QWGLNativeContext Class

Detailed Description

A class encapsulating a WGL context on Windows with desktop OpenGL (opengl32.dll)

Note: There is no binary compatibility guarantee for this class, meaning that an application using it is only guaranteed to work with the Qt version it was developed against.

QWGLNativeContext is a value class that can be passed to QOpenGLContext::setNativeHandle(). When creating a QOpenGLContext with the native handle set, no new context will get created. Instead, the provided handles are used, without taking ownership. This allows wrapping a context created by an external framework or rendering engine. The typical usage will be similar to the following snippet:

  #include <QtPlatformSupport/QWGLNativeContext>
  ...create and retrieve the WGL context and the corresponding window...
  QOpenGLContext *context = new QOpenGLContext;
  QWGLNativeContext nativeContext(hglrc, hwnd);
  context->setNativeHandle(QVariant::fromValue(nativeContext));
  context->create();
  ...

The window is needed because the its pixel format will be queried. When the adoption is successful, QOpenGLContext::format() will return a QSurfaceFormat describing this pixel format.

It is recommended to restrict the usage of QOpenGLContexts created this way. Various platform-specific behavior and issues may prevent such contexts to be made current with windows (surfaces) created by Qt due to non-matching pixel formats for example. A potentially safer solution is to use the wrapped context only to set up sharing and perform Qt-based rendering offscreen, using a separate, dedicated QOpenGLContext. The resulting textures are then accessible in the foreign context too.

  ...like above...
  QOpenGLContext *qtcontext = new QOpenGLContext;
  qtcontext->setShareContext(context);
  qtcontext->setFormat(context->format());
  qtcontext->create();
  ...use qtcontext for rendering with Qt...

In addition to being used with QOpenGLContext::setNativeHandle(), this class is used also to retrieve the native context handle, that is, a HGLRC value, from a QOpenGLContext. Calling QOpenGLContext::nativeHandle() returns a QVariant which, on Windows with opengl32.dll at least, will contain a QWGLNativeContext:

  QVariant nativeHandle = context->nativeHandle();
  if (!nativeHandle.isNull() && nativeHandle.canConvert<QWGLNativeContext>()) {
      QWGLNativeContext nativeContext = nativeHandle.value<QWGLNativeContext>();
      HGLRC hglrc = nativeContext.context();
      ...
  }