Qwt User's Guide  6.1.3
QwtWidgetOverlay Class Referenceabstract

An overlay for a widget. More...

#include <qwt_widget_overlay.h>

Inheritance diagram for QwtWidgetOverlay:

Public Types

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

Public Member Functions

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

Protected Member Functions

virtual void paintEvent (QPaintEvent *event)
virtual void resizeEvent (QResizeEvent *event)
virtual QRegion maskHint () const
 Calculate an approximation for the mask. More...
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:

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

See also

Member Enumeration Documentation

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.

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.

Constructor & Destructor Documentation

QwtWidgetOverlay::QwtWidgetOverlay ( QWidget *  widget)


widgetParent widget, where the overlay is aligned to

Member Function Documentation

virtual void QwtWidgetOverlay::drawOverlay ( QPainter *  painter) const
protectedpure virtual

Draw the widget overlay

bool QwtWidgetOverlay::eventFilter ( QObject *  object,
QEvent *  event 

Event filter.

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

objectObject to be filtered
See QObject::eventFilter()
QRegion QwtWidgetOverlay::maskHint ( ) const

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
QwtWidgetOverlay::MaskMode QwtWidgetOverlay::maskMode ( ) const
Mode how to find the mask for the overlay
See also
void QwtWidgetOverlay::paintEvent ( QPaintEvent *  event)

Paint event

eventPaint event
See also
QwtWidgetOverlay::RenderMode QwtWidgetOverlay::renderMode ( ) const
Render mode
See also
RenderMode, setRenderMode()
void QwtWidgetOverlay::resizeEvent ( QResizeEvent *  event)

Resize event

eventResize event
void QwtWidgetOverlay::setMaskMode ( MaskMode  mode)

Specify how to find the mask for the overlay.

modeNew mode
See also
void QwtWidgetOverlay::setRenderMode ( RenderMode  mode)

Set the render mode

modeRender mode
See also
RenderMode, renderMode()
void QwtWidgetOverlay::updateOverlay ( )

Recalculate the mask and repaint the overlay