QOpenGLFunctions Class

Detailed Description

OpenGL ES 2.0 defines a subset of the OpenGL specification that is common across many desktop and embedded OpenGL implementations. However, it can be difficult to use the functions from that subset because they need to be resolved manually on desktop systems.

QOpenGLFunctions provides a guaranteed API that is available on all OpenGL systems and takes care of function resolution on systems that need it. The recommended way to use QOpenGLFunctions is by direct inheritance:

     class MyGLWindow : public QWindow, protected QOpenGLFunctions
     {
         Q_OBJECT
     public:
         explicit MyGLWindow(QScreen *screen = nullptr);

     protected:
         void initializeGL();
         void paintGL();

         QOpenGLContext *m_context;
     };

     MyGLWindow(QScreen *screen)
       : QWindow(screen), QOpenGLWidget(parent)
     {
         setSurfaceType(OpenGLSurface);
         create();

         // Create an OpenGL context
         m_context = new QOpenGLContext;
         m_context->create();

         // Setup scene and render it
         initializeGL();
         paintGL();
     }

     void MyGLWindow::initializeGL()
     {
         m_context->makeCurrent(this);
         initializeOpenGLFunctions();
     }

The paintGL() function can then use any of the OpenGL ES 2.0 functions without explicit resolution, such as glActiveTexture() in the following example:

     void MyGLWindow::paintGL()
     {
         m_context->makeCurrent(this);
         glActiveTexture(GL_TEXTURE1);
         glBindTexture(GL_TEXTURE_2D, textureId);
         ...
         m_context->swapBuffers(this);
         m_context->doneCurrent();
     }

QOpenGLFunctions can also be used directly for ad-hoc invocation of OpenGL ES 2.0 functions on all platforms:

     QOpenGLFunctions glFuncs(QOpenGLContext::currentContext());
     glFuncs.glActiveTexture(GL_TEXTURE1);

An alternative approach is to query the context's associated QOpenGLFunctions instance. This is somewhat faster than the previous approach due to avoiding the creation of a new instance, but the difference is fairly small since the internal data structures are shared, and function resolving happens only once for a given context, regardless of the number of QOpenGLFunctions instances initialized for it.

     QOpenGLFunctions *glFuncs = QOpenGLContext::currentContext()->functions();
     glFuncs->glActiveTexture(GL_TEXTURE1);

QOpenGLFunctions provides wrappers for all OpenGL ES 2.0 functions, including the common subset of OpenGL 1.x and ES 2.0. While such functions, for example glClear() or glDrawArrays(), can be called also directly, as long as the application links to the platform-specific OpenGL library, calling them via QOpenGLFunctions enables the possibility of dynamically loading the OpenGL implementation.

The hasOpenGLFeature() and openGLFeatures() functions can be used to determine if the OpenGL implementation has a major OpenGL ES 2.0 feature. For example, the following checks if non power of two textures are available:

     QOpenGLFunctions funcs(QOpenGLContext::currentContext());
     bool npot = funcs.hasOpenGLFeature(QOpenGLFunctions::NPOTTextures);