBufferObjectPoolSize();
if (_displaySettings.valid()) maxBufferObjectPoolSize = std::max(maxBufferObjectPoolSize, _displaySettings->getMaxBufferObjectPoolSize());
if (_camera->getDisplaySettings()) maxBufferObjectPoolSize = std::max(maxBufferObjectPoolSize, _camera->getDisplaySettings()->getMaxBufferObjectPoolSize());
for(Contexts::iterator citr = contexts.begin();
citr != contexts.end();
++citr)
{
osg::GraphicsContext* gc = *citr;
// set the pool sizes, 0 the default will result in no GL object pools.
gc->getState()->setMaxTexturePoolSize(maxTexturePoolSize);
gc->getState()->setMaxBufferObjectPoolSize(maxBufferObjectPoolSize);
gc->realize();
if (_realizeOperation.valid() && gc->valid())
{
gc->makeCurrent();
(*_realizeOperation)(gc);
gc->releaseContext();
}
}
// attach contexts to _incrementalCompileOperation if attached.
if (_incrementalCompileOperation) _incrementalCompileOperation->assignContexts(contexts);
bool grabFocus = true;
if (grabFocus)
{
for(Contexts::iterator citr = contexts.begin();
citr != contexts.end();
++citr)
{
osgViewer::GraphicsWindow* gw = dynamic_cast<osgViewer::GraphicsWindow*>(*citr);
if (gw)
{
gw->grabFocusIfPointerInWindow();
}
}
}
// initialize the global timer to be relative to the current time.
osg::Timer::instance()->setStartTick();
// pass on the start tick to all the associated event queues
setStartTick(osg::Timer::instance()->getStartTick());
setUpThreading();
if (osg::DisplaySettings::instance()->getCompileContextsHint())
{
int numProcessors = OpenThreads::GetNumberOfProcessors();
int processNum = 0;
for(unsigned int i=0; i<= osg::GraphicsContext::getMaxContextID(); ++i)
{
osg::GraphicsContext* gc = osg::GraphicsContext::getOrCreateCompileContext(i);
if (gc)
{
gc->createGraphicsThread();
gc->getGraphicsThread()->setProcessorAffinity(processNum % numProcessors);
gc->getGraphicsThread()->startThread();
++processNum;
}
}
}
}1. 首先收集当前视景器Viewer中相机的渲染上下文，代码和isRealized中的实现一样
2. 如果不存在渲染上下文，我们进入第一段if判断中，可以看到主要进行的工作如下：
- 读取环境变量OSG_CONFIG_FILE，如果设置了OSG_CONFIG_FILE指向一个.view后缀的文件，可以创建该文件描述的窗口，格式如下：（在openscenegraph-data目录中的configuration目录下有.view类型的文件）
osgViewer::Viewer
{
setUpViewInWindow 100 200 600 400 0
}- 如果没有设置OSG_CONFIG_FILE环境变量，那么程序会检查 OSG_SCREEN和OSG_WINDOW这两个环境变量的值，OSG_SCREEN对应窗口的个数（值是一个整型数）OSG_WINDOW对应窗口的大小和位置，格式是（x, y, w, h) 分别是窗口左上角点坐标(x, y)以及窗口的长和宽(w, h)，具体来说是以下情况：
| Screen取值 | Window中width和height取值 | 窗口设置函数 |
| 大于或等于0 | 长和宽都大于0 | setUpViewInWindow(x, y, width, height, screenNum) |
| 小于0 | 长和宽都大于0 | setUpViewInWindow(x, y, width, height) |
| 大于0 | 长和宽为0或者负值 | setUpViewOnSingleScreen(screenNum) |
其他情况下窗口设置函数使用setUpViewAcrossAllScreens()
当窗口设置完成之后，osg会再次调用getContexts(contexts) 搜集目前可用的设备渲染上下文，如果这时候还没有窗口产生，也就是说设置窗口的各种方式都失败了，那么程序就会退出，如果成功，那么会针对已经常见的窗口进行一些设置，完成后续的操作
这三个函数的实现如下所示：
void View::setUpViewAcrossAllScreens()
{
apply(new osgViewer::AcrossAllScreens());
}
void View::setUpViewInWindow(int x, int y, int width, int height, unsigned int screenNum)
{
apply(new osgViewer::SingleWindow(x, y, width, height, screenNum));
}
void View::setUpViewOnSingleScreen(unsigned int screenNum)
{
apply(new osgViewer::SingleScreen(screenNum));
}
基类osgViewer::ViewConfig提供了一个虚函数 virtual void configure (osgViewer::View &) const，在子类中通过实现该函数来配置渲染的窗口大小和位置。
SingleScreen在实现过程中会调用SingleWindow来完成设置，AcrossAllScreen在未设置窗口的大小和显示屏数目的时候会被调用（一般就是我们默认使用osg的场景），那么它会调用SingleWindow来设置，在设置过程中会读取外接显示器的分辨率，产生一个全屏的窗口，代码如下：
//来自于osgViewer::SingleWindow::configure
if (traits->width<=0 || traits->height<=0 )
{
osg::GraphicsContext::ScreenIdentifier si;
si.readDISPLAY();
// displayNum has not been set so reset it to 0.
if (si.displayNum<0) si.displayNum = 0;
si.screenNum = _screenNum;
unsigned int width, height;
wsi->getScreenResolution(si, width, height);
if (traits->width<=0) traits->width = width;
if (traits->height<=0) traits->height = height;
}下面继续realize后面的代码，在完成窗口参数的设置之后，后面的代码无非是一些关于OpenGL缓冲区对象的设置等，暂时与窗口无关。在此之后进入到for循环之中，这里面就是窗口设置的内容了，实现的内容是对所有的相机中的设备上下文进行检查，并真正的产生窗口，窗口的产生调用在
gc->realize();我们查看一下它的实现：
bool GraphicsContext::realize()
{
if (realizeImplementation())
{
return true;
}
else
{
return false;
}
}bool GraphicsWindowWin32::realizeImplementation()
{
if (_realized) return true;
if (!_initialized)
{
init();
if (!_initialized) return false;
}
if (_traits.valid() && (_traits->sharedContext.valid() || _traits->vsync || _traits->swapGroupEnabled))
{
// make context current so we can test capabilities and set up context sharing
struct RestoreContext
{
RestoreContext()
{
_hdc = wglGetCurrentDC();
_hglrc = wglGetCurrentContext();
}
~RestoreContext()
{
wglMakeCurrent(_hdc,_hglrc);
}
protected:
HDC _hdc;
HGLRC _hglrc;
} restoreContext;
_realized = true;
bool result = makeCurrent();
_realized = false;
if (!result)
{
return false;
}
// set up sharing of contexts if required
GraphicsHandleWin32* graphicsHandleWin32 = dynamic_cast<GraphicsHandleWin32*>(_traits->sharedContext.get());
if (graphicsHandleWin32)
{
if (!wglShareLists(graphicsHandleWin32->getWGLContext(), getWGLContext()))
{
reportErrorForScreen("GraphicsWindowWin32::realizeImplementation() - Unable to share OpenGL context", _traits->screenNum, ::GetLastError());
return false;
}
}
// if vysnc should be on then enable it.
if (_traits->vsync)
{
setSyncToVBlank(_traits->vsync);
}
// If the swap group is active then enable it.
if (_traits->swapGroupEnabled)
{
setSwapGroup(_traits->swapGroupEnabled, _traits->swapGroup, _traits->swapBarrier);
}
}
if (_ownsWindow)
{
//
// Bring the window on top of other ones (including the taskbar if it covers it completely)
//
// NOTE: To cover the taskbar with a window that does not completely cover it, the HWND_TOPMOST
// Z-order must be used in the code below instead of HWND_TOP.
// @todo: This should be controlled through a flag in the traits (topMostWindow)
//
if (!::SetWindowPos(_hwnd,
HWND_TOP,
_windowOriginXToRealize,
_windowOriginYToRealize,
_windowWidthToRealize,
_windowHeightToRealize,
SWP_SHOWWINDOW))
{
reportErrorForScreen("GraphicsWindowWin32::realizeImplementation() - Unable to show window", _traits->screenNum, ::GetLastError());
return false;
}
if (!::UpdateWindow(_hwnd))
{
reportErrorForScreen("GraphicsWindowWin32::realizeImplementation() - Unable to update window", _traits->screenNum, ::GetLastError());
return false;
}
}
_realized = true;
// make sure the event queue has the correct window rectangle size and input range
getEventQueue()->syncWindowRectangleWithGraphcisContext();
return true;
}
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