- Qt 6.9
- Qt GUI
- C++ Classes
- QPaintDevice
- Qt 6.9.3 Reference Documentation

# QPaintDevice Class

The QPaintDevice class is the base class of objects that can be painted on with QPainter. More...

Header: | `#include <QPaintDevice>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Gui)` `target_link_libraries(mytarget PRIVATE Qt6::Gui)`
qmake: | `QT += gui`
Inherited By: | QImage, QOpenGLPaintDevice, QPagedPaintDevice, QPaintDeviceWindow, QPicture, QPixmap, and QWidget

- List of all members, including inherited members
- QPaintDevice is part of Painting Classes.

## Public Types

enum | PaintDeviceMetric { PdmWidth, PdmHeight, PdmWidthMM, PdmHeightMM, PdmNumColors, …, PdmDevicePixelRatioF_EncodedB }

## Public Functions

virtual | ~QPaintDevice()
int | colorCount() const
int | depth() const
qreal | devicePixelRatio() const
qreal | devicePixelRatioF() const
int | height() const
int | heightMM() const
int | logicalDpiX() const
int | logicalDpiY() const
virtual QPaintEngine * | paintEngine() const = 0
bool | paintingActive() const
int | physicalDpiX() const
int | physicalDpiY() const
int | width() const
int | widthMM() const

## Static Public Members

`(since 6.8)` int | encodeMetricF(QPaintDevice::PaintDeviceMetric metric, double value)

## Protected Functions

| QPaintDevice()
virtual int | metric(QPaintDevice::PaintDeviceMetric metric) const

## Detailed Description

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.

See also QPaintEngine, QPainter, Coordinate System, and Paint System.

## Member Type Documentation

### enum QPaintDevice::PaintDeviceMetric

Describes the various metrics of a paint device.

Constant | Value | Description
--- | --- | ---
`QPaintDevice::PdmWidth` | `1` | The width of the paint device in default coordinate system units (e.g. pixels for QPixmap and QWidget). See also width().
`QPaintDevice::PdmHeight` | `2` | The height of the paint device in default coordinate system units (e.g. pixels for QPixmap and QWidget). See also height().
`QPaintDevice::PdmWidthMM` | `3` | The width of the paint device in millimeters. See also widthMM().
`QPaintDevice::PdmHeightMM` | `4` | The height of the paint device in millimeters. See also heightMM().
`QPaintDevice::PdmNumColors` | `5` | The number of different colors available for the paint device. See also colorCount().
`QPaintDevice::PdmDepth` | `6` | The bit depth (number of bit planes) of the paint device. See also depth().
`QPaintDevice::PdmDpiX` | `7` | The horizontal resolution of the device in dots per inch. See also logicalDpiX().
`QPaintDevice::PdmDpiY` | `8` | The vertical resolution of the device in dots per inch. See also logicalDpiY().
`QPaintDevice::PdmPhysicalDpiX` | `9` | The horizontal resolution of the device in dots per inch. See also physicalDpiX().
`QPaintDevice::PdmPhysicalDpiY` | `10` | The vertical resolution of the device in dots per inch. See also physicalDpiY().
`QPaintDevice::PdmDevicePixelRatio` | `11` | The device pixel ratio for device. Common values are 1 for normal-dpi displays and 2 for high-dpi "retina" displays.
`QPaintDevice::PdmDevicePixelRatioScaled` | `12` | 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.

`QPaintDevice::PdmDevicePixelRatioF_EncodedA (since Qt 6.8)`

`13`

This enum item, together with the corresponding `B` item, are used together for the device pixel ratio of the device, as an encoded `double` floating point value. A QPaintDevice subclass that supports 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.

`QPaintDevice::PdmDevicePixelRatioF_EncodedB (since Qt 6.8)` | `14` | See PdmDevicePixelRatioF_EncodedA.

See also metric() and devicePixelRatio().

## Member Function Documentation

### `[noexcept protected]` QPaintDevice::QPaintDevice()

Constructs a paint device. This constructor can be invoked only from subclasses of QPaintDevice.

### `[virtual noexcept]` QPaintDevice::~QPaintDevice()

Destroys the paint device and frees window system resources.

### 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.

### int QPaintDevice::depth() const

Returns the bit depth (number of bit planes) of the paint device.

### 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.

### qreal QPaintDevice::devicePixelRatioF() const

Returns the device pixel ratio for the device as a floating point number.

### `[static, since 6.8]` int QPaintDevice::encodeMetricF(QPaintDevice::PaintDeviceMetric metric, double value)

Returns value encoded for the metric metric. Subclasses implementing metric() should use this function to encode

Constant | Description
--- | ---
`as` | an integer return value when the query metric specifies an encoded floating-point value.

This function was introduced in Qt 6.8.

### int QPaintDevice::height() const

Returns the height of the paint device in default coordinate system units (e.g. pixels for QPixmap and QWidget).

See also heightMM().

### 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.

See also height().

### 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.

See also logicalDpiY() and physicalDpiX().

### 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.

See also logicalDpiX() and physicalDpiY().

### `[virtual protected]` int QPaintDevice::metric(QPaintDevice::PaintDeviceMetric metric) const

Returns the metric information for the given paint device metric.

See also PaintDeviceMetric.

### `[pure virtual]` QPaintEngine *QPaintDevice::paintEngine() const

Returns a pointer to the paint engine used for drawing on the device.

### bool QPaintDevice::paintingActive() const

Returns `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 `false`.

See also QPainter::isActive().

### 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.

See also physicalDpiY() and logicalDpiX().

### 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.

See also physicalDpiX() and logicalDpiY().

### int QPaintDevice::width() const

Returns the width of the paint device in default coordinate system units (e.g. pixels for QPixmap and QWidget).

See also widthMM().

### 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.

See also width().