rotation of an animated QGraphicsItem

[darksaga]

hi there,

i've subclassed a QGraphicsItem to create a rotating & clickable icon for a QGraphicsScene,

rotation works fine, using QTimeLine & QGraphicsItemAnimation,

the problem that I'm facing now is that my item rotates around the coordinates 0/0,

how do I tell my item to rotate around its center: _with/2 / _height/2?

regards

ps: here's the code:

#ifndef ANIMATEICON_H
#define ANIMATEICON_H
 
#include <QObject>
#include <QGraphicsItem>
#include <QRectF>
#include <QPainter>
#include <QStyleOptionGraphicsItem>
#include <QWidget>
#include <QTimeLine>
#include <QGraphicsItemAnimation>
 
class AnimateIcon : public QObject, public QGraphicsItem
{
	Q_OBJECT
 
	public:
		AnimateIcon(QGraphicsItem *parent = 0, int width = 48, int height = 48) : QGraphicsItem(parent){
			_width = width;
			_height = height;
			_animation = new QGraphicsItemAnimation(this);
			_timeLine = new QTimeLine(10000, this);
 
			_animation->setItem(this);
			_animation->setTimeLine(_timeLine);
			for (int i = 0; i <= 360; i+=18)
				_animation->setRotationAt(i/360.0, i);
 
			_timeLine->setCurveShape(QTimeLine::LinearCurve);
			_timeLine->setLoopCount(0);
			_timeLine->start();
 
		}
		~AnimateIcon(void){}
		QRectF boundingRect(void) const{
			return QRectF(0, 0, _width, _height);
		}
		void paint(QPainter *painter, const QStyleOptionGraphicsItem *	option, QWidget *widget = 0){
			painter->drawPixmap(0, 0, _width, _height,QPixmap(":/icons/action.png"));
		}
 
	private:
		int _width;
		int _height;
		QGraphicsItemAnimation *_animation;
		QTimeLine *_timeLine;
};
#endif

#ifndef ANIMATEICON_H
#define ANIMATEICON_H

#include <QObject>
#include <QGraphicsItem>
#include <QRectF>
#include <QPainter>
#include <QStyleOptionGraphicsItem>
#include <QWidget>
#include <QTimeLine>
#include <QGraphicsItemAnimation>

class AnimateIcon : public QObject, public QGraphicsItem
{
	Q_OBJECT

	public:
		AnimateIcon(QGraphicsItem *parent = 0, int width = 48, int height = 48) : QGraphicsItem(parent){
			_width = width;
			_height = height;
			_animation = new QGraphicsItemAnimation(this);
			_timeLine = new QTimeLine(10000, this);
			
			_animation->setItem(this);
			_animation->setTimeLine(_timeLine);
			for (int i = 0; i <= 360; i+=18)
				_animation->setRotationAt(i/360.0, i);

			_timeLine->setCurveShape(QTimeLine::LinearCurve);
			_timeLine->setLoopCount(0);
			_timeLine->start();

		}
		~AnimateIcon(void){}
		QRectF boundingRect(void) const{
			return QRectF(0, 0, _width, _height);
		}
		void paint(QPainter *painter, const QStyleOptionGraphicsItem *	option, QWidget *widget = 0){
			painter->drawPixmap(0, 0, _width, _height,QPixmap(":/icons/action.png"));
		}

	private:
		int _width;
		int _height;
		QGraphicsItemAnimation *_animation;
		QTimeLine *_timeLine;
};
#endif

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[wysota]

Make its bounding rect QRect(-_width/2, -_height/2, _width, _height) and draw it accordingly. Simply speaking make 0,0 the middle of the item, not the top left corner.

[darksaga]

Make its bounding rect QRect(-_width/2, -_height/2, _width, _height) and draw it accordingly. Simply speaking make 0,0 the middle of the item, not the top left corner.

already thought about doing it this way,

but I thought, that there was some kind of function hidden inside the documentation I didn't find, something like setRotationAnchor()/setRotationCenter()

anyway, thx for your suggestion

[wysota]

No, there isn't. A matrix rotation is always around the origin.

[Bitto]

To rotate around an arbitrary point (x, y), or (width/2, height/2) in your case, do this:

item->setTransform(QTransform().translate(x, y).rotate(a).translate(-x, -y));

item->setTransform(QTransform().translate(x, y).rotate(a).translate(-x, -y));

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To scale around an arbitrary point (x, y), or (width/2, height/2) in your case, do this:

item->setTransform(QTransform().translate(x, y).scale(sx, sy).translate(-x, -y));

item->setTransform(QTransform().translate(x, y).scale(sx, sy).translate(-x, -y));

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I guess you see the pattern ;-).

[wysota]

I guess it's simpler to just move the origin

[Bitto]

That depends on the item - sometimes it's easier to replace one line than to rewrite all your coordinates ;-).

[momesana]

what about QGraphicsSvgItem. It already implements boundingRect() which returns 0,0 as Point of origin for one Item I'm testing here. Can I also handle that doing a transformation? Redefining boundingRect seemingly isn't an option here ...

Thanx in advance

[wysota]

That depends on the item - sometimes it's easier to replace one line than to rewrite all your coordinates ;-).

You can always do the translation in the item's paint routine. Also a single line

[Bitto]

momesana, you can just use the translate-rotate-translate trick from above, it works for any item and you don't have to modify it / subclass it / change its bounding rect or painting.

wysota, you'd have to translate the bounding rect, the shape, and the painter inside the paint() function. otherwise you'd get painting and collision artifacts.