QGraphicsEffect Class Reference
[QtGui module]

Detailed Description

The QGraphicsEffect class is the base class for all graphics effects.

Effects alter the appearance of elements by hooking into the rendering pipeline and operating between the source (e.g., a QGraphicsPixmapItem) and the destination device (e.g., QGraphicsView's viewport). Effects can be disabled by calling setEnabled(false). If effects are disabled, the source is rendered directly.

To add a visual effect to a QGraphicsItem, for example, you can use one of the standard effects, or alternately, create your own effect by creating a subclass of QGraphicsEffect. The effect can then be installed on the item using QGraphicsItem::setGraphicsEffect().

Qt provides the following standard effects:

• QGraphicsBlurEffect - blurs the item by a given radius
• QGraphicsDropShadowEffect - renders a dropshadow behind the item
• QGraphicsColorizeEffect - renders the item in shades of any given color
• QGraphicsOpacityEffect - renders the item with an opacity

For more information on how to use each effect, refer to the specific effect's documentation.

To create your own custom effect, create a subclass of QGraphicsEffect (or any other existing effects) and reimplement the virtual function draw(). This function is called whenever the effect needs to redraw. The draw() function takes the painter with which to draw as an argument. For more information, refer to the documenation for draw(). In the draw() function you can call sourcePixmap() to get a pixmap of the graphics effect source which you can then process.

If your effect changes, use update() to request for a redraw. If your custom effect changes the bounding rectangle of the source, e.g., a radial glow effect may need to apply an extra margin, you can reimplement the virtual boundingRectFor() function, and call updateBoundingRect() to notify the framework whenever this rectangle changes. The virtual sourceChanged() function is called to notify the effects that the source has changed in some way - e.g., if the source is a QGraphicsRectItem and its rectangle parameters have changed.

See also QGraphicsItem::setGraphicsEffect() and QWidget::setGraphicsEffect().

Member Type Documentation

 MyGraphicsEffect::draw(QPainter *painter)
 {
     ...
     QPoint offset;
     if (sourceIsPixmap()) {
         // No point in drawing in device coordinates (pixmap will be scaled anyways).
         const QPixmap pixmap = sourcePixmap(Qt::LogicalCoordinates, &offset);
         ...
         painter->drawPixmap(offset, pixmap);
     } else {
         // Draw pixmap in device coordinates to avoid pixmap scaling;
         const QPixmap pixmap = sourcePixmap(Qt::DeviceCoordinates, &offset);
         painter->setWorldTransform(QTransform());
         ...
         painter->drawPixmap(offset, pixmap);
     }
     ...
 }

 MyGraphicsOpacityEffect::draw(QPainter *painter)
 {
     // Fully opaque; draw directly without going through a pixmap.
     if (qFuzzyCompare(m_opacity, 1)) {
         drawSource(painter);
         return;
     }
     ...
 }