Qt 5.6 | Qt GUI | C++ Classes | QOffscreenSurface

Qt 5.6.2 Reference Documentation

# QOffscreenSurface Class

The QOffscreenSurface class represents an offscreen surface in the underlying platform. More...

Header: | #include <QOffscreenSurface>
qmake: | QT += gui
Since: | Qt 5.1
Inherits: | QSurface and

- List of all members, including inherited members

## Public Functions

| QOffscreenSurface(QScreen *targetScreen = Q_NULLPTR)
virtual | ~QOffscreenSurface()
void | create()
void | destroy()
bool | isValid() const
QSurfaceFormat | requestedFormat() const
QScreen * | screen() const
void | setFormat(const QSurfaceFormat &format)
void | setScreen(QScreen *newScreen)

## Reimplemented Public Functions

virtual QSurfaceFormat | format() const
virtual QSize | size() const
virtual SurfaceType | surfaceType() const

- 6 public functions inherited from QSurface

## Signals

void | screenChanged(QScreen *screen)

## Detailed Description

The QOffscreenSurface class represents an offscreen surface in the underlying platform.

QOffscreenSurface is intended to be used with QOpenGLContext to allow rendering with OpenGL in an arbitrary thread without the need to create a QWindow.

Even though the surface is typically renderable, the surface's pixels are not accessible. QOffscreenSurface should only be used to create OpenGL resources such as textures or framebuffer objects.

An application will typically use QOffscreenSurface to perform some time-consuming tasks in a separate thread in order to avoid stalling the main rendering thread. Resources created in the QOffscreenSurface's context can be shared with the main OpenGL context. Some common use cases are asynchronous texture uploads or rendering into a QOpenGLFramebufferObject.

How the offscreen surface is implemented depends on the underlying platform, but it will typically use a pixel buffer (pbuffer). If the platform doesn't implement or support offscreen surfaces, QOffscreenSurface will use an invisible QWindow internally.

Note: Due to the fact that QOffscreenSurface is backed by a QWindow on some platforms, cross-platform applications must ensure that create() is only called on the main (GUI) thread. The QOffscreenSurface is then safe to be used with makeCurrent() on other threads, but the initialization and destruction must always happen on the main (GUI) thread.

Note: In order to create an offscreen surface that is guaranteed to be compatible with a given context and window, make sure to set the format to the context's or the window's actual format, that is, the QSurfaceFormat returned from QOpenGLContext::format() or QWindow::format() after the context or window has been created. Passing the format returned from QWindow::requestedFormat() to setFormat() may result in an incompatible offscreen surface since the underlying windowing system interface may offer a different set of configurations for window and pbuffer surfaces.

Note: Some platforms may utilize a surfaceless context extension (for example EGL_KHR_surfaceless_context) when available. In this case there will be no underlying native surface. For the use cases of QOffscreenSurface (rendering to FBOs, texture upload) this is not a problem.

## Member Function Documentation

### QOffscreenSurface::QOffscreenSurface(QScreen *targetScreen = Q_NULLPTR)

Creates an offscreen surface for the targetScreen.

The underlying platform surface is not created until create() is called.

See also setScreen() and create().

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

Destroys the offscreen surface.

### void QOffscreenSurface::create()

Allocates the platform resources associated with the offscreen surface.

It is at this point that the surface format set using setFormat() gets resolved into an actual native surface.

Call destroy() to free the platform resources if necessary.

Note: Some platforms require this function to be called on the main (GUI) thread.

See also destroy().

### void QOffscreenSurface::destroy()

Releases the native platform resources associated with this offscreen surface.

See also create().

### `[virtual]` QSurfaceFormat QOffscreenSurface::format() const

Reimplemented from QSurface::format().

Returns the actual format of this offscreen surface.

After the offscreen surface has been created, this function will return the actual surface format of the surface. It might differ from the requested format if the requested format could not be fulfilled by the platform.

See also setFormat(), create(), and requestedFormat().

### bool QOffscreenSurface::isValid() const

Returns `true` if this offscreen surface is valid; otherwise returns `false`.

The offscreen surface is valid if the platform resources have been successfuly allocated.

See also create().

### QSurfaceFormat QOffscreenSurface::requestedFormat() const

Returns the requested surfaceformat of this offscreen surface.

If the requested format was not supported by the platform implementation, the requestedFormat will differ from the actual offscreen surface format.

This is the value set with setFormat().

See also setFormat() and format().

### QScreen *QOffscreenSurface::screen() const

Returns the screen to which the offscreen surface is connected.

See also setScreen().

### `[signal]` void QOffscreenSurface::screenChanged(QScreen *screen)

This signal is emitted when an offscreen surface's screen changes, either by being set explicitly with setScreen(), or automatically when the window's screen is removed.

### void QOffscreenSurface::setFormat(const QSurfaceFormat &format)

Sets the offscreen surface format.

The surface format will be resolved in the create() function. Calling this function after create() will not re-resolve the surface format of the native surface.

See also format(), create(), and destroy().

### void QOffscreenSurface::setScreen(QScreen *newScreen)

Sets the screen to which the offscreen surface is connected.

If the offscreen surface has been created, it will be recreated on the newScreen.

See also screen().

### `[virtual]` QSize QOffscreenSurface::size() const

Reimplemented from QSurface::size().

Returns the size of the offscreen surface.

### `[virtual]` SurfaceType QOffscreenSurface::surfaceType() const

Reimplemented from QSurface::surfaceType().

Returns the surface type of the offscreen surface.

The surface type of an offscreen surface is always QSurface::OpenGLSurface.