qtmultimedia/doc/src/cameraoverview.qdoc

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/*!
\page cameraoverview.html
\title Camera Overview
\brief Camera viewfinder, still image capture, and video recording.

The Qt Multimedia API provides a number of camera related classes, so you
can access images and videos from mobile device cameras or webcameras.
There are both C++ and QML apis for common tasks.

\section1 Camera Features

In order to use the camera classes a quick overview of the way a camera
works is needed.  If you're already familiar with this, you can skip ahead to
\l {camera-tldr}{Camera implementation details}.

[TBD - this needs a diagram]
* Camera features
 * lens -> sensors -> image processing -> capture/recording

\section2 The lens assembly
At one end of the camera assembly is the lens assembly (one or
more lenses, arranged to focus light onto the sensor).  The lens
themselves can sometimes be moved to adjust things like focus and zoom,
or they might be fixed in an arrangement to give a good balance between
objects in focus, and cost.

Some lens assemblies can automatically be adjusted so that
an object at different distances from the camera can be kept in focus.
This is usually done by measuring how sharp a particular area of the
frame is, and by adjusting the lens assembly until it is maximally
sharp.  In some cases the camera will always use the center of the
frame for this.  Other cameras may also allow the region to focus
to be specified (for "touch to zoom", or "face zoom" features).

\section2 The sensor
Once light arrives at the sensor, it gets converted into digital pixels.
This process can depend on a number of things but ultimately comes down
to two things - how long the conversion is allowed to take, and how
bright the light is.  The longer a conversion can take, the better the
quality.  Using a flash can assist with letting more light hit the sensor,
allowing it to convert pixels faster, giving better quality for the same
amount of time.  Conversely, allowing a longer conversion time can let you
take photos in darker environments, as long as the camera is steady.

\section2 Image processing
After the image has been captured by the sensor, the camera firmware performs
various image processing tasks on it to compensate for various sensor
characteristics, current lighting, and desired image properties.  Faster sensor
pixel conversion times tend to introduce digital noise, so some amount of image
processing can be done to remove this based on the camera sensor settings.

The color of the image can also be adjusted at this stage to compensate for
different light sources - fluorescent lights and sunlight give very different
appearances to the same object, so the image can be adjusted based on the
white balance of the picture (due to the different color temperatures of the
light sources).

Some forms of "special effects" can also be performed at this stage.  Black
and white, sepia, or "negative" style images can be produced.

\section2 Recording for posterity
Finally, once a perfectly focused, exposed and processed image has been
created, it can be put to good use.  Camera images can be further processed
by application code (for example, to detect barcodes, or to stitch together),
or saved to a common format like JPEG, or used to create a movie.  Many of
these tasks have classes to assist them.

\target camera-tldr
\section1 Camera Implementation Details
\section2 Viewfinder

While not strictly necessary, it's often useful to be able to see
what the camera is pointing at.  Most digital cameras allow an image
feed from the camera sensor at a lower resolution (usually up to
the size of the display of the camera) so you can compose
a photo or video, and then switch to a slower but higher resolution
mode for capturing the image.

Depending on whether you're using QML or C++, you can do this in multiple ways.
In QML, you can use the Camera and VideoOutput elements together to show a
simple viewfinder:

\qml
    Camera {
        id: camera
        // You can adjust various settings in here
    }

    VideoOutput {
        source: camera
    }
\endqml

In C++, your choice depends on whether you are using widgets, or QGraphicsView.
The \l QVideoWidget class is used in the widgets case, and \l QGraphicsVideoItem
is useful for QGraphicsView.

    \snippet doc/src/snippets/multimedia-snippets/camera.cpp Camera overview viewfinder

For advanced usage (like processing viewfinder frames as they come, to detect
objects or patterns), you can also derive from \l QAbstractVideoSurface and
set that as the viewfinder for the QCamera object.  In this case you will
need to render the viewfinder image yourself.

    \snippet doc/src/snippets/multimedia-snippets/camera.cpp Camera overview surface

\section2 Still Images

After setting up a viewfinder and finding something photogenic,
to capture an image we need to initialize a new QCameraImageCapture
object. All that is then needed is to start the camera, lock it so
that things are in focus and the settings are not different from the
viewfinder while the image capture occurs, capture the image, and
finally unlock the camera ready for the next photo.

    \snippet doc/src/snippets/multimedia-snippets/camera.cpp Camera overview capture

\section2 Movies

Previously we saw code that allowed the capture of a still image. Recording
video requires the use of a \l QMediaRecorder object.

To record video we need to create a camera object as before but this time as
well as creating a viewfinder, we will also initialize a media recorder object.

    \snippet doc/src/snippets/multimedia-snippets/camera.cpp Camera overview movie

Signals from the \e mediaRecorder can be connected to slots to react to
changes in the state of the recorder or error events. Recording itself
starts with the \l {QMediaRecorder::record()}{record()} function of
mediaRecorder being called, this causes the signal \l
{QMediaRecorder::stateChanged()}{stateChanged()} to be emitted. The
recording process can be changed with the \l {QMediaRecorder::record()}{record()},
\l {QMediaRecorder::stop()}{stop()} and \l {QMediaRecorder::setMuted()}{setMuted()}
slots in \l QMediaRecorder.

\section2 Controlling the imaging pipeline

Now that the basics of capturing images or movies are covered, there are a number
of ways to control the imaging pipeline to implement some interesting techniques.
As explained earlier, several physical and electronic elements combine to determine
the final images, and you can control them with different classes.

\section3 Focus and zoom

Focusing (and zoom) is managed primarily by the \l QCameraFocus class.
QCameraFocus allows the developer to set the general policy by means of the
enums for the \l {QCameraFocus::FocusMode}{FocusMode} and the
\l {QCameraFocus::FocusPointMode}{FocusPointMode}. \l {QCameraFocus::FocusMode}{FocusMode}
deals with settings such as \l {QCameraFocus::FocusMode}{AutoFocus},
\l {QCameraFocus::FocusMode}{ContinuousFocus} and \l {QCameraFocus::FocusMode}{InfinityFocus},
whereas \l {QCameraFocus::FocusMode}{FocusPointMode} deals with the
various focus zones within the view that are used for autofocus modes. \l {QCameraFocus::FocusMode}{FocusPointMode}
has support for face recognition (where the camera supports it), center focus and a custom
focus where the focus point can be specified.

For camera hardware that supports it, \l {QCameraFocus::FocusMode}{Macro focus} allows
imaging of things that are close to the sensor.  This is useful in applications like
barcode recognition, or business card scanning.

In addition to focus, QCameraFocus allows you to control any available optical or
digital zoom.  In general, optical zoom is higher quality, but more expensive to
manufacture, so the available zoom range might be limited (or fixed to unity).

\section3 Exposure, aperture, shutter speed and flash

There are a number of settings that affect the amount of light that hits the
camera sensor, and hence the quality of the resulting image.  The \l QCameraExposure
class allows you to adjust these settings.  You can use this class to implement
some techniques like High Dynamic Range (HDR) photos by locking the exposure
parameters (with \l {QCamera::lock()}), or motion blur by setting slow shutter speeds
with small apertures.

The main settings for automatic image taking are the \l {QCameraExposure::ExposureMode}{exposure mode}
and \l {QCameraExposure::FlashMode}{flash mode}.  Several other settings (aperture, ISO setting,
 shutter speed) are usually managed automatically but can also be overridden if desired.

You can also adjust the \l {QCameraExposure::meteringMode()} to control which parts
of the camera frame to measure exposure at.  Some camera implementations also allow
you to specify a specific point that should be used for exposure metering - this is
useful if you can let the user touch or click on an interesting part of the viewfinder,
and then use this point so that the image exposure is best at that point.

Finally, you can control the flash hardware (if present) using this class.  In some cases
the hardware may also double as a torch (typically when the flash is LED based, rather than
a xenon or other bulb).  See also the \l {Torch} QML element for an easy to use API for
torch functionality.

\section3 Image processing

The QCameraImageProcessing class lets you adjust the image processing
part of the pipeline.  This includes the \l {QCameraImageCapture::WhiteBalanceMode}{white balance}
(or color temperature), \l {QCameraImageCapture::contrast()}{contrast},
\l {QCameraImageCapture::saturation()}{saturation}, \l {QCameraImageCapture::setSharpening()}{sharpening}
and \l {QCameraImageCapture::setDenoisingLevel()}{denoising}.  Most cameras support automatic settings
for all of these, so you shouldn't need to adjust them unless the user wants a specific setting.

If you're taking a series of images (for example, to stitch them together for
a panoramic image), you should lock the image processing settings so that all the
images taken appear similar with \e {QCamera::lock(QCamera::LockWhiteBalance)}/

\section3 Canceling Asynchronous Operations

Various operations such as image capture and auto focusing occur
asynchrously. These operations can often be canceled by the start of a new
operation as long as this is supported by the camera. For image capture,
the operation can be canceled by calling
\l {QCameraImageCapture::cancelCapture()}{cancelCapture()}. For AutoFocus,
autoexposure or white balance cancellation can be done by calling
\e {QCamera::unlock(QCamera::LockFocus)}.

\section1 Examples

There are both C++ and QML examples available.

C++ Examples:
\list
    \o \l Camera
\endlist

QML Examples:
\list
    \o \l declarative-camera
    \o \l qmlvideofx
\endlist

\section1 Reference Documentation

\section2 Camera Classes

\annotatedlist multimedia_camera

\section2 QML Elements

\annotatedlist camera_qml

*/