Rotating pixmap

[jano_alex_es]

Hello, I want to rotate the pixmap of the QGraphicsPixmap item and show the item rotated. Rotating the item cause other sort of problems.

So far, I never had problems when rotating the pixmaps, but I don't know why now the rotation is not taking from the center and rotates the pixmap, it's like its rotated without the translation and I don't know why.

//m_pxOriginal is a copy of the original pixmap. Not rotated.
qreal rRadius = m_pxOriginal.width() / 2;
 
		//Rotate the previous rotation - 10 degrees
		m_iAngle -= 10;
 
QPixmap rotatedPix = m_pxOriginal .transformed(QTransform().translate(-rRadius, -rRadius).rotate(m_iAngle).translate(rRadius, rRadius));
 
		m_pxItem->setPixmap(rotatedPix);

//m_pxOriginal is a copy of the original pixmap. Not rotated.
qreal rRadius = m_pxOriginal.width() / 2;

		//Rotate the previous rotation - 10 degrees
		m_iAngle -= 10;

QPixmap rotatedPix = m_pxOriginal .transformed(QTransform().translate(-rRadius, -rRadius).rotate(m_iAngle).translate(rRadius, rRadius));

		m_pxItem->setPixmap(rotatedPix);

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Anything strange? :S thanks!

[SixDegrees]

Step one: get rid of the multiple transformations all packed onto a single line. Do them one at a time, examine each step and isolate the problem.

[jano_alex_es]

The point is the rotation of the pixmaps. I think there is something wrong, because the rotation of the item works fine (unfortunatly, I do need to create the rotation feeling through the pixmap rotation, item rotation is not a possibility). Maybe there is a problem when transforming from/to QImage.


Item rotation

m_pxItem->setTransformOriginPoint(rRadius, rRadius);
		m_pxItem->setRotation(m_iAngle);

m_pxItem->setTransformOriginPoint(rRadius, rRadius);
		m_pxItem->setRotation(m_iAngle);

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Pixmap rotation

QPixmap pix1 = m_pxOriginal.transformed(QTransform().translate(-rRadius, -rRadius));
		QPixmap pix2 = m_pxOriginal.transformed(QTransform().rotate(m_iAn  gle));
		QPixmap rotatedPix = pix2.transformed(QTransform().translate(rRadius, rRadius));
		m_pxItem->setPixmap(rotatedPix);
//Just rotating, without the traslation, shows the same bounding effect

QPixmap pix1 = m_pxOriginal.transformed(QTransform().translate(-rRadius, -rRadius));
		QPixmap pix2 = m_pxOriginal.transformed(QTransform().rotate(m_iAn  gle));
		QPixmap rotatedPix = pix2.transformed(QTransform().translate(rRadius, rRadius));
		m_pxItem->setPixmap(rotatedPix);
//Just rotating, without the traslation, shows the same bounding effect

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[jano_alex_es]

I was able to workaround the problem thanks drawing in a rotated QPainter.

QPixmap rotatedPixmap(m_pxOriginal.size());
		QPainter* p = new QPainter(&rotatedPixmap); 
		QSize size = m_pxOriginal.size();
		p->translate(size.height()/2,size.height()/2);
		p->rotate(m_iAngle);
		p->translate(-size.height()/2,-size.height()/2);
		p->drawPixmap(0, 0, m_pxOriginal);
		p->end();
		delete p;
		m_pxItem->setPixmap(rotatedPixmap);

QPixmap rotatedPixmap(m_pxOriginal.size());
		QPainter* p = new QPainter(&rotatedPixmap); 
		QSize size = m_pxOriginal.size();
		p->translate(size.height()/2,size.height()/2);
		p->rotate(m_iAngle);
		p->translate(-size.height()/2,-size.height()/2);
		p->drawPixmap(0, 0, m_pxOriginal);
		p->end();
		delete p;
		m_pxItem->setPixmap(rotatedPixmap);

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The rotation works fine! BUT a grey rectangle is painted:




Why is it created? and more important, how can I remove it?

[jano_alex_es]

Well, finally solved with the QPainter aproximation.

//Please note this method takes 2 mseg to finish for a 20x20 pixmap.
    QPixmap rotatedPixmap(m_pixOriginal.size());
    rotatedPixmap.fill(QColor::fromRgb(0, 0, 0, 0)); //the new pixmap must be transparent.
QPainter* p = new QPainter(&rotatedPixmap);
QSize size = m_pxOriginal.size();
p->translate(size.height()/2,size.height()/2);
p->rotate(m_iAngle);
p->translate(-size.height()/2,-size.height()/2);
p->drawPixmap(0, 0, m_pxOriginal);
p->end();
delete p;
m_pxItem->setPixmap(rotatedPixmap);

//Please note this method takes 2 mseg to finish for a 20x20 pixmap.
    QPixmap rotatedPixmap(m_pixOriginal.size());
    rotatedPixmap.fill(QColor::fromRgb(0, 0, 0, 0)); //the new pixmap must be transparent.
QPainter* p = new QPainter(&rotatedPixmap);
QSize size = m_pxOriginal.size();
p->translate(size.height()/2,size.height()/2);
p->rotate(m_iAngle);
p->translate(-size.height()/2,-size.height()/2);
p->drawPixmap(0, 0, m_pxOriginal);
p->end();
delete p;
m_pxItem->setPixmap(rotatedPixmap);

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Definitly there is a translation problem when rotating a QPixmap, I don't know if its because the conversion to/from QImage or the known small rotation (according the QtAssistan it's possible to solve it with QPixmap::trueMatrix, but It doesn't solve it in my code) but the pixmap rotation is an issue in Qt.