QAbstractVideoSurface Class

The QAbstractVideoSurface class is a base class for video presentation surfaces. More...

Header: #include <QAbstractVideoSurface>
qmake: QT += multimedia
Inherits: QObject

Public Types

enum Error { NoError, UnsupportedFormatError, IncorrectFormatError, StoppedError, ResourceError }


Public Functions

QAbstractVideoSurface(QObject *parent = nullptr)
virtual ~QAbstractVideoSurface()
QAbstractVideoSurface::Error error() const
bool isActive() const
virtual bool isFormatSupported(const QVideoSurfaceFormat &format) const
QSize nativeResolution() const
virtual QVideoSurfaceFormat nearestFormat(const QVideoSurfaceFormat &format) const
virtual bool present(const QVideoFrame &frame) = 0
virtual bool start(const QVideoSurfaceFormat &format)
virtual void stop()
virtual QList<QVideoFrame::PixelFormat> supportedPixelFormats(QAbstractVideoBuffer::HandleType type = QAbstractVideoBuffer::NoHandle) const = 0
QVideoSurfaceFormat surfaceFormat() const


void activeChanged(bool active)
void nativeResolutionChanged(const QSize &resolution)
void supportedFormatsChanged()
void surfaceFormatChanged(const QVideoSurfaceFormat &format)

Protected Functions

void setError(QAbstractVideoSurface::Error error)
void setNativeResolution(const QSize &resolution)

Detailed Description

The QAbstractVideoSurface class defines the standard interface that video producers use to inter-operate with video presentation surfaces. You can subclass this interface to receive video frames from sources like decoded media or cameras to perform your own processing.

A video surface presents a continuous stream of identically formatted QVideoFrame instances, where the format of each frame is compatible with a stream format supplied when starting a presentation. Each frame may have timestamp information that can be used by the surface to decide when to display that frame.

A list of pixel formats a surface can present is given by the supportedPixelFormats() function, and the isFormatSupported() function will test if a video surface format is supported. If a format is not supported the nearestFormat() function may be able to suggest a similar format. For example, if a surface supports fixed set of resolutions it may suggest the smallest supported resolution that contains the proposed resolution.

The start() function takes a supported format and enables a video surface. Once started a surface will begin displaying the frames it receives in the present() function. Surfaces may hold a reference to the buffer of a presented video frame until a new frame is presented or streaming is stopped. In addition, a video surface may hold a reference to a video frame until the end timestamp has passed. The stop() function will disable a surface and release any video buffers it holds references to.

Implementing a subclass of QAbstractVideoSurface

When implementing a subclass of this interface, there are only a handful of functions to implement, broken down into two classes:

  • Format related
  • Presentation related

For format related functionality, you just have to describe the pixel formats that you support (and the nearestFormat() function). For presentation related functionality, you have to implement the present() function, and the start() and stop() functions.

Note: You must call the base class implementation of start() and stop() in your implementation.

Member Type Documentation

enum QAbstractVideoSurface::Error

This enum describes the errors that may be returned by the error() function.

QAbstractVideoSurface::NoError0No error occurred.
QAbstractVideoSurface::UnsupportedFormatError1A video format was not supported.
QAbstractVideoSurface::IncorrectFormatError2A video frame was not compatible with the format of the surface.
QAbstractVideoSurface::StoppedError3The surface has not been started.
QAbstractVideoSurface::ResourceError4The surface could not allocate some resource.

Property Documentation

nativeResolution : const QSize

The native resolution of video surface. This is the resolution of video frames the surface can render with optimal quality and/or performance.

The native resolution is not always known and can be changed during playback.

Access functions:

QSize nativeResolution() const

Notifier signal:

void nativeResolutionChanged(const QSize &resolution)

Member Function Documentation

QAbstractVideoSurface::QAbstractVideoSurface(QObject *parent = nullptr)

Constructs a video surface with the given parent.

[signal] void QAbstractVideoSurface::activeChanged(bool active)

Signals that the active state of a video surface has changed.

See also isActive(), start(), and stop().

[signal] void QAbstractVideoSurface::nativeResolutionChanged(const QSize &resolution)

Signals the native resolution of video surface has changed.

Note: Notifier signal for property nativeResolution.

[signal] void QAbstractVideoSurface::supportedFormatsChanged()

Signals that the set of formats supported by a video surface has changed.

See also supportedPixelFormats() and isFormatSupported().

[signal] void QAbstractVideoSurface::surfaceFormatChanged(const QVideoSurfaceFormat &format)

Signals that the configured format of a video surface has changed.

See also surfaceFormat() and start().

[virtual] QAbstractVideoSurface::~QAbstractVideoSurface()

Destroys a video surface.

QAbstractVideoSurface::Error QAbstractVideoSurface::error() const

Returns the last error that occurred.

If a surface fails to start(), or stops unexpectedly this function can be called to discover what error occurred.

See also setError().

bool QAbstractVideoSurface::isActive() const

Indicates whether a video surface has been started.

Returns true if the surface has been started, and false otherwise.

[virtual] bool QAbstractVideoSurface::isFormatSupported(const QVideoSurfaceFormat &format) const

Tests a video surface format to determine if a surface can accept it.

Returns true if the format is supported by the surface, and false otherwise.

[virtual] QVideoSurfaceFormat QAbstractVideoSurface::nearestFormat(const QVideoSurfaceFormat &format) const

Returns a supported video surface format that is similar to format.

A similar surface format is one that has the same pixel format and handle type but may differ in some of the other properties. For example, if there are restrictions on the frame sizes a video surface can accept it may suggest a format with a larger frame size and a viewport the size of the original frame size.

If the format is already supported it will be returned unchanged, or if there is no similar supported format an invalid format will be returned.

[pure virtual] bool QAbstractVideoSurface::present(const QVideoFrame &frame)

Presents a video frame.

Returns true if the frame was presented, and false if an error occurred.

Not all surfaces will block until the presentation of a frame has completed. Calling present() on a non-blocking surface may fail if called before the presentation of a previous frame has completed. In such cases the surface may not return to a ready state until it has had an opportunity to process events.

If present() fails for any other reason the surface should immediately enter the stopped state and an error() value will be set.

A video surface must be in the started state for present() to succeed, and the format of the video frame must be compatible with the current video surface format.

See also error().

[protected] void QAbstractVideoSurface::setError(QAbstractVideoSurface::Error error)

Sets the value of error() to error.

This can be called by implementors of this interface to communicate what the most recent error was.

See also error().

[protected] void QAbstractVideoSurface::setNativeResolution(const QSize &resolution)

Set the video surface native resolution.

This function can be called by implementors of this interface to specify to frame producers what the native resolution of this surface is.

See also nativeResolution().

[virtual] bool QAbstractVideoSurface::start(const QVideoSurfaceFormat &format)

Starts a video surface presenting format frames.

Returns true if the surface was started, and false if an error occurred.

Note: You must call the base class implementation of start() at the end of your implementation.

See also isActive() and stop().

[virtual] void QAbstractVideoSurface::stop()

Stops a video surface presenting frames and releases any resources acquired in start().

Note: You must call the base class implementation of stop() at the start of your implementation.

See also isActive() and start().

[pure virtual] QList<QVideoFrame::PixelFormat> QAbstractVideoSurface::supportedPixelFormats(QAbstractVideoBuffer::HandleType type = QAbstractVideoBuffer::NoHandle) const

Returns a list of pixel formats a video surface can present for a given handle type.

The pixel formats returned for the QAbstractVideoBuffer::NoHandle type are valid for any buffer that can be mapped in read-only mode.

Types that are first in the list can be assumed to be faster to render.

QVideoSurfaceFormat QAbstractVideoSurface::surfaceFormat() const

Returns the format of a video surface.

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