QGLWidget Class

Detailed Description

QGLWidget provides functionality for displaying OpenGL graphics integrated into a Qt application. It is very simple to use. You inherit from it and use the subclass like any other QWidget, except that you have the choice between using QPainter and standard OpenGL rendering commands.

QGLWidget provides three convenient virtual functions that you can reimplement in your subclass to perform the typical OpenGL tasks:

• paintGL() - Renders the OpenGL scene. Gets called whenever the widget needs to be updated.
• resizeGL() - Sets up the OpenGL viewport, projection, etc. Gets called whenever the widget has been resized (and also when it is shown for the first time because all newly created widgets get a resize event automatically).
• initializeGL() - Sets up the OpenGL rendering context, defines display lists, etc. Gets called once before the first time resizeGL() or paintGL() is called.

Here is a rough outline of how a QGLWidget subclass might look:

 class MyGLDrawer : public QGLWidget
 {
     Q_OBJECT        // must include this if you use Qt signals/slots

 public:
     MyGLDrawer(QWidget *parent)
         : QGLWidget(parent) {}

 protected:

     void initializeGL() override
     {
         // Set up the rendering context, define display lists etc.:
         ...
         glClearColor(0.0, 0.0, 0.0, 0.0);
         glEnable(GL_DEPTH_TEST);
         ...
     }

     void resizeGL(int w, int h) override
     {
         // setup viewport, projection etc.:
         glViewport(0, 0, (GLint)w, (GLint)h);
         ...
         glFrustum(...);
         ...
     }

     void paintGL() override
     {
         // draw the scene:
         ...
         glRotatef(...);
         glMaterialfv(...);
         glBegin(GL_QUADS);
         glVertex3f(...);
         glVertex3f(...);
         ...
         glEnd();
         ...
     }

 };

If you need to trigger a repaint from places other than paintGL() (a typical example is when using timers to animate scenes), you should call the widget's updateGL() function.

Your widget's OpenGL rendering context is made current when paintGL(), resizeGL(), or initializeGL() is called. If you need to call the standard OpenGL API functions from other places (e.g. in your widget's constructor or in your own paint functions), you must call makeCurrent() first.

QGLWidget provides functions for requesting a new display format and you can also create widgets with customized rendering contexts.

You can also share OpenGL display lists between QGLWidget objects (see the documentation of the QGLWidget constructors for details).

Note that under Windows, the QGLContext belonging to a QGLWidget has to be recreated when the QGLWidget is reparented. This is necessary due to limitations on the Windows platform. This will most likely cause problems for users that have subclassed and installed their own QGLContext on a QGLWidget. It is possible to work around this issue by putting the QGLWidget inside a dummy widget and then reparenting the dummy widget, instead of the QGLWidget. This will side-step the issue altogether, and is what we recommend for users that need this kind of functionality.

On macOS, when Qt is built with Cocoa support, a QGLWidget can't have any sibling widgets placed ontop of itself. This is due to limitations in the Cocoa API and is not supported by Apple.

Overlays

The QGLWidget creates a GL overlay context in addition to the normal context if overlays are supported by the underlying system.

If you want to use overlays, you specify it in the format. (Note: Overlay must be requested in the format passed to the QGLWidget constructor.) Your GL widget should also implement some or all of these virtual methods:

• paintOverlayGL()
• resizeOverlayGL()
• initializeOverlayGL()

These methods work in the same way as the normal paintGL() etc. functions, except that they will be called when the overlay context is made current. You can explicitly make the overlay context current by using makeOverlayCurrent(), and you can access the overlay context directly (e.g. to ask for its transparent color) by calling overlayContext().

On X servers in which the default visual is in an overlay plane, non-GL Qt windows can also be used for overlays.

Painting Techniques

As described above, subclass QGLWidget to render pure 3D content in the following way:

• Reimplement the QGLWidget::initializeGL() and QGLWidget::resizeGL() to set up the OpenGL state and provide a perspective transformation.
• Reimplement QGLWidget::paintGL() to paint the 3D scene, calling only OpenGL functions to draw on the widget.

It is also possible to draw 2D graphics onto a QGLWidget subclass, it is necessary to reimplement QGLWidget::paintEvent() and do the following:

• Construct a QPainter object.
• Initialize it for use on the widget with the QPainter::begin() function.
• Draw primitives using QPainter's member functions.
• Call QPainter::end() to finish painting.

Threading

As of Qt version 4.8, support for doing threaded GL rendering has been improved. There are three scenarios that we currently support:

• 1. Buffer swapping in a thread.

  Swapping buffers in a double buffered context may be a synchronous, locking call that may be a costly operation in some GL implementations. Especially so on embedded devices. It's not optimal to have the CPU idling while the GPU is doing a buffer swap. In those cases it is possible to do the rendering in the main thread and do the actual buffer swap in a separate thread. This can be done with the following steps:

  1. Call doneCurrent() in the main thread when the rendering is finished.

  2. Call QGLContext::moveToThread(swapThread) to transfer ownership of the context to the swapping thread.

  3. Notify the swapping thread that it can grab the context.

  4. Make the rendering context current in the swapping thread with makeCurrent() and then call swapBuffers().

  5. Call doneCurrent() in the swapping thread.

  6. Call QGLContext::moveToThread(qApp->thread()) and notify the main thread that swapping is done.

  Doing this will free up the main thread so that it can continue with, for example, handling UI events or network requests. Even if there is a context swap involved, it may be preferable compared to having the main thread wait while the GPU finishes the swap operation. Note that this is highly implementation dependent.

• 2. Texture uploading in a thread.

  Doing texture uploads in a thread may be very useful for applications handling large amounts of images that needs to be displayed, like for instance a photo gallery application. This is supported in Qt through the existing bindTexture() API. A simple way of doing this is to create two sharing QGLWidgets. One is made current in the main GUI thread, while the other is made current in the texture upload thread. The widget in the uploading thread is never shown, it is only used for sharing textures with the main thread. For each texture that is bound via bindTexture(), notify the main thread so that it can start using the texture.

• 3. Using QPainter to draw into a QGLWidget in a thread.

  In Qt 4.8, it is possible to draw into a QGLWidget using a QPainter in a separate thread. Note that this is also possible for QGLPixelBuffers and QGLFramebufferObjects. Since this is only supported in the GL 2 paint engine, OpenGL 2.0 or OpenGL ES 2.0 is required.

  QGLWidgets can only be created in the main GUI thread. This means a call to doneCurrent() is necessary to release the GL context from the main thread, before the widget can be drawn into by another thread. You then need to call QGLContext::moveToThread() to transfer ownership of the context to the thread in which you want to make it current. Also, the main GUI thread will dispatch resize and paint events to a QGLWidget when the widget is resized, or parts of it becomes exposed or needs redrawing. It is therefore necessary to handle those events because the default implementations inside QGLWidget will try to make the QGLWidget's context current, which again will interfere with any threads rendering into the widget. Reimplement QGLWidget::paintEvent() and QGLWidget::resizeEvent() to notify the rendering thread that a resize or update is necessary, and be careful not to call the base class implementation. If you are rendering an animation, it might not be necessary to handle the paint event at all since the rendering thread is doing regular updates. Then it would be enough to reimplement QGLWidget::paintEvent() to do nothing.

As a general rule when doing threaded rendering: be aware that binding and releasing contexts in different threads have to be synchronized by the user. A GL rendering context can only be current in one thread at any time. If you try to open a QPainter on a QGLWidget and the widget's rendering context is current in another thread, it will fail.

In addition to this, rendering using raw GL calls in a separate thread is supported.

OpenGL is a trademark of Silicon Graphics, Inc. in the United States and other countries.