The Graphics View Framework

Item Coordinates

Items live in their own local coordinate system. Their coordinates are usually centered around its center point (0, 0), and this is also the center for all transformations. Geometric primitives in the item coordinate system are often referred to as item points, item lines, or item rectangles.

When creating a custom item, item coordinates are all you need to worry about; QGraphicsScene and QGraphicsView will perform all transformations for you. This makes it very easy to implement custom items. For example, if you receive a mouse press or a drag enter event, the event position is given in item coordinates. The QGraphicsItem::contains() virtual function, which returns true if a certain point is inside your item, and false otherwise, takes a point argument in item coordinates. Similarly, an item's bounding rect and shape are in item coordinates.

At item's position is the coordinate of the item's center point in its parent's coordinate system; sometimes referred to as parent coordinates. The scene is in this sense regarded as all parent-less items' "parent". Top level items' position are in scene coordinates.

Child coordinates are relative to the parent's coordinates. If the child is untransformed, the difference between a child coordinate and a parent coordinate is the same as the distance between the items in parent coordinates. For example: If an untransformed child item is positioned precisely in its parent's center point, then the two items' coordinate systems will be identical. If the child's position is (10, 0), however, the child's (0, 10) point will correspond to its parent's (10, 10) point.

Because items' position and transformation are relative to the parent, child items' coordinates are unaffected by the parent's transformation, although the parent's transformation implicitly transforms the child. In the above example, even if the parent is rotated and scaled, the child's (0, 10) point will still correspond to the parent's (10, 10) point. Relative to the scene, however, the child will follow the parent's transformation and position. If the parent is scaled (2x, 2x), the child's position will be at scene coordinate (20, 0), and its (10, 0) point will correspond to the point (40, 0) on the scene.

With QGraphicsItem::pos() being one of the few exceptions, QGraphicsItem's functions operate in item coordinates, regardless of the item, or any of its parents' transformation. For example, an item's bounding rect (i.e. QGraphicsItem::boundingRect()) is always given in item coordinates.

 class View : public QGraphicsView
 {
 Q_OBJECT
     ...
 public slots:
     void zoomIn() { scale(1.2, 1.2); }
     void zoomOut() { scale(1 / 1.2, 1 / 1.2); }
     void rotateLeft() { rotate(-10); }
     void rotateRight() { rotate(10); }
     ...
 };

 QGraphicsScene scene;
 scene.addRect(QRectF(0, 0, 100, 200), Qt::black, QBrush(Qt::green));

 QPrinter printer;
 if (QPrintDialog(&printer).exec() == QDialog::Accepted) {
     QPainter painter(&printer);
     painter.setRenderHint(QPainter::Antialiasing);
     scene->render(&painter);
 }

 QGraphicsScene scene;
 scene.addRect(QRectF(0, 0, 100, 200), Qt::black, QBrush(Qt::green));

 QPixmap pixmap;
 QPainter painter(&pixmap);
 painter.setRenderHint(QPainter::Antialiasing);
 scene->render(&painter);
 painter.end();

 pixmap.save("scene.png");

 void CustomItem::mousePressEvent(QGraphicsSceneMouseEvent *event)
 {
     QMimeData *data = new QMimeData;
     data->setColor(Qt::green);

     QDrag *drag = new QDrag(event->widget());
     drag->setMimeData(data);
     drag->start();
 }

 QGraphicsView view(&scene);
 view.setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));