Archived:Opacity effect with the Qt graphics view framework

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Qt Quick should be used for all UI development on mobile devices. The approach described in this article (based on C++ for the Qt UI) is deprecated.

This article shows how to use the opacity effect using the Qt QGraphicsOpacityEffect.

Article Metadata

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Code Example

Source file: Media:GraphicsEffect-Opacity.zip

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Compatibility

Platform(s): any

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Article

Keywords: QGraphicsOpacityEffect

Created: gnuton (24 Dec 2010)

Last edited: hamishwillee (11 Oct 2012)

Introduction

Qt Graphics view framework offers a convenient way to apply graphics effects to Graphics Items.

Code

This is a minimal application which changes the opacity of a QGraphicsPixmap using the QGraphicsOpacityEffect.

The item opacity can be set with two methods:

• setOpacity(): it accepts values from 0.0 (image completely transparent) to 1.0.
• setOpacityBrush(): it supports gradients or masks

The following example shows how to use a mask to set the opacity of the given pixmap item.

#include <QtGui/QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsEffect>
#include <QBrush>
#include <QBitmap>
#include <QGraphicsPixmapItem>
 
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(":/cappella_sistina.jpg"));
    QGraphicsOpacityEffect opacity;
    opacity.setOpacityMask(QPixmap(":/star.png").mask());
    p->setGraphicsEffect(&opacity);
 
    view.show();
    return a.exec();
}

Source code

You can download the code from here: Media:GraphicsEffect-Opacity.zip