QOpenGLWindow Class

The QOpenGLWindow class is a convenience subclass of QWindow to perform OpenGL painting. More...

Header: #include <QOpenGLWindow>
CMake: find_package(Qt6 REQUIRED COMPONENTS OpenGL)
target_link_libraries(mytarget PRIVATE Qt6::OpenGL)
qmake: QT += opengl
Inherits: QPaintDeviceWindow

Public Types

enum UpdateBehavior { NoPartialUpdate, PartialUpdateBlit, PartialUpdateBlend }

Public Functions

QOpenGLWindow(QOpenGLWindow::UpdateBehavior updateBehavior = NoPartialUpdate, QWindow *parent = nullptr)
QOpenGLWindow(QOpenGLContext *shareContext, QOpenGLWindow::UpdateBehavior updateBehavior = NoPartialUpdate, QWindow *parent = nullptr)
virtual ~QOpenGLWindow()
QOpenGLContext *context() const
GLuint defaultFramebufferObject() const
void doneCurrent()
QImage grabFramebuffer()
bool isValid() const
void makeCurrent()
QOpenGLContext *shareContext() const
QOpenGLWindow::UpdateBehavior updateBehavior() const


void frameSwapped()

Protected Functions

virtual void initializeGL()
virtual void paintGL()
virtual void paintOverGL()
virtual void paintUnderGL()
virtual void resizeGL(int w, int h)

Reimplemented Protected Functions

virtual void paintEvent(QPaintEvent *event) override
virtual void resizeEvent(QResizeEvent *event) override

Detailed Description

QOpenGLWindow is an enhanced QWindow that allows easily creating windows that perform OpenGL rendering using an API that is compatible with QOpenGLWidget Unlike QOpenGLWidget, QOpenGLWindow has no dependency on the widgets module and offers better performance.

A typical application will subclass QOpenGLWindow and reimplement the following virtual functions:

  • initializeGL() to perform OpenGL resource initialization
  • resizeGL() to set up the transformation matrices and other window size dependent resources
  • paintGL() to issue OpenGL commands or draw using QPainter

To schedule a repaint, call the update() function. Note that this will not immediately result in a call to paintGL(). Calling update() multiple times in a row will not change the behavior in any way.

This is a slot so it can be connected to a QTimer::timeout() signal to perform animation. Note however that in the modern OpenGL world it is a much better choice to rely on synchronization to the vertical refresh rate of the display. See setSwapInterval() on a description of the swap interval. With a swap interval of 1, which is the case on most systems by default, the swapBuffers() call, that is executed internally by QOpenGLWindow after each repaint, will block and wait for vsync. This means that whenever the swap is done, an update can be scheduled again by calling update(), without relying on timers.

To request a specific configuration for the context, use setFormat() like for any other QWindow. This allows, among others, requesting a given OpenGL version and profile, or enabling depth and stencil buffers.

Note: It is up to the application to ensure depth and stencil buffers are requested from the underlying windowing system interface. Without requesting a non-zero depth buffer size there is no guarantee that a depth buffer will be available, and as a result depth testing related OpenGL operations may fail to function as expected.

Commonly used depth and stencil buffer size requests are 24 and 8, respectively. For example, a QOpenGLWindow subclass could do this in its constructor:

QSurfaceFormat format;

Unlike QWindow, QOpenGLWindow allows opening a painter on itself and perform QPainter-based drawing.

QOpenGLWindow supports multiple update behaviors. The default, NoPartialUpdate is equivalent to a regular, OpenGL-based QWindow. In contrast, PartialUpdateBlit and PartialUpdateBlend are more in line with QOpenGLWidget's way of working, where there is always an extra, dedicated framebuffer object present. These modes allow, by sacrificing some performance, redrawing only a smaller area on each paint and having the rest of the content preserved from of the previous frame. This is useful for applications than render incrementally using QPainter, because this way they do not have to redraw the entire window content on each paintGL() call.

Similarly to QOpenGLWidget, QOpenGLWindow supports the Qt::AA_ShareOpenGLContexts attribute. When enabled, the OpenGL contexts of all QOpenGLWindow instances will share with each other. This allows accessing each other's shareable OpenGL resources.

For more information on graphics in Qt, see Graphics.

Member Type Documentation

enum QOpenGLWindow::UpdateBehavior

This enum describes the update strategy of the QOpenGLWindow.

QOpenGLWindow::NoPartialUpdate0Indicates that the entire window surface will redrawn on each update and so no additional framebuffers are needed. This is the setting used in most cases and is equivalent to how drawing directly via QWindow would function.
QOpenGLWindow::PartialUpdateBlit1Indicates that the drawing performed in paintGL() does not cover the entire window. In this case an extra framebuffer object is created under the hood, and rendering performed in paintGL() will target this framebuffer. This framebuffer is then blitted onto the window surface's default framebuffer after each paint. This allows having QPainter-based drawing code in paintGL() which only repaints a smaller area at a time, because, unlike NoPartialUpdate, the previous content is preserved.
QOpenGLWindow::PartialUpdateBlend2Similar to PartialUpdateBlit, but instead of using framebuffer blits, the contents of the extra framebuffer is rendered by drawing a textured quad with blending enabled. This, unlike PartialUpdateBlit, allows alpha blended content and works even when the glBlitFramebuffer is not available. Performance-wise this setting is likely to be somewhat slower than PartialUpdateBlit.

Member Function Documentation

[explicit] QOpenGLWindow::QOpenGLWindow(QOpenGLWindow::UpdateBehavior updateBehavior = NoPartialUpdate, QWindow *parent = nullptr)

Constructs a new QOpenGLWindow with the given parent and updateBehavior.

See also QOpenGLWindow::UpdateBehavior.

[explicit] QOpenGLWindow::QOpenGLWindow(QOpenGLContext *shareContext, QOpenGLWindow::UpdateBehavior updateBehavior = NoPartialUpdate, QWindow *parent = nullptr)

Constructs a new QOpenGLWindow with the given parent and updateBehavior. The QOpenGLWindow's context will share with shareContext.

See also QOpenGLWindow::UpdateBehavior and shareContext.

[virtual noexcept] QOpenGLWindow::~QOpenGLWindow()

Destroys the QOpenGLWindow instance, freeing its resources.

The OpenGLWindow's context is made current in the destructor, allowing for safe destruction of any child object that may need to release OpenGL resources belonging to the context provided by this window.

Warning: if you have objects wrapping OpenGL resources (such as QOpenGLBuffer, QOpenGLShaderProgram, etc.) as members of a QOpenGLWindow subclass, you may need to add a call to makeCurrent() in that subclass' destructor as well. Due to the rules of C++ object destruction, those objects will be destroyed before calling this function (but after that the destructor of the subclass has run), therefore making the OpenGL context current in this function happens too late for their safe disposal.

See also makeCurrent.

QOpenGLContext *QOpenGLWindow::context() const

Returns The QOpenGLContext used by this window or 0 if not yet initialized.

GLuint QOpenGLWindow::defaultFramebufferObject() const

The framebuffer object handle used by this window.

When the update behavior is set to NoPartialUpdate, there is no separate framebuffer object. In this case the returned value is the ID of the default framebuffer.

Otherwise the value of the ID of the framebuffer object or 0 if not yet initialized.

void QOpenGLWindow::doneCurrent()

Releases the context.

It is not necessary to call this function in most cases, since the widget will make sure the context is bound and released properly when invoking paintGL().

See also makeCurrent().

[signal] void QOpenGLWindow::frameSwapped()

This signal is emitted after the potentially blocking buffer swap has been done. Applications that wish to continuously repaint synchronized to the vertical refresh, should issue an update() upon this signal. This allows for a much smoother experience compared to the traditional usage of timers.

QImage QOpenGLWindow::grabFramebuffer()

Returns a copy of the framebuffer.

Note: This is a potentially expensive operation because it relies on glReadPixels() to read back the pixels. This may be slow and can stall the GPU pipeline.

Note: When used together with update behavior NoPartialUpdate, the returned image may not contain the desired content when called after the front and back buffers have been swapped (unless preserved swap is enabled in the underlying windowing system interface). In this mode the function reads from the back buffer and the contents of that may not match the content on the screen (the front buffer). In this case the only place where this function can safely be used is paintGL() or paintOverGL().

[virtual protected] void QOpenGLWindow::initializeGL()

This virtual function is called once before the first call to paintGL() or resizeGL(). Reimplement it in a subclass.

This function should set up any required OpenGL resources and state.

There is no need to call makeCurrent() because this has already been done when this function is called. Note however that the framebuffer, in case partial update mode is used, is not yet available at this stage, so avoid issuing draw calls from here. Defer such calls to paintGL() instead.

See also paintGL() and resizeGL().

bool QOpenGLWindow::isValid() const

Returns true if the window's OpenGL resources, like the context, have been successfully initialized. Note that the return value is always false until the window becomes exposed (shown).

void QOpenGLWindow::makeCurrent()

Prepares for rendering OpenGL content for this window by making the corresponding context current and binding the framebuffer object, if there is one, in that context.

It is not necessary to call this function in most cases, because it is called automatically before invoking paintGL(). It is provided nonetheless to support advanced, multi-threaded scenarios where a thread different than the GUI or main thread may want to update the surface or framebuffer contents. See QOpenGLContext for more information on threading related issues.

This function is suitable for calling also when the underlying platform window is already destroyed. This means that it is safe to call this function from a QOpenGLWindow subclass' destructor. If there is no native window anymore, an offscreen surface is used instead. This ensures that OpenGL resource cleanup operations in the destructor will always work, as long as this function is called first.

See also QOpenGLContext, context(), paintGL(), and doneCurrent().

[override virtual protected] void QOpenGLWindow::paintEvent(QPaintEvent *event)

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

Paint event handler. Calls paintGL().

See also paintGL().

[virtual protected] void QOpenGLWindow::paintGL()

This virtual function is called whenever the window contents 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.

Before invoking this function, the context and the framebuffer, if there is one, are bound, and the viewport is set up by a call to glViewport(). No other state is set and no clearing or drawing is performed by the framework.

Note: When using a partial update behavior, like PartialUpdateBlend, the output of the previous paintGL() call is preserved and, after the additional drawing performed in the current invocation of the function, the content is blitted or blended over the content drawn directly to the window in paintUnderGL().

See also initializeGL(), resizeGL(), paintUnderGL(), paintOverGL(), and UpdateBehavior.

[virtual protected] void QOpenGLWindow::paintOverGL()

This virtual function is called after each invocation of paintGL().

When the update mode is set to NoPartialUpdate, there is no difference between this function and paintGL(), performing rendering in either of them leads to the same result.

Like paintUnderGL(), rendering in this function targets the default framebuffer of the window, regardless of the update behavior. It gets called after paintGL() has returned and the blit (PartialUpdateBlit) or quad drawing (PartialUpdateBlend) has been done.

See also paintGL(), paintUnderGL(), and UpdateBehavior.

[virtual protected] void QOpenGLWindow::paintUnderGL()

The virtual function is called before each invocation of paintGL().

When the update mode is set to NoPartialUpdate, there is no difference between this function and paintGL(), performing rendering in either of them leads to the same result.

The difference becomes significant when using PartialUpdateBlend, where an extra framebuffer object is used. There, paintGL() targets this additional framebuffer object, which preserves its contents, while paintUnderGL() and paintOverGL() target the default framebuffer, i.e. directly the window surface, the contents of which is lost after each displayed frame.

Note: Avoid relying on this function when the update behavior is PartialUpdateBlit. This mode involves blitting the extra framebuffer used by paintGL() onto the default framebuffer after each invocation of paintGL(), thus overwriting all drawing generated in this function.

See also paintGL(), paintOverGL(), and UpdateBehavior.

[override virtual protected] void QOpenGLWindow::resizeEvent(QResizeEvent *event)

Reimplements: QWindow::resizeEvent(QResizeEvent *ev).

Resize event handler. Calls resizeGL().

See also resizeGL().

[virtual protected] void QOpenGLWindow::resizeGL(int w, int h)

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

Note: This is merely a convenience function in order to provide an API that is compatible with QOpenGLWidget. Unlike with QOpenGLWidget, derived classes are free to choose to override resizeEvent() instead of this function.

Note: Avoid issuing OpenGL commands from this function as there may not be a context current when it is invoked. If it cannot be avoided, call makeCurrent().

Note: Scheduling updates from here is not necessary. The windowing systems will send expose events that trigger an update automatically.

See also initializeGL() and paintGL().

QOpenGLContext *QOpenGLWindow::shareContext() const

Returns The QOpenGLContext requested to be shared with this window's QOpenGLContext.

QOpenGLWindow::UpdateBehavior QOpenGLWindow::updateBehavior() const

Returns the update behavior for this QOpenGLWindow.

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