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

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:

CodePudding user response：

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 doubles) to ints as it is done in the code by forcing it to QPoints instead of QPointFs. Just use QPointF whenever you are dealing with scene coordinates.