Inverted colors effect with the Qt graphics view framework

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Article Metadata

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Code Example

Source file: Media:‎GraphicsEffect-Inverted.zip

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Compatibility

Platform(s): any

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Article

Keywords: QGraphicsEffect, QGraphicsView

Created: gnuton (24 Dec 2010)

Last edited: hamishwillee (11 Oct 2012)

Introduction

This article shows how to inverts the color of a graphics item by applying custom graphics effect. Qt Graphics View Framework offers a convenient way to apply graphics effect to Graphics Items.

Code

Since the invert effect is not available in Qt, QGraphicsEffect has been subclassed to achieve the desired effect.

#ifndef QGRAPHICSINVERTCOLORSEFFECT_H
#define QGRAPHICSINVERTCOLORSEFFECT_H
 
#include <QGraphicsEffect>
#include <QPainter>
 
class QGraphicsInvertColorsEffect : public QGraphicsEffect {
public:
    QGraphicsInvertColorsEffect(QObject *parent = 0) : QGraphicsEffect(parent)
    {
    }
 
    ~QGraphicsInvertColorsEffect()
    {
    }
 
protected:
    void draw(QPainter *painter)
    {
        QPoint offset;
 
        QPixmap pixmap = sourcePixmap(Qt::LogicalCoordinates, &offset);
        if (pixmap.isNull())
          return;
 
        QImage img = pixmap.toImage();
        img.invertPixels();
        painter->drawImage(boundingRect(), img);
    }
};
#endif // QGRAPHICSINVERTCOLORSEFFECT_H

And the new class has been used in this way

#include <QtGui/QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsPixmapItem>
#include <QGraphicsEffect>
#include "QGraphicsInvertColorEffect.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    //a.setGraphicsSystem("raster");
    QGraphicsView view;
    view.setScene(new QGraphicsScene);
 
    QGraphicsPixmapItem *p = view.scene()->addPixmap(QPixmap(":/meditate.png"));
 
    QGraphicsInvertColorsEffect opacity;
    p->setGraphicsEffect(&opacity);
 
    view.show();
    return a.exec();
}

Source code

Code is available here: Media:‎GraphicsEffect-Inverted.zip