How to rotate QGraphicsItem right after mouse?

[Valderman]

Hello!
My application have QGraphicsScene and custom QGraphicsItem on it.
I wrote the following code for my custom graphics item rotation:

// FigureScene is inherited from QGraphicsScene
void FigureScene::mousePressEvent(QGraphicsSceneMouseEvent* mouseEvent)
{
	// Remember last press coordinate
	if (mouseEvent->button() == Qt::RightButton)
	{
        	lastRightMousePressed = mouseEvent->screenPos();
	}
}

void FigureScene::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent)
{
	if (mouseEvent->button() == Qt::RightButton)
	{
		const auto sceneCoord = mouseEvent->scenePos();

		foreach (QGraphicsItem* item, m_parentScene->selectedItems())
        	{
			QTransform transform;
			QPointF	   center = item->sceneBoundingRect().center();
        	
			transform.translate(center.x(), center.y());
        	
 			qreal angle
                              = qAtan2(lastRightMousePressed.y() - sceneCoord.y(),
                                       lastRightMousePressed.x() - sceneCoord.x());
        	
			transform.rotate(angle);
			transform.translate(-center.x(), -center.y());
        	
			item->setPos(transform.map(item->pos()));
			item->setRotation(item->rotation() + angle);
        	}
	}
}

And my figure rotating something like this:

The problem is that when I rotate the mouse clockwise or counterclockwise, the figure rotates only counterclockwise.

I would like my figure to rotate right after the mouse.
How can I achieve that?

[SGaist]

Hi,

You need to keep track of the last position of the mouse so your can determine in which direction it's going and adjust the rotation direction accordingly.

[Chris Kawa]

Instead of doing additive rotation each move I would modify item's origin and remember its rotation on press in a map and then just set rotation on move. This would make the item follow cursor with pixel accuracy.

In press:

itemRotations.clear(); // this is a QMap<QGraphicsItem*, qreal>
const QPointF mousePos = mouseEvent->scenePos();

foreach (QGraphicsItem* item, selectedItems())
{
    const QPointF itemOrigin = item->sceneBoundingRect().center();
    const qreal angle = qAtan2(mousePos.y() - itemOrigin.y(),
                               mousePos.x() - itemOrigin.x());

    item->setTransformOriginPoint(itemOrigin);
    itemRotations[item] = item->rotation() - qRadiansToDegrees(angle);
}

and in move:

const QPointF mousePos = mouseEvent->scenePos();

foreach (QGraphicsItem* item, selectedItems())
{
    const QPointF itemOrigin = item->transformOriginPoint();
    const qreal angle = qAtan2(mousePos.y() - itemOrigin.y(),
                               mousePos.x() - itemOrigin.x());

    item->setRotation(itemRotations[item] + qRadiansToDegrees(angle));
}

[Valderman]

@Chris-Kawa, Hello and thank you so much for your answer!
But I don't get it. Here is code that you propose:

void FigureScene::onRightMousePressEvent(QGraphicsSceneMouseEvent* mouseEvent)
{
	itemRotations.clear();
	const QPointF mousePos = mouseEvent->scenePos();

	foreach (QGraphicsItem* item, selectedItems())
	{
		const QPointF itemOrigin = item->sceneBoundingRect().center();
		const qreal angle 
                    = qAtan2(mousePos.y() - itemOrigin.y(), mousePos.x() - itemOrigin.x());

		item->setTransformOriginPoint(itemOrigin);
		itemRotations[item] = item->rotation() - qRadiansToDegrees(angle);
	}
}

void FigureScene::onRightMouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent)
{
	const QPointF mousePos = mouseEvent->scenePos();

	foreach (QGraphicsItem* item, selectedItems())
	{
		const QPointF itemOrigin = item->transformOriginPoint();
		const qreal angle
                      = qAtan2(mousePos.y() - itemOrigin.y(),  mousePos.x() - itemOrigin.x());

		item->setRotation(itemRotations[item] + qRadiansToDegrees(angle));
	}
}

Here is what happens:

Figure just running away right after movement.
Is it something wrong with setting position?

[johngod]

@Valderman

I did something like that in a project of mine, but with a difference that the rectangle does not rotate in his center but rotates relative to a center defined in a first click position called commandInitialPos, then you do a second click to start rotating, then you move the mouse to rotate.
I am posting the relevant code I used may it can be helpfull for you to adapt

the relevant pseudo code and link to the project:
https://bitbucket.org/joaodeusmorgado/techdrawstudio/src/142960ab5c809ddcbd20fb40f294e01d85c850d5/qml/qmlEntities/EntityTemplate.qml#lines-117

//first click - define the center of rotation
onCommandCprStart: function(pos) {
    commandInitialPos = pos //assume that pos is the mouse click position
}

https://bitbucket.org/joaodeusmorgado/techdrawstudio/src/142960ab5c809ddcbd20fb40f294e01d85c850d5/qml/qmlEntities/RectangleThick.qml#lines-39

//second click - start rotation position
onCommandCprUpdate: function(pos, ....){
    rotateStartPoint = pos //save the start rotation position
	//calculates and saves the start angle
	rotateStartAngle = mathLib.angleFromPoints(commandInitialPos, rotateStartPoint) 
	rectP0aux = re.p0 // here I just saving the rectangle initiall position vertexs
	rectP1aux = re.p1
	rectP2aux = re.p2
	rectP3aux = re.p3
}

https://bitbucket.org/joaodeusmorgado/techdrawstudio/src/142960ab5c809ddcbd20fb40f294e01d85c850d5/qml/qmlEntities/RectangleThick.qml#lines-75

onCommandMtm: function(pos, ....){//this is equivalent to mouse move 
	rotateUpdateAngle = mathLib.angleFromPoints(commandInitialPos, pos)

	var angle = rotateUpdateAngle - rotateStartAngle
    // lets do it baby, lets rotate it, yeahhhh, updates the rectangles vertices, which is the rotation
	re.p0 = mathLib.rotate2DFromPoint(commandInitialPos, angle, rectP0aux)
	re.p1 = mathLib.rotate2DFromPoint(commandInitialPos, angle, rectP1aux)
	re.p2 = mathLib.rotate2DFromPoint(commandInitialPos, angle, rectP2aux)
	re.p3 = mathLib.rotate2DFromPoint(commandInitialPos, angle, rectP3aux)
}

https://bitbucket.org/joaodeusmorgado/techdrawstudio/src/142960ab5c809ddcbd20fb40f294e01d85c850d5/mathlib.cpp#lines-103

QVector3D MathLib::rotate2DFromPoint(const QVector3D &center, const float &angle, const QVector3D &p) const
{
    float cosAngle = qCos(angle);
    float sinAngle = qSin(angle);
    float x = cosAngle*p.x() - sinAngle*p.y() + center.x() - cosAngle*center.x() + sinAngle*center.y();
    float y = sinAngle*p.x() + cosAngle*p.y() + center.y() - sinAngle*center.x() - cosAngle*center.y();
    return QVector3D(x, y, p.z());
}

This is the rectangle rotating: