- Qt 5.5
- Qt Multimedia
- Multimedia Backend Development
- Qt 5.5.1 Reference Documentation

# Multimedia Backend Development

## Overview

A multimedia backend provides the glue between platform specific libraries, and Qt Multimedia. In some cases the available cross-platform Multimedia APIs or implementations are not sufficient, or not immediately available on a certain platform. Alternatively, the multimedia implementation on a platform might expose certain extra properties or functionality that other platforms do not, or a finer degree of control might be possible. For these cases, it is possible to use extended controls directly.

In addition, if you plan to port the Qt Multimedia APIs to a new platform, you do this by implementing certain control and service classes, as detailed below.

## Extending the API

For the developer who wishes to extend the functionality of the Qt Multimedia classes there are several classes of particular importance. The classes providing default functionality are QMediaService, QMediaServiceProvider and QMediaControl. Some of these classes are not in the public API since they are very seldom useful to application developers.

To extend the Multimedia API you would use the following three classes or classes derived from them.

- QMediaServiceProvider is used by the top level client class to request a service. The top level class knowing what kind of service it needs.
- QMediaService provides a service and when asked by the top level object, say a component, will return a QMediaControl object.
- QMediaControl allows the control of the service using a known interface.

Consider a developer creating, for example, a media player class called MyPlayer. It may have special requirements beyond ordinary media players and so may need a custom service and a custom control. We can subclass QMediaServiceProvider to create our MyServiceProvider class. Also we will create a MyMediaService, and the MyMediaControl to manipulate the media service.

The MyPlayer object calls MyServiceProvider::requestService() to get an instance of MyMediaService. Then the MyPlayer object calls this service object it has just received and calling requestControl() it will receive the control object derived from QMediaControl.

Now we have all the parts necessary for our media application. We have the service provider, the service it provides and the control used to manipulate the service. Since our MyPlayer object has instances of the service and its control then it would be possible for these to be used by associated classes that could do additional actions, perhaps with their own control since the parameter to requestControl() is a zero-terminated string, const char *, for the interface.

### Adding a Media Service Provider

In general, adding a new media service provider is outside the scope of this documentation. For best results, consult the existing provider source code, and seek assistance on the relevant mailing lists and IRC channels.

### Classes for service implementers.

QAudioDecoderControl | Access to the audio decoding functionality of a QMediaService
QAudioEncoderSettingsControl | Access to the settings of a media service that performs audio encoding
QAudioInputSelectorControl | Audio input selector media control
QAudioOutputSelectorControl | Audio output selector media control
QCameraCaptureBufferFormatControl | Control for setting the capture buffer format
QCameraCaptureDestinationControl | Control for setting capture destination
QCameraControl | Abstract base class for classes that control still cameras or video cameras
QCameraExposureControl | Allows controlling camera exposure parameters
QCameraFeedbackControl | Allows controlling feedback (sounds etc) during camera operation
QCameraFlashControl | Allows controlling a camera's flash
QCameraFocusControl | Supplies control for focusing related camera parameters
QCameraImageCaptureControl | Control interface for image capture services
QCameraImageProcessingControl | Abstract class for controlling image processing parameters, like white balance, contrast, saturation, sharpening and denoising
QCameraInfoControl | Camera info media control
QCameraLocksControl | Abstract base class for classes that control still cameras or video cameras
QCameraViewfinderSettingsControl | Abstract class for controlling camera viewfinder parameters
QCameraViewfinderSettingsControl2 | Access to the viewfinder settings of a camera media service
QCameraZoomControl | Supplies control for optical and digital camera zoom
QImageEncoderControl | Access to the settings of a media service that performs image encoding
QMediaAudioProbeControl | Allows control over probing audio data in media objects
QMediaAvailabilityControl | Supplies a control for reporting availability of a service
QMediaContainerControl | Access to the output container format of a QMediaService
QMediaControl | Base interface for media service controls
QMediaGaplessPlaybackControl | Access to the gapless playback related control of a QMediaService
QMediaNetworkAccessControl | Allows the setting of the Network Access Point for media related activities
QMediaPlayerControl | Access to the media playing functionality of a QMediaService
QMediaRecorderControl | Access to the recording functionality of a QMediaService
QMediaService | Common base class for media service implementations
QMediaStreamsControl | Media stream selection control
QMediaVideoProbeControl | Allows control over probing video frames in media objects
QMetaDataReaderControl | Read access to the meta-data of a QMediaService's media
QMetaDataWriterControl | Write access to the meta-data of a QMediaService's media
QRadioDataControl | Access to the RDS functionality of the radio in the QMediaService
QRadioTunerControl | Access to the radio tuning functionality of a QMediaService
QVideoDeviceSelectorControl | Video device selector media control
QVideoEncoderSettingsControl | Access to the settings of a media service that performs video encoding
QVideoRendererControl | Media control for rendering video to a QAbstractVideoSurface
QVideoWindowControl | Media control for rendering video to a window