Manipulating Rotations in Qt Graphics: QGraphicsRotation and Beyond

What is QGraphicsRotation?

• It's specifically designed for use with QGraphicsItem objects, which are the building blocks of Qt's graphics scene framework.
• In Qt, QGraphicsRotation is a class within the Qt Widgets module that provides a way to rotate items in a scene.

How does it work?

• By default, the rotation axis is (0, 0, 1), which corresponds to rotation around the Z-axis.
• You can set the rotation angle using the setAngle function, and the axis of rotation using either the axis property (holding a QVector3D) or the setAxis convenience function (taking a Qt::Axis member).
• QGraphicsRotation represents a rotation transformation around a specified axis.

Example

#include <QtWidgets>
#include <QtOpenGL>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a scene
    QGraphicsScene scene;

    // Create a rectangle item
    QGraphicsRectItem *rect = new QGraphicsRectItem(0, 0, 100, 50);

    // Add the rectangle to the scene
    scene.addItem(rect);

    // Create a QGraphicsRotation object
    QGraphicsRotation *rotation = new QGraphicsRotation();

    // Set the rotation angle to 45 degrees
    rotation->setAngle(45);

    // Apply the rotation to the rectangle item
    rect->setTransform(rotation->matrix());

    // Create a view to display the scene
    QGraphicsView *view = new QGraphicsView(&scene);
    view->show();

    return app.exec();
}

In this example:

1. We create a rectangle item.
2. We create a QGraphicsRotation object and set its angle to 45 degrees.
3. We retrieve the rotation matrix using rotation->matrix().
4. We apply the rotation matrix to the rectangle item using its setTransform function.
5. Finally, we display the scene in a QGraphicsView.

Key points

• The rotation axis can be customized for different scenarios.
• It offers a convenient way to define and apply rotations.
• QGraphicsRotation is used in conjunction with QGraphicsItem objects for scene manipulation.

• For more complex transformations, consider using the QTransform class.
• Combining multiple transformations (e.g., translation, scaling, rotation) can be achieved by chaining the corresponding matrix operations.

---

Rotating around a different axis

#include <QtWidgets>
#include <QtOpenGL>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a scene
    QGraphicsScene scene;

    // Create a rectangle item
    QGraphicsRectItem *rect = new QGraphicsRectItem(0, 0, 100, 50);

    // Add the rectangle to the scene
    scene.addItem(rect);

    // Create a QGraphicsRotation object
    QGraphicsRotation *rotation = new QGraphicsRotation();

    // Set the rotation angle to 45 degrees
    rotation->setAngle(45);

    // Rotate around the X-axis
    rotation->setAxis(Qt::XAxis);

    // Apply the rotation to the rectangle item
    rect->setTransform(rotation->matrix());

    // Create a view to display the scene
    QGraphicsView *view = new QGraphicsView(&scene);
    view->show();

    return app.exec();
}

This example rotates the rectangle around the X-axis instead of the default Z-axis.

Animating rotation

#include <QtWidgets>
#include <QtOpenGL>
#include <QTimer>

class RotatingItem : public QGraphicsRectItem {
public:
    RotatingItem(QGraphicsScene *scene, int width, int height)
        : QGraphicsRectItem(0, 0, width, height), scene(scene), angle(0) {
        timer = new QTimer(this);
        connect(timer, &QTimer::timeout, this, &RotatingItem::rotate);
        timer->start(10); // Update every 10 milliseconds
    }

private:
    QGraphicsScene *scene;
    QGraphicsRotation *rotation = new QGraphicsRotation();
    int angle;

    void rotate() {
        angle = (angle + 5) % 360; // Rotate by 5 degrees each update
        rotation->setAngle(angle);
        setTransform(rotation->matrix());
        scene->update();
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a scene
    QGraphicsScene scene;

    // Create a rotating rectangle item
    RotatingItem *rect = new RotatingItem(&scene, 100, 50);

    // Add the rectangle to the scene
    scene.addItem(rect);

    // Create a view to display the scene
    QGraphicsView *view = new QGraphicsView(&scene);
    view->show();

    return app.exec();
}

This example creates a custom RotatingItem class that inherits from QGraphicsRectItem. It uses a QTimer to update the rotation angle periodically, simulating a continuously rotating rectangle.

Combining rotations with other transformations

#include <QtWidgets>
#include <QtOpenGL>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a scene
    QGraphicsScene scene;

    // Create a rectangle item
    QGraphicsRectItem *rect = new QGraphicsRectItem(0, 0, 100, 50);

    // Add the rectangle to the scene
    scene.addItem(rect);

    // Create transformations
    QGraphicsRotation *rotation = new QGraphicsRotation();
    rotation->setAngle(30);
    QTransform scale;
    scale.scale(0.5, 0.75); // Scale down and stretch vertically

    // Combine transformations (rotation applied first, then scaling)
    QTransform combinedTransform = rotation->matrix() * scale;

    // Apply the combined transformation to the rectangle item
    rect->setTransform(combinedTransform);

    // Create a view to display the scene
    QGraphicsView *view = new QGraphicsView(&scene);
    view->show();

    return app.exec();
}

This example demonstrates combining a rotation with a scaling transformation. The order of application matters in this case (rotation before scaling).

---

QTransform Class

• Might require more code compared to QGraphicsRotation for simple rotations.
• Offers finer control over the transformation matrix.
• You can use it to perform rotations, translations, scaling, shearing, and other operations on graphics items.
• Qt's QTransform class provides a more general approach to manipulating 2D and 3D transformations.

QGraphicsItem::setTransformOrigin

• Can be combined with QGraphicsItem::setTransform (using a QTransform object) for more complex scenarios.
• Useful for creating rotations that pivot around a specific point within the item.
• This method allows you to specify the origin point around which transformations (including rotation) are applied.

Custom Transformations (QPainter)

• Offers the most control but requires more low-level programming.
• This involves manipulating the drawing operations within a paint event.
• For highly customized rotations or transformations that don't fit into existing classes, you can use the QPainter class directly.

• For highly customized transformations, the QPainter class offers the most flexibility.
• If you need to pivot transformations around a specific point within the item, use QGraphicsItem::setTransformOrigin in conjunction with QTransform or QPainter.
• For more complex transformations or finer control, consider QTransform.
• If you need basic rotation functionality, QGraphicsRotation is a good choice for its simplicity.

Alternative	Description	Pros	Cons
QTransform	General transformation class for 2D and 3D	More control over transformations, can combine rotations with other operations	More complex to use for basic rotations
QGraphicsItem::setTransformOrigin	Sets the origin point for transformations applied to an item	Allows rotation around a specific point within the item	Requires additional setup with QTransform or QPainter for transformations
QPainter (custom)	Low-level manipulation of drawing operations within a paint event	Provides the most control over transformations	Requires more code and understanding of low-level graphics programming