Transforming Items with Precision: QGraphicsItem::transformOriginPoint() in Qt Widgets

Understanding Transform Origin Point

In Qt's graphics framework, QGraphicsItem represents items that can be displayed and manipulated within a scene. These items can undergo various transformations like scaling, rotation, and shearing. The transformOriginPoint() function plays a crucial role in determining how these transformations are applied.

What it Does

• By default, the transform origin point is set to the item's center.
• Returns a QPointF object that specifies the point relative to the item's bounding rectangle where transformations are centered.

Impact on Transformations

• Similar logic applies to scaling and shearing transformations. The scaling or shearing is centered around this point.
• When you rotate an item, it's rotated around this point. Imagine a pin stuck at this point, and the item pivots around it during rotation.

Example

#include <QtWidgets>

class MyItem : public QGraphicsItem {
public:
    MyItem(const QPixmap &pixmap) : m_pixmap(pixmap) {}

    QRectF boundingRect() const override {
        return m_pixmap.rect();
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override {
        painter->drawPixmap(0, 0, m_pixmap);
    }

    // Set the transform origin point to the top-left corner
    void setTopLeftTransformOrigin() {
        setTransformOriginPoint(QPoint(0, 0));
    }

private:
    QPixmap m_pixmap;
};

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

    QGraphicsScene scene;
    QGraphicsPixmapItem *item = new QGraphicsPixmapItem(QPixmap("image.png"));
    scene.addItem(item);

    item->setTopLeftTransformOrigin(); // Set origin to top-left
    item->setRotation(45); // Now rotation is centered at top-left

    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

In this example, by setting the transform origin to the top-left corner using setTopLeftTransformOrigin(), the rotation of 45 degrees will be centered around that point, resulting in a different visual effect compared to the default center-based rotation.

Key Points

• Understanding the transform origin point is essential for creating visually accurate and interactive graphics applications.
• You can modify the transform origin point using setTransformOriginPoint() to achieve specific visual effects during item manipulation.

• The concept of transform origin points is not unique to Qt and applies to various graphics libraries and frameworks.
• While QGraphicsItem::transformOriginPoint() is primarily used with Qt Widgets, it's also relevant in the broader Qt framework, including QtWidgets and Qt Quick.

---

Rotating a Hand Around Its Wrist

#include <QtWidgets>

class HandItem : public QGraphicsItem {
public:
    HandItem(const QPixmap &handPixmap, const QPixmap &wristPixmap)
        : m_handPixmap(handPixmap), m_wristPixmap(wristPixmap) {}

    QRectF boundingRect() const override {
        return m_handPixmap.rect();
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override {
        painter->drawPixmap(0, 0, m_handPixmap);
        painter->drawPixmap(-m_wristPixmap.width() / 2, boundingRect().height() - m_wristPixmap.height(), m_wristPixmap);
    }

    void setWristRotation(qreal angle) {
        setTransformOriginPoint(QPoint(-m_wristPixmap.width() / 2, boundingRect().height() - m_wristPixmap.height() / 2));
        setRotation(angle);
    }

private:
    QPixmap m_handPixmap;
    QPixmap m_wristPixmap;
};

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

    QGraphicsScene scene;
    HandItem *hand = new HandItem(QPixmap("hand.png"), QPixmap("wrist.png"));
    scene.addItem(hand);

    hand->setWristRotation(45); // Rotate hand around wrist

    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

This example creates a hand item with a separate wrist image. By setting the transform origin point to the wrist's center, the hand rotates around that point when applying setRotation().

Scaling an Image from Its Bottom-Right Corner

#include <QtWidgets>

class ScalableImage : public QGraphicsPixmapItem {
public:
    ScalableImage(const QPixmap &pixmap) : QGraphicsPixmapItem(pixmap) {}

    void setBottomRightScaling() {
        setTransformOriginPoint(boundingRect().bottomRight());
        setScale(2.0); // Now scales from bottom-right corner
    }
};

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

    QGraphicsScene scene;
    ScalableImage *image = new ScalableImage(QPixmap("image.png"));
    scene.addItem(image);

    image->setBottomRightScaling();

    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

This example demonstrates setting the transform origin to the image's bottom-right corner before scaling it. This way, the scaling effect appears as if it originates from that point.

• Experiment with different origin points and transformations to achieve the visual behavior you need.
• These are just a few examples, and the specific use of setTransformOriginPoint() will depend on your desired visual effect.

---

1. • Break down your item into smaller sub-items and manipulate them individually. You can position these sub-items to create the illusion of a custom transform origin point for the overall item. This approach requires more code but can be more flexible for complex visuals.

2. Custom QGraphicsEffect
   

   • For more advanced scenarios, consider creating a custom QGraphicsEffect class that modifies the painting process of your item. This would allow you to implement custom transformation behavior based on a defined origin point within the effect. However, this approach has a steeper learning curve.

Choosing the Right Approach

• For highly customized behavior, a custom effect might be necessary.
• If you need more control over individual item components, consider item composition.
• For simple origin point adjustments, combining transformations is often sufficient.

Additional Considerations

• Evaluate the complexity of your desired effect and choose the approach that balances ease of implementation with the level of control you need.
• These alternatives might require slightly more code compared to directly using setTransformOriginPoint().