qpaintdevice.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \class QPaintDevice
    \brief The QPaintDevice class is the base class of objects that
    can be painted on with QPainter.
 
    \ingroup painting
    \inmodule QtGui
 
    A paint device is an abstraction of a two-dimensional space that
    can be drawn on using a QPainter.  Its default coordinate system has
    its origin located at the top-left position. X increases to the
    right and Y increases downwards. The unit is one pixel.
 
    The drawing capabilities of QPaintDevice are currently implemented
    by the QWidget, QImage, QPixmap, QPicture, and QPrinter subclasses.
 
    To implement support for a new backend, you must derive from
    QPaintDevice and reimplement the virtual paintEngine() function to
    tell QPainter which paint engine should be used to draw on this
    particular device. Note that you also must create a corresponding
    paint engine to be able to draw on the device, i.e derive from
    QPaintEngine and reimplement its virtual functions.
 
    \warning Qt requires that a QGuiApplication object exists before
    any paint devices can be created. Paint devices access window
    system resources, and these resources are not initialized before
    an application object is created.
 
    The QPaintDevice class provides several functions returning the
    various device metrics: The depth() function returns its bit depth
    (number of bit planes). The height() function returns its height
    in default coordinate system units (e.g. pixels for QPixmap and
    QWidget) while heightMM() returns the height of the device in
    millimeters. Similiarily, the width() and widthMM() functions
    return the width of the device in default coordinate system units
    and in millimeters, respectively. Alternatively, the protected
    metric() function can be used to retrieve the metric information
    by specifying the desired PaintDeviceMetric as argument.
 
    The logicalDpiX() and logicalDpiY() functions return the
    horizontal and vertical resolution of the device in dots per
    inch. The physicalDpiX() and physicalDpiY() functions also return
    the resolution of the device in dots per inch, but note that if
    the logical and physical resolution differ, the corresponding
    QPaintEngine must handle the mapping. Finally, the colorCount()
    function returns the number of different colors available for the
    paint device.
 
    \sa QPaintEngine, QPainter, {Coordinate System}, {Paint System}
*/
 
/*!
    \enum QPaintDevice::PaintDeviceMetric
 
    Describes the various metrics of a paint device.
 
    \value PdmWidth The width of the paint device in default
    coordinate system units (e.g. pixels for QPixmap and QWidget). See
    also width().
 
    \value PdmHeight The height of the paint device in default
    coordinate system units (e.g. pixels for QPixmap and QWidget). See
    also height().
 
    \value PdmWidthMM The width of the paint device in millimeters. See
    also widthMM().
 
    \value PdmHeightMM  The height of the paint device in millimeters. See
    also heightMM().
 
    \value PdmNumColors The number of different colors available for
    the paint device. See also colorCount().
 
    \value PdmDepth The bit depth (number of bit planes) of the paint
    device. See also depth().
 
    \value PdmDpiX The horizontal resolution of the device in dots per
    inch. See also logicalDpiX().
 
    \value PdmDpiY  The vertical resolution of the device in dots per inch. See
    also logicalDpiY().
 
    \value PdmPhysicalDpiX The horizontal resolution of the device in
    dots per inch. See also physicalDpiX().
 
    \value PdmPhysicalDpiY The vertical resolution of the device in
    dots per inch. See also physicalDpiY().
 
    \value PdmDevicePixelRatio The device pixel ratio for device. Common
    values are 1 for normal-dpi displays and 2 for high-dpi "retina"
    displays.
 
    \value PdmDevicePixelRatioScaled The scaled device pixel ratio for the device.
    This is identical to PdmDevicePixelRatio, except that the value is scaled by a
    constant factor in order to support paint devices with fractional scale factors.
    The constant scaling factor used is devicePixelRatioFScale(). This enum value
    has been introduced in Qt 5.6.
 
    \value [since 6.8] PdmDevicePixelRatioF_EncodedA This enum item, together with the
    corresponding \c B item, are used together for the device pixel ratio of the device, as an
    encoded \c double floating point value. A QPaintDevice subclass that supports \e fractional DPR
    values should implement support for these two enum items in its override of the metric()
    function. The return value is expected to be the result of the encodeMetricF() function.
 
    \value [since 6.8] PdmDevicePixelRatioF_EncodedB See PdmDevicePixelRatioF_EncodedA.
 
    \sa metric(), devicePixelRatio()
*/
 
/*!
    \fn QPaintDevice::QPaintDevice()
 
    Constructs a paint device. This constructor can be invoked only from
    subclasses of QPaintDevice.
*/
 
/*!
    \fn QPaintDevice::~QPaintDevice()
 
    Destroys the paint device and frees window system resources.
*/
 
/*!
    \fn int QPaintDevice::devType() const
 
    \internal
 
    Returns the device type identifier, which is QInternal::Widget
    if the device is a QWidget, QInternal::Pixmap if it's a
    QPixmap, QInternal::Printer if it's a QPrinter,
    QInternal::Picture if it's a QPicture, or
    QInternal::UnknownDevice in other cases.
*/
 
/*!
    \fn bool QPaintDevice::paintingActive() const
 
    Returns \c true if the device is currently being painted on, i.e. someone has
    called QPainter::begin() but not yet called QPainter::end() for
    this device; otherwise returns \c false.
 
    \sa QPainter::isActive()
*/
 
/*!
    \fn QPaintEngine *QPaintDevice::paintEngine() const
 
    Returns a pointer to the paint engine used for drawing on the
    device.
*/
 
/*!
    \fn int QPaintDevice::metric(PaintDeviceMetric metric) const
 
    Returns the metric information for  the given paint device \a metric.
 
    \sa PaintDeviceMetric
*/
 
/*!
    \fn int QPaintDevice::width() const
 
    Returns the width of the paint device in default coordinate system
    units (e.g. pixels for QPixmap and QWidget).
 
    \sa widthMM()
*/
 
/*!
    \fn int QPaintDevice::height() const
 
    Returns the height of the paint device in default coordinate
    system units (e.g. pixels for QPixmap and QWidget).
 
    \sa heightMM()
*/
 
/*!
    \fn int QPaintDevice::widthMM() const
 
    Returns the width of the paint device in millimeters. Due to platform
    limitations it may not be possible to use this function to determine
    the actual physical size of a widget on the screen.
 
    \sa width()
*/
 
/*!
    \fn int QPaintDevice::heightMM() const
 
    Returns the height of the paint device in millimeters. Due to platform
    limitations it may not be possible to use this function to determine
    the actual physical size of a widget on the screen.
 
    \sa height()
*/
 
/*!
    \fn int QPaintDevice::colorCount() const
 
    Returns the number of different colors available for the paint
    device. If the number of colors available is too great to be
    represented by the int data type, then INT_MAX will be returned
    instead.
*/
 
/*!
    \fn int QPaintDevice::depth() const
 
    Returns the bit depth (number of bit planes) of the paint device.
*/
 
/*!
    \fn int QPaintDevice::logicalDpiX() const
 
    Returns the horizontal resolution of the device in dots per inch,
    which is used when computing font sizes. For X11, this is usually
    the same as could be computed from widthMM().
 
    Note that if the logicalDpiX() doesn't equal the physicalDpiX(),
    the corresponding QPaintEngine must handle the resolution mapping.
 
    \sa logicalDpiY(), physicalDpiX()
*/
 
/*!
    \fn int QPaintDevice::logicalDpiY() const
 
    Returns the vertical resolution of the device in dots per inch,
    which is used when computing font sizes. For X11, this is usually
    the same as could be computed from heightMM().
 
    Note that if the logicalDpiY() doesn't equal the physicalDpiY(),
    the corresponding QPaintEngine must handle the resolution mapping.
 
    \sa logicalDpiX(), physicalDpiY()
*/
 
/*!
    \fn int QPaintDevice::physicalDpiX() const
 
    Returns the horizontal resolution of the device in dots per inch.
    For example, when printing, this resolution refers to the physical
    printer's resolution. The logical DPI on the other hand, refers to
    the resolution used by the actual paint engine.
 
    Note that if the physicalDpiX() doesn't equal the logicalDpiX(),
    the corresponding QPaintEngine must handle the resolution mapping.
 
    \sa physicalDpiY(), logicalDpiX()
*/
 
/*!
    \fn int QPaintDevice::physicalDpiY() const
 
    Returns the horizontal resolution of the device in dots per inch.
    For example, when printing, this resolution refers to the physical
    printer's resolution. The logical DPI on the other hand, refers to
    the resolution used by the actual paint engine.
 
    Note that if the physicalDpiY() doesn't equal the logicalDpiY(),
    the corresponding QPaintEngine must handle the resolution mapping.
 
    \sa physicalDpiX(), logicalDpiY()
*/
 
/*!
    \fn qreal QPaintDevice::devicePixelRatio() const
 
    Returns the device pixel ratio for device.
 
    Common values are 1 for normal-dpi displays and 2 for high-dpi
    "retina" displays.
*/
 
/*!
    \fn qreal QPaintDevice::devicePixelRatioF() const
 
    Returns the device pixel ratio for the device as a floating point number.
 
    \since 5.6
*/
 
/*!
    \fn qreal QPaintDevice::devicePixelRatioFScale()
 
    \internal
 
    Returns the scaling factor used for \c PdmDevicePixelRatioScaled. Classes
    that are not QPaintDevice subclasses are implementing metric(), and need to
    access this constant. Since it's a qreal, it cannot be an enum, and an inline
    function is more efficient than a static member variable.
 
    \since 5.6
*/
 
/*!
    \fn int QPaintDevice::encodeMetricF(PaintDeviceMetric metric, double value)
 
    Returns \a value encoded for the metric \a metric. Subclasses implementing metric() should use
    this function to encode \value as an integer return value when the query metric specifies an
    encoded floating-point value.
 
    \since 6.8
*/