How to rotate a QGraphicsPixmap around a point according to mouseMoveEvent?

Question

I want rotate a QGraphicsPixmapItem around a point according to mouse position.

So i tried this:

void Game::mouseMoveEvent(QMouseEvent* e){
  setMouseTracking(true);
QPoint midPos((sceneRect().width() / 2), 0), currPos;

currPos = QPoint(mapToScene(e->pos()).x(), mapToScene(e->pos()).y());


QPoint itemPos((midPos.x() - cannon->scenePos().x()), (midPos.y() - cannon->scenePos().y()));

double angle = atan2(currPos.y(), midPos.x()) - atan2(midPos.y(), currPos.x());
cannon->setTransformOriginPoint(itemPos);
cannon->setRotation(angle); }

But the pixmap moves a few of pixels.

I want a result like this:

Answer 1

Besides the mixup of degrees and radians that @rafix07 pointed out there is a bug in the angle calculation. You basically need the angle of the line from midPos to currPos which you calculate by

double angle = atan2(currPos.y() - midPos.y(), currPos.x() - midPos.x());

Additionally the calculation of the transformation origin assumes the wrong coordinate system. The origin must be given in the coordinate system of the item in question (see QGraphicsItem::setTransformOriginPoint ), not in scene coordinates. Since you want to rotate around the center of that item it would just be:

QPointF itemPos(cannon->boundingRect().center());

Then there is the question whether midPos is actually the point highlighted in your image in the middle of the canon. The y-coordinate is set to 0 which would normally be the edge of the screen, but your coordinate system may be different. I would assume the itemPos calculated above is just the right point, you only need to map it to scene coordinates ( cannon->mapToScene(itemPos) ).

Lastly I would strongly advise against rounding scene coordinates (which are double s) to int s as it is done in the code by forcing it to QPoint s instead of QPointF s. Just use QPointF whenever you are dealing with scene coordinates.