QPixmap blur when QGraphicsView scale

[shanshan]

Hello,

thanks for your fast reply. But this explains that one step scale, that 1.2, 1.3, 1.4... only one image pixel is interpolated. But our case is that we scaled many steps, the image is zoomed 1.5^9 times to 38.4434, but not only one pixel at 39 is interpolated, but multiple of them.
Thant means that when the picture of (1*2) is zoomed to (1,77), (1-38) should be black, but they are not. about( 20-38) are all random and they change when we drag the image. That is why we thought it is a bug of qt

---

Added after 7 minutes:

The pixmap is 1024*768, the line is at 767,
the line stays between two pixel when zoom 1.5^8 = 25.6289, get higher at 1.5^9 = 38.4434 and move back between two pixel at 1.5^10=57.665

this is just an sample that to point out the problem, our users need to see the circles or results inside exact each image pixel, cannot accept them move around and back.

qt1.jpg

[wysota]

I have no idea what you do with your image but it seems Qt behaves correctly, here is a sample app that demonstrates this:

#include <QtWidgets>
 
class GraphicsView : public QGraphicsView {
public:
  GraphicsView() { m_scale = 1; }
protected:
  void wheelEvent(QWheelEvent *ev) {
    if(ev->delta() > 0) {
      m_scale *= 1.5;
      qDebug() << "+1.5" << m_scale;
      scale(1.5, 1.5);
    } else {
      m_scale /= 1.5;
      qDebug() << "-1.5" << m_scale;
      scale(1/1.5, 1/1.5);
    }
  }
private:
  qreal m_scale;
};
 
int main(int argc, char **argv) {
  QApplication app(argc, argv);
 
  QGraphicsScene scene;
  QPixmap px(2,2);
  px.fill(Qt::white);
  QPainter p(&px);
  p.setPen(Qt::black);
  p.drawPoint(0,0);
  p.drawPoint(1,1);
  p.end();
  scene.addPixmap(px);
  GraphicsView view;
  view.setScene(&scene);
  view.show();
  return app.exec();
}

#include <QtWidgets>

class GraphicsView : public QGraphicsView {
public:
  GraphicsView() { m_scale = 1; }
protected:
  void wheelEvent(QWheelEvent *ev) {
    if(ev->delta() > 0) {
      m_scale *= 1.5;
      qDebug() << "+1.5" << m_scale;
      scale(1.5, 1.5);
    } else {
      m_scale /= 1.5;
      qDebug() << "-1.5" << m_scale;
      scale(1/1.5, 1/1.5);
    }
  }
private:
  qreal m_scale;
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  
  QGraphicsScene scene;
  QPixmap px(2,2);
  px.fill(Qt::white);
  QPainter p(&px);
  p.setPen(Qt::black);
  p.drawPoint(0,0);
  p.drawPoint(1,1);
  p.end();
  scene.addPixmap(px);
  GraphicsView view;
  view.setScene(&scene);
  view.show();
  return app.exec();
}

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You can see that with small scales the squares are not even but the situation improves as the scales get larger.

[shanshan]

What I did is a big project and which handled image is like this

MLGraphicsView( MLGraphicsScene * scene, QWidget * parent = 0, Qt::WindowFlags f = 0 )
		:QGraphicsView( scene )
		,m_scale(1.0)
	{
		setDragMode( QGraphicsView::ScrollHandDrag );
	}
	~MLGraphicsView()
	{}
protected: 
    //Take over the interaction
    virtual void wheelEvent(QWheelEvent* event)
	{
		setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
		setResizeAnchor(QGraphicsView::AnchorUnderMouse);
		// Scale the view / do the zoom
		double scaleFactor = 1.5;
 
		if(event->delta() > 0) {
			// Zoom in
			scale(scaleFactor, scaleFactor);
			m_scale *= scaleFactor;
 
		} else {
			scale(1.0 / scaleFactor, 1.0 / scaleFactor);
			m_scale /= scaleFactor;
		}
 
// some other part
  		QGraphicsScene* gScene = this->getView()->scene();
 
		gScene->clear();
 
		QPixmap pixmap = m_visDocument->getImag( camId );
		if( !pixmap )
		{
			ART_LOG_ERROR( logger, "Camera " << camId << " is empty. " );
			continue;
		}
		QGraphicsPixmapItem * qpixmap = gScene->addPixmap( pixmap );
                 QGraphicsLineItem* line = new QGraphicsLineItem( -size, 0, size, 0 );
               gScene->addItem(line);

MLGraphicsView( MLGraphicsScene * scene, QWidget * parent = 0, Qt::WindowFlags f = 0 )
		:QGraphicsView( scene )
		,m_scale(1.0)
	{
		setDragMode( QGraphicsView::ScrollHandDrag );
	}
	~MLGraphicsView()
	{}
protected: 
    //Take over the interaction
    virtual void wheelEvent(QWheelEvent* event)
	{
		setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
		setResizeAnchor(QGraphicsView::AnchorUnderMouse);
		// Scale the view / do the zoom
		double scaleFactor = 1.5;

		if(event->delta() > 0) {
			// Zoom in
			scale(scaleFactor, scaleFactor);
			m_scale *= scaleFactor;

		} else {
			scale(1.0 / scaleFactor, 1.0 / scaleFactor);
			m_scale /= scaleFactor;
		}

// some other part
  		QGraphicsScene* gScene = this->getView()->scene();

		gScene->clear();

		QPixmap pixmap = m_visDocument->getImag( camId );
		if( !pixmap )
		{
			ART_LOG_ERROR( logger, "Camera " << camId << " is empty. " );
			continue;
		}
		QGraphicsPixmapItem * qpixmap = gScene->addPixmap( pixmap );
                 QGraphicsLineItem* line = new QGraphicsLineItem( -size, 0, size, 0 );
               gScene->addItem(line);

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what you did is just a 2*2 pixmap and with the same color, you can try load a bigger windows picture, and then zoom it in in bigger levels, maybe you will see what I get.

Like the 1.5^9 = 38.4434 times, that if qt is doing right, 1-38 pixel should be same color, but 39 is random. But in my img you saw, it lookes like 1-25 is sampled correct, 25-38is not. It is obviously wrong!

[wysota]

Can you post a minimal compilable example reproducing the problem?

[shanshan]

Hello, the code is as followed,

I work with the windows default pixtures and
here is the screenshot
when scale factor 25.6289, no problem, when 38.** or 57.6**, the line is not in the correct postion, and when drag the scene with mouse, the pixels blured and changed.

#include <QtWidgets>
#include <QGraphicsView>
#include <QWheelEvent>
#include <QApplication>
 
 
class GraphicsView : public QGraphicsView 
{
 
public:
	GraphicsView ( QWidget* parent = 0 )
		:QGraphicsView( parent )
		,m_scale(1.0)
	{
	 		setDragMode( QGraphicsView::ScrollHandDrag );
	}
protected:
	void wheelEvent(QWheelEvent *ev)
	{
		setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
		setResizeAnchor(QGraphicsView::AnchorUnderMouse);
		if(ev->delta() > 0) {
		m_scale *= 1.5;
		qDebug() << "+1.5" << m_scale;
		scale(1.5, 1.5);
		} else {
		m_scale /= 1.5;
		qDebug() << "-1.5" << m_scale;
		scale(1/1.5, 1/1.5);
	}
 }
private:
 qreal m_scale;
};
int main(int argc, char **argv)
{
	QApplication app(argc, argv);
	QGraphicsScene scene;
	QPixmap px; 
 
	QString fileName = QFileDialog::getOpenFileName(0, QString("Open File"), QDir::currentPath(), "Image Files (*.png *.jpg *.bmp)");
	if (!fileName.isEmpty())
	{
		QFileInfo fi( fileName );
		if( fi.exists())
		{
			bool err = px.load( fi.absoluteFilePath());
			if (!err)
				return 0;
		}
	}
 
	scene.addPixmap(px);
	QGraphicsLineItem* line = new QGraphicsLineItem( px.width()-10,px.height()-1,px.width(),px.height()-1 );
	QPen pen(Qt::red, 1.0 );
	pen.setCosmetic( true );
	line->setPen( pen );
	scene.addItem( line );
	GraphicsView view;
	view.setScene(&scene);
	view.show();
	return app.exec();
}

#include <QtWidgets>
#include <QGraphicsView>
#include <QWheelEvent>
#include <QApplication>


class GraphicsView : public QGraphicsView 
{

public:
	GraphicsView ( QWidget* parent = 0 )
		:QGraphicsView( parent )
		,m_scale(1.0)
	{
	 		setDragMode( QGraphicsView::ScrollHandDrag );
	}
protected:
	void wheelEvent(QWheelEvent *ev)
	{
		setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
		setResizeAnchor(QGraphicsView::AnchorUnderMouse);
		if(ev->delta() > 0) {
		m_scale *= 1.5;
		qDebug() << "+1.5" << m_scale;
		scale(1.5, 1.5);
		} else {
		m_scale /= 1.5;
		qDebug() << "-1.5" << m_scale;
		scale(1/1.5, 1/1.5);
	}
 }
private:
 qreal m_scale;
};
int main(int argc, char **argv)
{
	QApplication app(argc, argv);
	QGraphicsScene scene;
	QPixmap px; 
 	
	QString fileName = QFileDialog::getOpenFileName(0, QString("Open File"), QDir::currentPath(), "Image Files (*.png *.jpg *.bmp)");
	if (!fileName.isEmpty())
	{
		QFileInfo fi( fileName );
		if( fi.exists())
		{
			bool err = px.load( fi.absoluteFilePath());
			if (!err)
				return 0;
		}
	}

	scene.addPixmap(px);
	QGraphicsLineItem* line = new QGraphicsLineItem( px.width()-10,px.height()-1,px.width(),px.height()-1 );
	QPen pen(Qt::red, 1.0 );
	pen.setCosmetic( true );
	line->setPen( pen );
	scene.addItem( line );
	GraphicsView view;
	view.setScene(&scene);
	view.show();
	return app.exec();
}

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qttest1.jpg qttest2.jpgqttest3.jpgqttest4.jpg