How to Set Mouse Collision Box for QGraphicsPixmapItem? [QT/C++]

Question

When re-implementing any mouse event function in a QGraphicsPixmapItem such as mousePressEvent, mouseReleaseEvent, mouseMoveEvent, the graphics item uses pixel-perfect collision. For example, for the mousePressEvent to be triggered, the mouse must be exactly on top of a visible pixel in the pixmap when clicked.

On the other hand, I want the collision to be a generalized box based on the pixmap's width and height: [0, 0, width, height].

How can this be accomplished?

(Some sample code cause people seem to like that):

class MyGraphicsItem: public QGraphicsPixmapItem {
public:
  MyGraphicsItem(): QGraphicsPixmapItem() {}

protected:
  void mousePressEvent(QGraphicsSceneMouseEvent* event) {
    // do stuff. Will be called when clicked exactly on the image
    QGraphicsPixmapItem::mousePressEvent(event);
  }
}

@SteakOverflow Oh that was it! In fact, you could take it one step further and use QGraphicsPixmapItem::shape() to create a rectangle shape based on the dimensions. Thanks so much! — Griffort

Griffort Griffort · Accepted Answer · 2018-03-21T14:29:56

Thanks to SteakOverflow for a point in the right direction.

Aiiright, here's what you gotta do:

class MyGraphicsItem: public QGraphicsPixmapItem {
public:
  MyGraphicsItem(): QGraphicsPixmapItem() {
    // Change shape mode in constructor
    setShapeMode(QGraphicsPixmapItem::BoundingRectShape);
  }

protected:
  void mousePressEvent(QGraphicsSceneMouseEvent* event) {
    // do stuff. Will be called when clicked exactly on the image
    QGraphicsPixmapItem::mousePressEvent(event);
  }
}

Alternatively, you can do:

class MyGraphicsItem: public QGraphicsPixmapItem {
public:
  MyGraphicsItem(): QGraphicsPixmapItem() {}

protected:
  void mousePressEvent(QGraphicsSceneMouseEvent* event) {
    // do stuff. Will be called when clicked exactly on the image
    QGraphicsPixmapItem::mousePressEvent(event);
  }

  // ::shape defines collision. 
  // This will make it a rect based on general dimensions of the pixmap
  QPainterPath shape() const {
    if(!pixmap().isNull()) {
        QPainterPath path;
        path.addRect(0, 0, pixmap().width(), pixmap().height());
        return path;
    } else {
        return QGraphicsPixmapItem::shape();
    }
  }
}

How to Set Mouse Collision Box for QGraphicsPixmapItem? [QT/C++]

1 Answers