QPaintEngine Class

The QPaintEngine class provides an abstract definition of how QPainter draws to a given device on a given platform. More...

Header: #include <QPaintEngine>
qmake: QT += gui

Public Types

enum DirtyFlag { DirtyPen, DirtyBrush, DirtyBrushOrigin, DirtyFont, ..., AllDirty }
flags DirtyFlags
enum PaintEngineFeature { AlphaBlend, Antialiasing, BlendModes, BrushStroke, ..., AllFeatures }
flags PaintEngineFeatures
enum PolygonDrawMode { OddEvenMode, WindingMode, ConvexMode, PolylineMode }
enum Type { X11, Windows, MacPrinter, CoreGraphics, ..., Direct2D }

Public Functions

QPaintEngine(QPaintEngine::PaintEngineFeatures caps = ...)
virtual ~QPaintEngine()
virtual bool begin(QPaintDevice *pdev) = 0
virtual void drawEllipse(const QRectF &rect)
virtual void drawEllipse(const QRect &rect)
virtual void drawImage(const QRectF &rectangle, const QImage &image, const QRectF &sr, Qt::ImageConversionFlags flags)
virtual void drawLines(const QLineF *lines, int lineCount)
virtual void drawLines(const QLine *lines, int lineCount)
virtual void drawPath(const QPainterPath &path)
virtual void drawPixmap(const QRectF &r, const QPixmap &pm, const QRectF &sr) = 0
virtual void drawPoints(const QPointF *points, int pointCount)
virtual void drawPoints(const QPoint *points, int pointCount)
virtual void drawPolygon(const QPointF *points, int pointCount, QPaintEngine::PolygonDrawMode mode)
virtual void drawPolygon(const QPoint *points, int pointCount, QPaintEngine::PolygonDrawMode mode)
virtual void drawRects(const QRectF *rects, int rectCount)
virtual void drawRects(const QRect *rects, int rectCount)
virtual void drawTextItem(const QPointF &p, const QTextItem &textItem)
virtual void drawTiledPixmap(const QRectF &rect, const QPixmap &pixmap, const QPointF &p)
virtual bool end() = 0
bool hasFeature(QPaintEngine::PaintEngineFeatures feature) const
bool isActive() const
QPaintDevice * paintDevice() const
QPainter * painter() const
void setActive(bool state)
virtual QPaintEngine::Type type() const = 0
virtual void updateState(const QPaintEngineState &state) = 0

Detailed Description

The QPaintEngine class provides an abstract definition of how QPainter draws to a given device on a given platform.

Qt provides several premade implementations of QPaintEngine for the different painter backends we support. The primary paint engine provided is the raster paint engine, which contains a software rasterizer which supports the full feature set on all supported platforms. This is the default for painting on QWidget-based classes in e.g. on Windows, X11 and macOS, it is the backend for painting on QImage and it is used as a fallback for paint engines that do not support a certain capability. In addition we provide QPaintEngine implementations for OpenGL (accessible through QGLWidget) and printing (which allows using QPainter to draw on a QPrinter object).

If one wants to use QPainter to draw to a different backend, one must subclass QPaintEngine and reimplement all its virtual functions. The QPaintEngine implementation is then made available by subclassing QPaintDevice and reimplementing the virtual function QPaintDevice::paintEngine().

QPaintEngine is created and owned by the QPaintDevice that created it.

See also QPainter, QPaintDevice::paintEngine(), and Paint System.

Member Type Documentation

enum QPaintEngine::DirtyFlag
flags QPaintEngine::DirtyFlags

Constant Value Description
QPaintEngine::DirtyPen 0x0001 The pen is dirty and needs to be updated.
QPaintEngine::DirtyBrush 0x0002 The brush is dirty and needs to be updated.
QPaintEngine::DirtyBrushOrigin 0x0004 The brush origin is dirty and needs to updated.
QPaintEngine::DirtyFont 0x0008 The font is dirty and needs to be updated.
QPaintEngine::DirtyBackground 0x0010 The background is dirty and needs to be updated.
QPaintEngine::DirtyBackgroundMode 0x0020 The background mode is dirty and needs to be updated.
QPaintEngine::DirtyTransform 0x0040 The transform is dirty and needs to be updated.
QPaintEngine::DirtyClipRegion 0x0080 The clip region is dirty and needs to be updated.
QPaintEngine::DirtyClipPath 0x0100 The clip path is dirty and needs to be updated.
QPaintEngine::DirtyHints 0x0200 The render hints is dirty and needs to be updated.
QPaintEngine::DirtyCompositionMode 0x0400 The composition mode is dirty and needs to be updated.
QPaintEngine::DirtyClipEnabled 0x0800 Whether clipping is enabled or not is dirty and needs to be updated.
QPaintEngine::DirtyOpacity 0x1000 The constant opacity has changed and needs to be updated as part of the state change in QPaintEngine::updateState().
QPaintEngine::AllDirty 0xffff Convenience enum used internally.

These types are used by QPainter to trigger lazy updates of the various states in the QPaintEngine using QPaintEngine::updateState().

A paint engine must update every dirty state.

The DirtyFlags type is a typedef for QFlags<DirtyFlag>. It stores an OR combination of DirtyFlag values.

enum QPaintEngine::PaintEngineFeature
flags QPaintEngine::PaintEngineFeatures

This enum is used to describe the features or capabilities that the paint engine has. If a feature is not supported by the engine, QPainter will do a best effort to emulate that feature through other means and pass on an alpha blended QImage to the engine with the emulated results. Some features cannot be emulated: AlphaBlend and PorterDuff.

Constant Value Description
QPaintEngine::AlphaBlend 0x00000080 The engine can alpha blend primitives.
QPaintEngine::Antialiasing 0x00000400 The engine can use antialising to improve the appearance of rendered primitives.
QPaintEngine::BlendModes 0x00008000 The engine supports blending modes.
QPaintEngine::BrushStroke 0x00000800 The engine supports drawing strokes that contain brushes as fills, not just solid colors (e.g. a dashed gradient line of width 2).
QPaintEngine::ConicalGradientFill 0x00000040 The engine supports conical gradient fills.
QPaintEngine::ConstantOpacity 0x00001000 The engine supports the feature provided by QPainter::setOpacity().
QPaintEngine::LinearGradientFill 0x00000010 The engine supports linear gradient fills.
QPaintEngine::MaskedBrush 0x00002000 The engine is capable of rendering brushes that has a texture with an alpha channel or a mask.
QPaintEngine::ObjectBoundingModeGradients 0x00010000 The engine has native support for gradients with coordinate mode QGradient::ObjectBoundingMode. Otherwise, if QPaintEngine::PatternTransform is supported, object bounding mode gradients are converted to gradients with coordinate mode QGradient::LogicalMode and a brush transform for the coordinate mapping.
QPaintEngine::PainterPaths 0x00000200 The engine has path support.
QPaintEngine::PaintOutsidePaintEvent 0x20000000 The engine is capable of painting outside of paint events.
QPaintEngine::PatternBrush 0x00000008 The engine is capable of rendering brushes with the brush patterns specified in Qt::BrushStyle.
QPaintEngine::PatternTransform 0x00000002 The engine has support for transforming brush patterns.
QPaintEngine::PerspectiveTransform 0x00004000 The engine has support for performing perspective transformations on primitives.
QPaintEngine::PixmapTransform 0x00000004 The engine can transform pixmaps, including rotation and shearing.
QPaintEngine::PorterDuff 0x00000100 The engine supports Porter-Duff operations
QPaintEngine::PrimitiveTransform 0x00000001 The engine has support for transforming drawing primitives.
QPaintEngine::RadialGradientFill 0x00000020 The engine supports radial gradient fills.
QPaintEngine::RasterOpModes 0x00020000 The engine supports bitwise raster operations.
QPaintEngine::AllFeatures 0xffffffff All of the above features. This enum value is usually used as a bit mask.

The PaintEngineFeatures type is a typedef for QFlags<PaintEngineFeature>. It stores an OR combination of PaintEngineFeature values.

enum QPaintEngine::PolygonDrawMode

Constant Value Description
QPaintEngine::OddEvenMode 0 The polygon should be drawn using OddEven fill rule.
QPaintEngine::WindingMode 1 The polygon should be drawn using Winding fill rule.
QPaintEngine::ConvexMode 2 The polygon is a convex polygon and can be drawn using specialized algorithms where available.
QPaintEngine::PolylineMode 3 Only the outline of the polygon should be drawn.

enum QPaintEngine::Type

Constant Value Description
QPaintEngine::X11 0  
QPaintEngine::Windows 1  
QPaintEngine::MacPrinter 4  
QPaintEngine::CoreGraphics 3 macOS's Quartz2D (CoreGraphics)
QPaintEngine::QuickDraw 2 macOS's QuickDraw
QPaintEngine::QWindowSystem 5 Qt for Embedded Linux
QPaintEngine::PostScript 6 (No longer supported)
QPaintEngine::OpenGL 7  
QPaintEngine::Picture 8 QPicture format
QPaintEngine::SVG 9 Scalable Vector Graphics XML format
QPaintEngine::Raster 10  
QPaintEngine::Direct3D 11 Windows only, Direct3D based engine
QPaintEngine::Pdf 12 Portable Document Format
QPaintEngine::OpenVG 13  
QPaintEngine::User 50 First user type ID
QPaintEngine::MaxUser 100 Last user type ID
QPaintEngine::OpenGL2 14  
QPaintEngine::PaintBuffer 15  
QPaintEngine::Blitter 16  
QPaintEngine::Direct2D 17 Windows only, Direct2D based engine

Member Function Documentation

QPaintEngine::QPaintEngine(QPaintEngine::PaintEngineFeatures caps = ...)

Creates a paint engine with the featureset specified by caps.

[virtual] QPaintEngine::~QPaintEngine()

Destroys the paint engine.

[pure virtual] bool QPaintEngine::begin(QPaintDevice *pdev)

Reimplement this function to initialise your paint engine when painting is to start on the paint device pdev. Return true if the initialization was successful; otherwise return false.

See also end() and isActive().

[virtual] void QPaintEngine::drawEllipse(const QRectF &rect)

Reimplement this function to draw the largest ellipse that can be contained within rectangle rect.

The default implementation calls drawPolygon().

[virtual] void QPaintEngine::drawEllipse(const QRect &rect)

The default implementation of this function calls the floating point version of this function

[virtual] void QPaintEngine::drawImage(const QRectF &rectangle, const QImage &image, const QRectF &sr, Qt::ImageConversionFlags flags)

Reimplement this function to draw the part of the image specified by the sr rectangle in the given rectangle using the given conversion flags flags, to convert it to a pixmap.

[virtual] void QPaintEngine::drawLines(const QLineF *lines, int lineCount)

The default implementation splits the list of lines in lines into lineCount separate calls to drawPath() or drawPolygon() depending on the feature set of the paint engine.

[virtual] void QPaintEngine::drawLines(const QLine *lines, int lineCount)

This is an overloaded function.

The default implementation converts the first lineCount lines in lines to a QLineF and calls the floating point version of this function.

[virtual] void QPaintEngine::drawPath(const QPainterPath &path)

The default implementation ignores the path and does nothing.

[pure virtual] void QPaintEngine::drawPixmap(const QRectF &r, const QPixmap &pm, const QRectF &sr)

Reimplement this function to draw the part of the pm specified by the sr rectangle in the given r.

[virtual] void QPaintEngine::drawPoints(const QPointF *points, int pointCount)

Draws the first pointCount points in the buffer points

[virtual] void QPaintEngine::drawPoints(const QPoint *points, int pointCount)

Draws the first pointCount points in the buffer points

The default implementation converts the first pointCount QPoints in points to QPointFs and calls the floating point version of drawPoints.

[virtual] void QPaintEngine::drawPolygon(const QPointF *points, int pointCount, QPaintEngine::PolygonDrawMode mode)

Reimplement this virtual function to draw the polygon defined by the pointCount first points in points, using mode mode.

Note: At least one of the drawPolygon() functions must be reimplemented.

[virtual] void QPaintEngine::drawPolygon(const QPoint *points, int pointCount, QPaintEngine::PolygonDrawMode mode)

This is an overloaded function.

Reimplement this virtual function to draw the polygon defined by the pointCount first points in points, using mode mode.

Note: At least one of the drawPolygon() functions must be reimplemented.

[virtual] void QPaintEngine::drawRects(const QRectF *rects, int rectCount)

Draws the first rectCount rectangles in the buffer rects. The default implementation of this function calls drawPath() or drawPolygon() depending on the feature set of the paint engine.

[virtual] void QPaintEngine::drawRects(const QRect *rects, int rectCount)

This is an overloaded function.

The default implementation converts the first rectCount rectangles in the buffer rects to a QRectF and calls the floating point version of this function.

[virtual] void QPaintEngine::drawTextItem(const QPointF &p, const QTextItem &textItem)

This function draws the text item textItem at position p. The default implementation of this function converts the text to a QPainterPath and paints the resulting path.

[virtual] void QPaintEngine::drawTiledPixmap(const QRectF &rect, const QPixmap &pixmap, const QPointF &p)

Reimplement this function to draw the pixmap in the given rect, starting at the given p. The pixmap will be drawn repeatedly until the rect is filled.

[pure virtual] bool QPaintEngine::end()

Reimplement this function to finish painting on the current paint device. Return true if painting was finished successfully; otherwise return false.

See also begin() and isActive().

bool QPaintEngine::hasFeature(QPaintEngine::PaintEngineFeatures feature) const

Returns true if the paint engine supports the specified feature; otherwise returns false.

bool QPaintEngine::isActive() const

Returns true if the paint engine is actively drawing; otherwise returns false.

See also setActive().

QPaintDevice *QPaintEngine::paintDevice() const

Returns the device that this engine is painting on, if painting is active; otherwise returns 0.

QPainter *QPaintEngine::painter() const

Returns the paint engine's painter.

void QPaintEngine::setActive(bool state)

Sets the active state of the paint engine to state.

See also isActive().

[pure virtual] QPaintEngine::Type QPaintEngine::type() const

Reimplement this function to return the paint engine Type.

[pure virtual] void QPaintEngine::updateState(const QPaintEngineState &state)

Reimplement this function to update the state of a paint engine.

When implemented, this function is responsible for checking the paint engine's current state and update the properties that are changed. Use the QPaintEngineState::state() function to find out which properties that must be updated, then use the corresponding get function to retrieve the current values for the given properties.

See also QPaintEngineState.

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