Qwt User's Guide  6.1.3
QwtPlotRasterItem Class Referenceabstract

A class, which displays raster data. More...

#include <qwt_plot_rasteritem.h>

Inheritance diagram for QwtPlotRasterItem:

Public Types

enum  CachePolicy { NoCache, PaintCache }
 Cache policy The default policy is NoCache. More...
enum  PaintAttribute { PaintInDeviceResolution = 1 }
typedef QFlags< PaintAttributePaintAttributes
 Paint attributes.
- Public Types inherited from QwtPlotItem
enum  RttiValues {
  Rtti_PlotItem = 0, Rtti_PlotGrid, Rtti_PlotScale, Rtti_PlotLegend,
  Rtti_PlotMarker, Rtti_PlotCurve, Rtti_PlotSpectroCurve, Rtti_PlotIntervalCurve,
  Rtti_PlotHistogram, Rtti_PlotSpectrogram, Rtti_PlotSVG, Rtti_PlotTradingCurve,
  Rtti_PlotBarChart, Rtti_PlotMultiBarChart, Rtti_PlotShape, Rtti_PlotTextLabel,
  Rtti_PlotZone, Rtti_PlotUserItem = 1000
 Runtime type information. More...
enum  ItemAttribute { Legend = 0x01, AutoScale = 0x02, Margins = 0x04 }
 Plot Item Attributes. More...
enum  ItemInterest { ScaleInterest = 0x01, LegendInterest = 0x02 }
 Plot Item Interests. More...
enum  RenderHint { RenderAntialiased = 0x1 }
 Render hints. More...
typedef QFlags< ItemAttributeItemAttributes
 Plot Item Attributes.
typedef QFlags< ItemInterestItemInterests
 Plot Item Interests.
typedef QFlags< RenderHintRenderHints
 Render hints.

Public Member Functions

 QwtPlotRasterItem (const QString &title=QString::null)
 QwtPlotRasterItem (const QwtText &title)
virtual ~QwtPlotRasterItem ()
void setPaintAttribute (PaintAttribute, bool on=true)
bool testPaintAttribute (PaintAttribute) const
void setAlpha (int alpha)
 Set an alpha value for the raster data. More...
int alpha () const
void setCachePolicy (CachePolicy)
CachePolicy cachePolicy () const
void invalidateCache ()
virtual void draw (QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &rect) const
 Draw the raster data. More...
virtual QRectF pixelHint (const QRectF &) const
 Pixel hint. More...
virtual QwtInterval interval (Qt::Axis) const
virtual QRectF boundingRect () const
- Public Member Functions inherited from QwtPlotItem
 QwtPlotItem (const QwtText &title=QwtText())
virtual ~QwtPlotItem ()
 Destroy the QwtPlotItem.
void attach (QwtPlot *plot)
 Attach the item to a plot. More...
void detach ()
 This method detaches a QwtPlotItem from any QwtPlot it has been associated with. More...
QwtPlotplot () const
 Return attached plot.
void setTitle (const QString &title)
void setTitle (const QwtText &title)
const QwtTexttitle () const
virtual int rtti () const
void setItemAttribute (ItemAttribute, bool on=true)
bool testItemAttribute (ItemAttribute) const
void setItemInterest (ItemInterest, bool on=true)
bool testItemInterest (ItemInterest) const
void setRenderHint (RenderHint, bool on=true)
bool testRenderHint (RenderHint) const
void setRenderThreadCount (uint numThreads)
uint renderThreadCount () const
void setLegendIconSize (const QSize &)
QSize legendIconSize () const
double z () const
void setZ (double z)
 Set the z value. More...
void show ()
 Show the item.
void hide ()
 Hide the item.
virtual void setVisible (bool)
bool isVisible () const
void setAxes (int xAxis, int yAxis)
void setXAxis (int axis)
int xAxis () const
 Return xAxis.
void setYAxis (int axis)
int yAxis () const
 Return yAxis.
virtual void itemChanged ()
virtual void legendChanged ()
virtual void getCanvasMarginHint (const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasSize, double &left, double &top, double &right, double &bottom) const
 Calculate a hint for the canvas margin. More...
virtual void updateScaleDiv (const QwtScaleDiv &, const QwtScaleDiv &)
 Update the item to changes of the axes scale division. More...
virtual void updateLegend (const QwtPlotItem *, const QList< QwtLegendData > &)
 Update the item to changes of the legend info. More...
QRectF scaleRect (const QwtScaleMap &, const QwtScaleMap &) const
 Calculate the bounding scale rectangle of 2 maps. More...
QRectF paintRect (const QwtScaleMap &, const QwtScaleMap &) const
 Calculate the bounding paint rectangle of 2 maps. More...
virtual QList< QwtLegendDatalegendData () const
 Return all information, that is needed to represent the item on the legend. More...
virtual QwtGraphic legendIcon (int index, const QSizeF &) const

Protected Member Functions

virtual QImage renderImage (const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &area, const QSize &imageSize) const =0
 Render an image. More...
virtual QwtScaleMap imageMap (Qt::Orientation, const QwtScaleMap &map, const QRectF &area, const QSize &imageSize, double pixelSize) const
 Calculate a scale map for painting to an image. More...
- Protected Member Functions inherited from QwtPlotItem
QwtGraphic defaultIcon (const QBrush &, const QSizeF &) const
 Return a default icon from a brush. More...

Detailed Description

A class, which displays raster data.

Raster data is a grid of pixel values, that can be represented as a QImage. It is used for many types of information like spectrograms, cartograms, geographical maps ...

Often a plot has several types of raster data organized in layers. ( f.e a geographical map, with weather statistics ). Using setAlpha() raster items can be stacked easily.

QwtPlotRasterItem is only implemented for images of the following formats: QImage::Format_Indexed8, QImage::Format_ARGB32.

See also

Member Enumeration Documentation

Cache policy The default policy is NoCache.


renderImage() is called each time the item has to be repainted


renderImage() is called, whenever the image cache is not valid, or the scales, or the size of the canvas has changed.

This type of cache is useful for improving the performance of hide/show operations or manipulations of the alpha value. All other situations are handled by the canvas backing store.

Attributes to modify the drawing algorithm.

See also
setPaintAttribute(), testPaintAttribute()

When the image is rendered according to the data pixels ( QwtRasterData::pixelHint() ) it can be expanded to paint device resolution before it is passed to QPainter. The expansion algorithm rounds the pixel borders in the same way as the axis ticks, what is usually better than the scaling algorithm implemented in Qt. Disabling this flag might make sense, to reduce the size of a document/file. If this is possible for a document format depends on the implementation of the specific QPaintEngine.

Member Function Documentation

int QwtPlotRasterItem::alpha ( ) const
Alpha value of the raster item
See also
QRectF QwtPlotRasterItem::boundingRect ( ) const
Bounding rectangle of the data
See also

Reimplemented from QwtPlotItem.

QwtPlotRasterItem::CachePolicy QwtPlotRasterItem::cachePolicy ( ) const
Cache policy
See also
CachePolicy, setCachePolicy()
void QwtPlotRasterItem::draw ( QPainter *  painter,
const QwtScaleMap xMap,
const QwtScaleMap yMap,
const QRectF &  canvasRect 
) const

Draw the raster data.

xMapX-Scale Map
yMapY-Scale Map
canvasRectContents rectangle of the plot canvas

Implements QwtPlotItem.

Reimplemented in QwtPlotSpectrogram.

QwtScaleMap QwtPlotRasterItem::imageMap ( Qt::Orientation  orientation,
const QwtScaleMap map,
const QRectF &  area,
const QSize &  imageSize,
double  pixelSize 
) const

Calculate a scale map for painting to an image.

orientationOrientation, Qt::Horizontal means a X axis
mapScale map for rendering the plot item
areaArea to be painted on the image
imageSizeImage size
pixelSizeWidth/Height of a data pixel
Calculated scale map
QwtInterval QwtPlotRasterItem::interval ( Qt::Axis  axis) const
Bounding interval for an axis

This method is intended to be reimplemented by derived classes. The default implementation returns an invalid interval.

axisX, Y, or Z axis

Reimplemented in QwtPlotSpectrogram.

void QwtPlotRasterItem::invalidateCache ( )

Invalidate the paint cache

See also
QRectF QwtPlotRasterItem::pixelHint ( const QRectF &  area) const

Pixel hint.

The geometry of a pixel is used to calculated the resolution and alignment of the rendered image.

Width and height of the hint need to be the horizontal and vertical distances between 2 neighbored points. The center of the hint has to be the position of any point ( it doesn't matter which one ).

Limiting the resolution of the image might significantly improve the performance and heavily reduce the amount of memory when rendering a QImage from the raster data.

The default implementation returns an empty rectangle (QRectF()), meaning, that the image will be rendered in target device ( f.e screen ) resolution.

areaIn most implementations the resolution of the data doesn't depend on the requested area.
Bounding rectangle of a pixel
See also
render(), renderImage()

Reimplemented in QwtPlotSpectrogram.

virtual QImage QwtPlotRasterItem::renderImage ( const QwtScaleMap xMap,
const QwtScaleMap yMap,
const QRectF &  area,
const QSize &  imageSize 
) const
protectedpure virtual

Render an image.

An implementation of render() might iterate over all pixels of imageRect. Each pixel has to be translated into the corresponding position in scale coordinates using the maps. This position can be used to look up a value in a implementation specific way and to map it into a color.

xMapX-Scale Map
yMapY-Scale Map
areaRequested area for the image in scale coordinates
imageSizeRequested size of the image
Rendered image

Implemented in QwtPlotSpectrogram.

void QwtPlotRasterItem::setAlpha ( int  alpha)

Set an alpha value for the raster data.

Often a plot has several types of raster data organized in layers. ( f.e a geographical map, with weather statistics ). Using setAlpha() raster items can be stacked easily.

The alpha value is a value [0, 255] to control the transparency of the image. 0 represents a fully transparent color, while 255 represents a fully opaque color.

alphaAlpha value
  • alpha >= 0
    All alpha values of the pixels returned by renderImage() will be set to alpha, beside those with an alpha value of 0 (invalid pixels).
  • alpha < 0 The alpha values returned by renderImage() are not changed.

The default alpha value is -1.

See also
void QwtPlotRasterItem::setCachePolicy ( QwtPlotRasterItem::CachePolicy  policy)

Change the cache policy

The default policy is NoCache

policyCache policy
See also
CachePolicy, cachePolicy()
void QwtPlotRasterItem::setPaintAttribute ( PaintAttribute  attribute,
bool  on = true 

Specify an attribute how to draw the raster item

attributePaint attribute
onOn/Off /sa PaintAttribute, testPaintAttribute()
bool QwtPlotRasterItem::testPaintAttribute ( PaintAttribute  attribute) const
True, when attribute is enabled
See also
PaintAttribute, setPaintAttribute()