QGLWidget Class | Qt OpenGL

Qt 5.13 | Qt OpenGL | C++ Classes | QGLWidget

Qt 5.13.2 Reference Documentation

# QGLWidget Class

The QGLWidget class is a widget for rendering OpenGL graphics. More...

Header: | #include <QGLWidget>
qmake: | QT += opengl
Inherits: | QWidget

This class is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

- List of all members, including inherited members
- Obsolete members

## Public Functions

| QGLWidget(const QGLFormat &format, QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
| QGLWidget(QGLContext *context, QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
| QGLWidget(QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
virtual | ~QGLWidget()
GLuint | bindTexture(const QImage &image, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA)
GLuint | bindTexture(const QImage &image, GLenum target, GLint format, QGLContext::BindOptions options)
GLuint | bindTexture(const QPixmap &pixmap, GLenum target, GLint format, QGLContext::BindOptions options)
GLuint | bindTexture(const QPixmap &pixmap, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA)
GLuint | bindTexture(const QString &fileName)
const QGLColormap & | colormap() const
QGLContext * | context() const
void | deleteTexture(GLuint id)
void | doneCurrent()
bool | doubleBuffer() const
void | drawTexture(const QRectF &target, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)
void | drawTexture(const QPointF &point, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)
QGLFormat | format() const
QImage | grabFrameBuffer(bool withAlpha = false)
bool | isSharing() const
bool | isValid() const
void | makeCurrent()
void | makeOverlayCurrent()
const QGLContext * | overlayContext() const
void | qglClearColor(const QColor &c) const
void | qglColor(const QColor &c) const
QPixmap | renderPixmap(int w = 0, int h = 0, bool useContext = false)
void | renderText(int x, int y, const QString &str, const QFont &font = QFont())
void | renderText(double x, double y, double z, const QString &str, const QFont &font = QFont())
void | setColormap(const QGLColormap &cmap)
void | swapBuffers()

## Public Slots

virtual void | updateGL()
virtual void | updateOverlayGL()

## Static Public Members

QImage | convertToGLFormat(const QImage &img)

## Protected Functions

bool | autoBufferSwap() const
virtual void | glDraw()
virtual void | glInit()
virtual void | initializeGL()
virtual void | initializeOverlayGL()
virtual void | paintGL()
virtual void | paintOverlayGL()
virtual void | resizeGL(int width, int height)
virtual void | resizeOverlayGL(int width, int height)
void | setAutoBufferSwap(bool on)

## Reimplemented Protected Functions

virtual bool | event(QEvent *e) override
virtual void | paintEvent(QPaintEvent *event) override
virtual void | resizeEvent(QResizeEvent *event) override

## 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.

Note: This class is part of the legacy Qt OpenGL module and, like the other `QGL` classes, should be avoided in the new applications. Instead, starting from Qt 5.4, prefer using QOpenGLWidget and the `QOpenGL` classes.

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.

See also QOpenGLWidget and QGLPixelBuffer.

## Member Function Documentation

### QGLWidget::QGLWidget(const QGLFormat &format, QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())

Constructs an OpenGL widget with parent parent.

The format argument specifies the desired rendering options. If the underlying OpenGL/Window system cannot satisfy all the features requested in format, the nearest subset of features will be used. After creation, the format() method will return the actual format obtained.

The widget will be invalid if the system has no OpenGL support.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

See also QGLFormat::defaultFormat() and isValid().

### QGLWidget::QGLWidget(QGLContext *context, QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())

Constructs an OpenGL widget with parent parent.

The context argument is a pointer to the QGLContext that you wish to be bound to this widget. This allows you to pass in your own QGLContext sub-classes.

The widget will be invalid if the system has no OpenGL support.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

See also QGLFormat::defaultFormat() and isValid().

### QGLWidget::QGLWidget(QWidget *parent = nullptr, const QGLWidget *shareWidget = nullptr, Qt::WindowFlags f = Qt::WindowFlags())

Constructs an OpenGL widget with a parent widget.

The default format is used. The widget will be invalid if the system has no OpenGL support.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

See also QGLFormat::defaultFormat() and Textures Example.

### `[virtual slot]` void QGLWidget::updateGL()

Updates the widget by calling glDraw().

### `[virtual slot]` void QGLWidget::updateOverlayGL()

Updates the widget's overlay (if any). Will cause the virtual function paintOverlayGL() to be executed.

The widget's rendering context will become the current context and initializeGL() will be called if it hasn't already been called.

### `[virtual]` QGLWidget::~QGLWidget()

Destroys the widget.

### `[protected]` bool QGLWidget::autoBufferSwap() const

Returns `true` if the widget is doing automatic GL buffer swapping; otherwise returns `false`.

See also setAutoBufferSwap().

### GLuint QGLWidget::bindTexture(const QImage &image, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA)

Calls QGLContext:::bindTexture(image, target, format) on the currently set context.

See also deleteTexture().

### GLuint QGLWidget::bindTexture(const QImage &image, GLenum target, GLint format, QGLContext::BindOptions options)

This is an overloaded function.

The binding options are a set of options used to decide how to bind the texture to the context.

This function was introduced in Qt 4.6.

### GLuint QGLWidget::bindTexture(const QPixmap &pixmap, GLenum target, GLint format, QGLContext::BindOptions options)

This is an overloaded function.

Generates and binds a 2D GL texture to the current context, based on pixmap. The generated texture id is returned and can be used in

The binding options are a set of options used to decide how to bind the texture to the context.

This function was introduced in Qt 4.6.

### GLuint QGLWidget::bindTexture(const QPixmap &pixmap, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA)

Calls QGLContext:::bindTexture(pixmap, target, format) on the currently set context.

See also deleteTexture().

### GLuint QGLWidget::bindTexture(const QString &fileName)

This is an overloaded function.

Calls QGLContext::bindTexture(fileName) on the currently set context.

See also deleteTexture().

### const QGLColormap &QGLWidget::colormap() const

Returns the colormap for this widget.

Usually it is only top-level widgets that can have different colormaps installed. Asking for the colormap of a child widget will return the colormap for the child's top-level widget.

If no colormap has been set for this widget, the QGLColormap returned will be empty.

See also setColormap() and QGLColormap::isEmpty().

### QGLContext *QGLWidget::context() const

Returns the context of this widget.

It is possible that the context is not valid (see isValid()), for example, if the underlying hardware does not support the format attributes that were requested.

See also setContext().

### `[static]` QImage QGLWidget::convertToGLFormat(const QImage &img)

Converts the image img into the unnamed format expected by OpenGL functions such as glTexImage2D(). The returned image is not usable as a QImage, but QImage::width(), QImage::height() and QImage::bits() may be used with OpenGL. The GL format used is `GL_RGBA`.

### void QGLWidget::deleteTexture(GLuint id)

Calls QGLContext::deleteTexture(id) on the currently set context.

See also bindTexture().

### void QGLWidget::doneCurrent()

Makes no GL context the current context. Normally, you do not need to call this function; QGLContext calls it as necessary. However, it may be useful in multithreaded environments.

### bool QGLWidget::doubleBuffer() const

Returns `true` if the contained GL rendering context has double buffering; otherwise returns `false`.

See also QGLFormat::doubleBuffer().

### void QGLWidget::drawTexture(const QRectF &target, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)

Calls the corresponding QGLContext::drawTexture() with target, textureId, and textureTarget for this widget's context.

This function was introduced in Qt 4.4.

### void QGLWidget::drawTexture(const QPointF &point, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)

Calls the corresponding QGLContext::drawTexture() with point, textureId, and textureTarget for this widget's context.

This function was introduced in Qt 4.4.

### `[override virtual protected]` bool QGLWidget::event(QEvent *e)

Reimplements: QWidget::event(QEvent *event).

### QGLFormat QGLWidget::format() const

Returns the format of the contained GL rendering context.

See also setFormat().

### `[virtual protected]` void QGLWidget::glDraw()

Executes the virtual function paintGL().

The widget's rendering context will become the current context and initializeGL() will be called if it hasn't already been called.

### `[virtual protected]` void QGLWidget::glInit()

Initializes OpenGL for this widget's context. Calls the virtual function initializeGL().

### QImage QGLWidget::grabFrameBuffer(bool withAlpha = false)

Returns an image of the frame buffer. If withAlpha is true the alpha channel is included.

Depending on your hardware, you can explicitly select which color buffer to grab with a glReadBuffer() call before calling this function.

On QNX the back buffer is not preserved when swapBuffers() is called. The back buffer where this function reads from, might thus not contain the same content as the front buffer. In order to retrieve what is currently visible on the screen, swapBuffers() has to be executed prior to this function call.

### `[virtual protected]` void QGLWidget::initializeGL()

This virtual function is called once before the first call to paintGL() or resizeGL(), and then once whenever the widget has been assigned a new QGLContext. Reimplement it in a subclass.

This function should set up any required OpenGL context rendering flags, defining display lists, etc.

There is no need to call makeCurrent() because this has already been done when this function is called.

### `[virtual protected]` void QGLWidget::initializeOverlayGL()

This virtual function is used in the same manner as initializeGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that initializeOverlayGL() is called once before the first call to paintOverlayGL() or resizeOverlayGL(). Reimplement it in a subclass.

This function should set up any required OpenGL context rendering flags, defining display lists, etc. for the overlay context.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

### bool QGLWidget::isSharing() const

Returns `true` if this widget's GL context is shared with another GL context, otherwise false is returned. Context sharing might not be possible if the widgets use different formats.

See also format().

### bool QGLWidget::isValid() const

Returns `true` if the widget has a valid GL rendering context; otherwise returns `false`. A widget will be invalid if the system has no OpenGL support.

### void QGLWidget::makeCurrent()

Makes this widget the current widget for OpenGL operations, i.e. makes the widget's rendering context the current OpenGL rendering context.

### void QGLWidget::makeOverlayCurrent()

Makes the overlay context of this widget current. Use this if you need to issue OpenGL commands to the overlay context outside of initializeOverlayGL(), resizeOverlayGL(), and paintOverlayGL().

Does nothing if this widget has no overlay.

See also makeCurrent().

### const QGLContext *QGLWidget::overlayContext() const

Returns the overlay context of this widget, or `nullptr` if this widget has no overlay.

See also context().

### `[override virtual protected]` void QGLWidget::paintEvent(QPaintEvent *event)

Reimplements: QWidget::paintEvent(QPaintEvent *event).

Handles paint events passed in the event parameter. Will cause the virtual paintGL() function to be called.

The widget's rendering context will become the current context and initializeGL() will be called if it hasn't already been called.

### `[virtual protected]` void QGLWidget::paintGL()

This virtual function is called whenever the widget needs to be painted. Reimplement it in a subclass.

There is no need to call makeCurrent() because this has already been done when this function is called.

### `[virtual protected]` void QGLWidget::paintOverlayGL()

This virtual function is used in the same manner as paintGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that paintOverlayGL() is called whenever the widget's overlay needs to be painted. Reimplement it in a subclass.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

### void QGLWidget::qglClearColor(const QColor &c) const

Convenience function for specifying the clearing color to OpenGL. Calls glClearColor (in RGBA mode) or glClearIndex (in color-index mode) with the color c. Applies to this widgets GL context.

See also qglColor(), QGLContext::currentContext(), and QColor.

### void QGLWidget::qglColor(const QColor &c) const

Convenience function for specifying a drawing color to OpenGL. Calls glColor4 (in RGBA mode) or glIndex (in color-index mode) with the color c. Applies to this widgets GL context.

Note: This function is not supported on OpenGL/ES 2.0 systems.

See also qglClearColor(), QGLContext::currentContext(), and QColor.

### QPixmap QGLWidget::renderPixmap(int w = 0, int h = 0, bool useContext = false)

Renders the current scene on a pixmap and returns the pixmap.

You can use this method on both visible and invisible QGLWidget objects.

Internally the function renders into a framebuffer object and performs pixel readback. This has a performance penalty, meaning that this function is not suitable to be called at a high frequency.

After creating and binding the framebuffer object, the function will call initializeGL(), resizeGL(), and paintGL(). On the next normal update initializeGL() and resizeGL() will be triggered again since the size of the destination pixmap and the QGLWidget's size may differ.

The size of the pixmap will be w pixels wide and h pixels high unless one of these parameters is 0 (the default), in which case the pixmap will have the same size as the widget.

Care must be taken when using framebuffer objects in paintGL() in combination with this function. To switch back to the default framebuffer, use QGLFramebufferObject::bindDefault(). Binding FBO 0 is wrong since renderPixmap() uses a custom framebuffer instead of the one provided by the windowing system.

useContext is ignored. Historically this parameter enabled the usage of the existing GL context. This is not supported anymore since additional contexts are never created.

Overlays are not rendered onto the pixmap.

If the GL rendering context and the desktop have different bit depths, the result will most likely look surprising.

Note that the creation of display lists, modifications of the view frustum etc. should be done from within initializeGL(). If this is not done, the temporary QGLContext will not be initialized properly, and the rendered pixmap may be incomplete/corrupted.

### void QGLWidget::renderText(int x, int y, const QString &str, const QFont &font = QFont())

Renders the string str into the GL context of this widget.

x and y are specified in window coordinates, with the origin in the upper left-hand corner of the window. If font is not specified, the currently set application font will be used to render the string. To change the color of the rendered text you can use the glColor() call (or the qglColor() convenience function), just before the renderText() call.

Note: This function clears the stencil buffer.

Note: This function is not supported on OpenGL/ES systems.

Note: This function temporarily disables depth-testing when the text is drawn.

Note: This function can only be used inside a QPainter::beginNativePainting()/QPainter::endNativePainting() block if a painter is active on the QGLWidget.

### void QGLWidget::renderText(double x, double y, double z, const QString &str, const QFont &font = QFont())

This is an overloaded function.

x, y and z are specified in scene or object coordinates relative to the currently set projection and model matrices. This can be useful if you want to annotate models with text labels and have the labels move with the model as it is rotated etc.

Note: This function is not supported on OpenGL/ES systems.

Note: If depth testing is enabled before this function is called, then the drawn text will be depth-tested against the models that have already been drawn in the scene. Use `glDisable(GL_DEPTH_TEST)` before calling this function to annotate the models without depth-testing the text.

Note: This function can only be used inside a QPainter::beginNativePainting()/QPainter::endNativePainting() block if a painter is active on the QGLWidget.

### `[override virtual protected]` void QGLWidget::resizeEvent(QResizeEvent *event)

Reimplements: QWidget::resizeEvent(QResizeEvent *event).

Handles resize events that are passed in the event parameter. Calls the virtual function resizeGL().

### `[virtual protected]` void QGLWidget::resizeGL(int width, int height)

This virtual function is called whenever the widget has been resized. The new size is passed in width and height. Reimplement it in a subclass.

There is no need to call makeCurrent() because this has already been done when this function is called.

### `[virtual protected]` void QGLWidget::resizeOverlayGL(int width, int height)

This virtual function is used in the same manner as paintGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that resizeOverlayGL() is called whenever the widget has been resized. The new size is passed in width and height. Reimplement it in a subclass.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

### `[protected]` void QGLWidget::setAutoBufferSwap(bool on)

If on is true automatic GL buffer swapping is switched on; otherwise it is switched off.

If on is true and the widget is using a double-buffered format, the background and foreground GL buffers will automatically be swapped after each paintGL() call.

The buffer auto-swapping is on by default.

See also autoBufferSwap(), doubleBuffer(), and swapBuffers().

### void QGLWidget::setColormap(const QGLColormap &cmap)

Set the colormap for this widget to cmap. Usually it is only top-level widgets that can have colormaps installed.

See also colormap().

### void QGLWidget::swapBuffers()

Swaps the screen contents with an off-screen buffer. This only works if the widget's format specifies double buffer mode.

Normally, there is no need to explicitly call this function because it is done automatically after each widget repaint, i.e. each time after paintGL() has been executed.

See also doubleBuffer(), setAutoBufferSwap(), and QGLFormat::setDoubleBuffer().