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QwtWidgetOverlay(3) Qwt User's Guide QwtWidgetOverlay(3)


QwtWidgetOverlay - An overlay for a widget.


#include <qwt_widget_overlay.h>

Inherits QWidget.

Public Types

enum MaskMode { NoMask, MaskHint, AlphaMask }
Mask mode. enum RenderMode { AutoRenderMode, CopyAlphaMask, DrawOverlay }
Render mode.

Public Slots

void updateOverlay ()

Public Member Functions

QwtWidgetOverlay (QWidget *)
Constructor. virtual ~QwtWidgetOverlay ()
Destructor. void setMaskMode (MaskMode)
Specify how to find the mask for the overlay. MaskMode maskMode () const
void setRenderMode (RenderMode)
RenderMode renderMode () const
virtual bool eventFilter (QObject *, QEvent *) override
Event filter.

Protected Member Functions

virtual void paintEvent (QPaintEvent *) override
virtual void resizeEvent (QResizeEvent *) override
virtual QRegion maskHint () const
Calculate an approximation for the mask. virtual void drawOverlay (QPainter *painter) const =0

Detailed Description

An overlay for a widget.

The main use case of an widget overlay is to avoid heavy repaint operation of the widget below.

F.e. in combination with the plot canvas an overlay avoid replots as the content of the canvas can be restored from its backing store.

QwtWidgetOverlay is an abstract base class. Deriving classes are supposed to reimplement the following methods:

  • drawOverlay()
  • maskHint()

Internally QwtPlotPicker uses overlays for displaying the rubber band and the tracker text.

See also


Definition at line 40 of file qwt_widget_overlay.h.

Member Enumeration Documentation

enum QwtWidgetOverlay::MaskMode

Mask mode. When using masks the widget below gets paint events for the masked regions of the overlay only. Otherwise Qt triggers full repaints. On less powerful hardware ( f.e embedded systems ) - or when using the raster paint engine on a remote desktop - bit blitting is a noticeable operation, that needs to be avoided.

If and how to mask depends on how expensive the calculation of the mask is and how many pixels can be excluded by the mask.

The default setting is MaskHint.

See also

setMaskMode(), maskMode()


Don't use a mask.
Use maskHint() as mask. For many situations a fast approximation is good enough and it is not necessary to build a more detailed mask ( f.e the bounding rectangle of a text ).
Calculate a mask by checking the alpha values. Sometimes it is not possible to give a fast approximation and the mask needs to be calculated by drawing the overlay and testing the result.

When a valid maskHint() is available only pixels inside this approximation are checked.

Definition at line 60 of file qwt_widget_overlay.h.

enum QwtWidgetOverlay::RenderMode

Render mode. For calculating the alpha mask the overlay has already been painted to a temporary QImage. Instead of rendering the overlay twice this buffer can be copied for drawing the overlay.

On graphic systems using the raster paint engine ( QWS, Windows ) it means usually copying some memory only. On X11 it results in an expensive operation building a pixmap and for simple overlays it might not be recommended.


The render mode has no effect, when maskMode() != AlphaMask.


Copy the buffer, when using the raster paint engine.
Always copy the buffer.
Never copy the buffer.

Definition at line 102 of file qwt_widget_overlay.h.

Constructor & Destructor Documentation

QwtWidgetOverlay::QwtWidgetOverlay (QWidget * widget) [explicit]



widget Parent widget, where the overlay is aligned to

Definition at line 126 of file qwt_widget_overlay.cpp.

Member Function Documentation

virtual void QwtWidgetOverlay::drawOverlay (QPainter * painter) const [protected], [pure virtual]

Draw the widget overlay


painter Painter

bool QwtWidgetOverlay::eventFilter (QObject * object, QEvent * event) [override], [virtual]

Event filter. Resize the overlay according to the size of the parent widget.


object Object to be filtered
event Event


See QObject::eventFilter()

Definition at line 389 of file qwt_widget_overlay.cpp.

QRegion QwtWidgetOverlay::maskHint () const [protected], [virtual]

Calculate an approximation for the mask.

  • MaskHint The hint is used as mask.
  • AlphaMask The hint is used to speed up the algorithm for calculating a mask from non transparent pixels
  • NoMask The hint is unused.

The default implementation returns an invalid region indicating no hint.


Hint for the mask

Definition at line 373 of file qwt_widget_overlay.cpp.

QwtWidgetOverlay::MaskMode QwtWidgetOverlay::maskMode () const


Mode how to find the mask for the overlay

See also


Definition at line 167 of file qwt_widget_overlay.cpp.

void QwtWidgetOverlay::paintEvent (QPaintEvent * event) [override], [protected], [virtual]

Paint event


event Paint event

See also


Definition at line 261 of file qwt_widget_overlay.cpp.

QwtWidgetOverlay::RenderMode QwtWidgetOverlay::renderMode () const


Render mode

See also

RenderMode, setRenderMode()

Definition at line 187 of file qwt_widget_overlay.cpp.

void QwtWidgetOverlay::resizeEvent (QResizeEvent * event) [override], [protected], [virtual]

Resize event


event Resize event

Definition at line 323 of file qwt_widget_overlay.cpp.

void QwtWidgetOverlay::setMaskMode (MaskMode mode)

Specify how to find the mask for the overlay.


mode New mode

See also


Definition at line 154 of file qwt_widget_overlay.cpp.

void QwtWidgetOverlay::setRenderMode (RenderMode mode)

Set the render mode


mode Render mode

See also

RenderMode, renderMode()

Definition at line 178 of file qwt_widget_overlay.cpp.

void QwtWidgetOverlay::updateOverlay () [slot]

Recalculate the mask and repaint the overlay

Definition at line 195 of file qwt_widget_overlay.cpp.


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Sun Jul 18 2021 Version 6.2.0