QGraphicsTransform Class Reference

Detailed Description

The QGraphicsTransform class is an abstract base class for building advanced transformations on QGraphicsItems.

As an alternative to QGraphicsItem::transform, QGraphicsTransform lets you create and control advanced transformations that can be configured independently using specialized properties.

QGraphicsItem allows you to assign any number of QGraphicsTransform instances to one QGraphicsItem. Each QGraphicsTransform is applied in order, one at a time, to the QGraphicsItem it's assigned to.

QGraphicsTransform is particularly useful for animations. Whereas QGraphicsItem::setTransform() lets you assign any transform directly to an item, there is no direct way to interpolate between two different transformations (e.g., when transitioning between two states, each for which the item has a different arbitrary transform assigned). Using QGraphicsTransform you can interpolate the property values of each independent transformation. The resulting operation is then combined into a single transform which is applied to QGraphicsItem.

Transformations are computed in true 3D space using QMatrix4x4. When the transformation is applied to a QGraphicsItem, it will be projected back to a 2D QTransform. When multiple QGraphicsTransform objects are applied to a QGraphicsItem, all of the transformations are computed in true 3D space, with the projection back to 2D only occurring after the last QGraphicsTransform is applied. The exception to this is QGraphicsRotation, which projects back to 2D after each rotation to preserve the perspective effect around the X and Y axes.

If you want to create your own configurable transformation, you can create a subclass of QGraphicsTransform (or any or the existing subclasses), and reimplement the pure virtual applyTo() function, which takes a pointer to a QMatrix4x4. Each operation you would like to apply should be exposed as properties (e.g., customTransform->setVerticalShear(2.5)). Inside you reimplementation of applyTo(), you can modify the provided transform respectively.

QGraphicsTransform can be used together with QGraphicsItem::setTransform(), QGraphicsItem::setRotation(), and QGraphicsItem::setScale().