A sample highlighting QGraphicsEffect

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Revision as of 22:46, 3 March 2015 by Simow (talk | contribs) (Corrected Layout after Wiki Conversion)
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While learning Qt, I wrote this sample highlighting graphics effect to be used with rectangular QGraphicsPixmapItem... and I use this post just to learn how to publish a wiki page here too.. ;-)


  1. ifndef HIGHLIGHT_H
  2. define HIGHLIGHT_H
  1. include <QtGui>

class HighlightEffect : public QGraphicsEffect {

Q_PROPERTY( QColor color READ color WRITE setColor )
Q_PROPERTY( QPointF offset READ offset WRITE setOffset )
 HighlightEffect( qreal offset = 1.5 );
 virtual QRectF boundingRectFor( const QRectF& sourceRect; ) const;
 QColor color() const { return mColor;}
 void setColor( QColor& color; ) { mColor = color;}
 QPointF offset() const { return mOffset;}
 void setOffset( QPointF offset ) { mOffset = offset;}
 virtual void draw( QPainter *painter ); // , QGraphicsEffectSource *source );
 QColor mColor;
 QPointF mOffset;


  1. endif // HIGHLIGHT_H


  1. include "highlight.h"

HighlightEffect::HighlightEffect( qreal offset ) :

mColor( 255, 255, 0, 128 ), // yellow, semi-transparent
mOffset( offset, offset ) {}

QRectF HighlightEffect::boundingRectFor( const QRectF& sourceRect; ) const {

return sourceRect.adjusted( -mOffset.x(), -mOffset.y(), mOffset.x(), mOffset.y() );


void HighlightEffect::draw( QPainter* painter ) {

QPoint offset;
QPixmap pixmap;
// if ( sourceIsPixmap() ) // doesn't seems to work, return false
// {
// No point in drawing in device coordinates (pixmap will be scaled anyways).
pixmap = sourcePixmap( Qt::LogicalCoordinates, &offset; );
QRectF bound = boundingRectFor( pixmap.rect() );
painter->setPen( Qt::NoPen );
painter->setBrush( mColor );
QPointF p( offset.x()-mOffset.x(), offset.y()-mOffset.y() );
bound.moveTopLeft( p );
painter->drawRoundedRect( bound, 5, 5, Qt::RelativeSize );
painter->drawPixmap( offset, pixmap );


The strange thing in the implementation is that the sourceIsPixmap function returns always false, even if the documentation states that Returns true if the source effectively is a pixmap, e.g., a QGraphicsPixmapItem. I asked the Qt gurus what can be going on here...

The default color is pale yellow, semi-transparent and the border size is set to 1.5. To add the effect simply add this:

Pixmap *item = new Pixmap(kineticPix);
HighlightEffect *effect = new HighlightEffect();
item->setGraphicsEffect( effect );

Here the modified animated tiles example with the effect turned on:


Animated tiles example with QGraphicsPixmapItem highlighting

Hope someone finds this useful.