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:
alt text