QItems alignment problem when using scaling with custom graphicsView and graphicsScene

[lugemononeurone]

Hi,

I'm developping on WinXP, Visual Studio 2008 and Qt5. I'm a newbie in Qt

I am trying to visualize an image. I must be able to zoom in/out in that image. When the cursor hovers the image, a vertical and an horizontal lines, are drawn at the cursor position. Next to the cursor position, I have a box displaying some data (i.e. rgba pixel values).

To manage most parts of these requirements, I subclassed QGraphicsView and also custom QGraphicsScene.

My subclassed view manages the zoom by overriding wheelEvent(QWheelEvent* event).

My subclassed scene contains the scene items and manages the mouse events when hovering. The member lines, box and text ignore the scaling.

@
m_pRectInfo_O->setFlag(QGraphicsItem::ItemIgnoresTransformations, true);
@

Also note that the lines have a position offset of 0.5 pixel, to be aligned with the image pixel.

@
m_pHLine_O->setPos(0.5, y+0.5); // a line has no width so, +0.5 to paint the line on the pixel and not at the border of two pixels
m_pVLine_O->setPos(x+0.5,0.5);
@

All is working quite well except that when I zoom in, some misalignment can occur between the vertical/horizontal lines and the image pixels (see the image below).

I don't understand this behavior as all the positions specified are in the "scene" world, and the scale come from the view. What am I doing wrong ?

Best regards,

Julien

!https://scontent-a-ams.xx.fbcdn.net/hphotos-ash3/555965_10152062159384903_178021155_n.jpg(image)!

!https://scontent-a-ams.xx.fbcdn.net/hphotos-frc3/1457707_10152062159374903_55035940_n.jpg(misalignment)!

[lugemononeurone]

My subclassed view code is

ImageAnalyzisGraphicsView.h

@

#ifndef IMAGEANALYZISGRAPHICSVIEW_H
#define IMAGEANALYZISGRAPHICSVIEW_H

#include <QGraphicsView>
#include <QGraphicsRectItem>
#include "ImageAnalyzisGraphicsScene.h"

class ImageAnalyzisGraphicsView : public QGraphicsView
{
public:
ImageAnalyzisGraphicsView(QWidget* parent = NULL);

ImageAnalyzisGraphicsScene* mScene_O;
protected:

//Take over the interaction
virtual void wheelEvent(QWheelEvent* event);
};

#endif // IMAGEANALYZISGRAPHICSVIEW_H

@

ImageAnalyzisGraphicsView.cpp

@

#include "ImageAnalyzisGraphicsView.h"

//Qt includes
#include <QGraphicsScene>
#include <QGraphicsTextItem>
#include <QTextStream>
#include <QScrollBar>
#include <QMouseEvent>
#include <QWheelEvent>
#include <QDebug>

/**

• Sets up the subclassed QGraphicsView
  /
  ImageAnalyzisGraphicsView::ImageAnalyzisGraphicsView(QWidget parent) : QGraphicsView(parent) {

  setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing |QPainter::SmoothPixmapTransform);
  setAlignment(Qt::AlignLeft|Qt::AlignTop);

  //Set-up the scene
  mScene_O = new ImageAnalyzisGraphicsScene(this);
  setScene(mScene_O);

  //Use ScrollHand Drag Mode to enable Panning
  setDragMode(ScrollHandDrag);
  // Enable mouse event
  setMouseTracking(true);

  viewport()->setProperty("cursor", QVariant(QCursor(Qt::CrossCursor)));
  }

/**

• Zoom the view in and out.
  /
  void ImageAnalyzisGraphicsView::wheelEvent(QWheelEvent event) {

  setTransformationAnchor(QGraphicsView::AnchorUnderMouse);

  // Scale the view / do the zoom
  double scaleFactor = 1.15;
  if(event->delta() > 0) {
  // Zoom in
  scale(scaleFactor, scaleFactor); //scale all the scene : some QGraphicsItem can ignore the scale transformation by setFlag(QGraphicsItem::ItemIgnoresTransformations, true);

  } else {
  // Zooming out
  scale(1.0 / scaleFactor, 1.0 / scaleFactor); //scale all the scene
  }

  // Don't call superclass handler here
  // as wheel is normally used for moving scrollbars
  }

@

[lugemononeurone]

My subclassed scene is

ImageAnalyzisGraphicsScene.h

@

#ifndef IMAGEANALYZISGRAPHICSSCENE_H
#define IMAGEANALYZISGRAPHICSSCENE_H

#include <QGraphicsScene>
#include <QGraphicsLineItem>
#include <QGraphicsRectItem>
#include <QGraphicsTextItem>
#include <QGraphicsSimpleTextItem>
#include <QPainter>
#include <QEvent>
#include <QHoverEvent>
#include <QDebug>
#include <QGraphicsSceneMouseEvent>
#include <QCursor>

class ImageAnalyzisGraphicsScene : public QGraphicsScene
{
Q_OBJECT

public:
ImageAnalyzisGraphicsScene(QObject * parent = 0);

public:
void LoadMyImage(QString myImg_Str);

protected:

virtual void mouseMoveEvent(QGraphicsSceneMouseEvent * mouseEvent);
//virtual void mousePressEvent(QMouseEvent * event)
//virtual void mouseReleaseEvent(QMouseEvent * event)

private:

bool mInitialized_B;

public:
QGraphicsRectItem *m_pRectInfo_O;
QGraphicsTextItem *m_pPixelInfo_O;

QGraphicsLineItem *m_pVLine_O;
QGraphicsLineItem *m_pHLine_O;

QGraphicsPixmapItem * m_pGraphPixmap_O;

};

#endif // IMAGEANALYZISGRAPHICSSCENE_H

@

[lugemononeurone]

ImageAnalyzisGraphicsScene.cpp

@
#include "ImageAnalyzisGraphicsView.h"

ImageAnalyzisGraphicsScene::ImageAnalyzisGraphicsScene( QObject * parent /= 0/ ):QGraphicsScene(parent)
{
mInitialized_B=false;
setBackgroundBrush( Qt::black );
setSceneRect(QRectF(0,0, 200,200)); //forcing the size of the scene
}

void ImageAnalyzisGraphicsScene::LoadMyImage( QString myImg_Str )
{
if(!mInitialized_B)
{
/*
Create the QGraphicsItems
*/
QPixmap Pixmap_O;
m_pGraphPixmap_O =addPixmap(Pixmap_O);

  m_pRectInfo_O = addRect(QRectF(0, 0,150,30),QPen(Qt::black), QBrush(Qt::black));//, QBrush(Qt::black));//QPen(Qt::black),
  m_pRectInfo_O->setFlag(QGraphicsItem::ItemIgnoresTransformations, true);
  m_pPixelInfo_O = addText("POS(255,255)\nRGBA(255,255,255,255)");  // Initialized with the max text size
  m_pPixelInfo_O->setFlag(QGraphicsItem::ItemIgnoresTransformations, true);
  m_pPixelInfo_O->setDefaultTextColor(QColor(Qt::white));
  m_pPixelInfo_O->setHtml(QString("<div >") + "" + "</div>"); // n'a pas d'effet

  m_pHLine_O= addLine(QLine(0,0,width(),0),  QPen(Qt::white));
  m_pVLine_O= addLine(QLine(0,0,0,height()),  QPen(Qt::white));   

  /*
     lines opacity management : don't work
  */
  double tmp =m_pHLine_O->effectiveOpacity() ;
  //m_pHLine_O->setFlag ( QGraphicsItem::ItemIgnoresParentOpacity); // ne semble pas fonctionner
  m_pHLine_O->setOpacity(0.25);
  m_pVLine_O->setOpacity(0.25);
  tmp =m_pHLine_O->effectiveOpacity() ;

}

QPixmap Pixmap_O;
Pixmap_O.load(myImg_Str);
m_pGraphPixmap_O->setPixmap(Pixmap_O);
setSceneRect(m_pGraphPixmap_O->boundingRect()); //forcing the size of the scene with the image size

//resize the lines
m_pHLine_O->setLine(0,0,width(),0);
m_pVLine_O->setLine(0,0,0,height());
m_pHLine_O->setPos(0, 0);
m_pVLine_O->setPos(0, 0);

m_pPixelInfo_O->setPos(0,0);
m_pGraphPixmap_O->setPos(0,0);

mInitialized_B=true;
}

void ImageAnalyzisGraphicsScene::mouseMoveEvent( QGraphicsSceneMouseEvent * mouseEvent )
{
if(mInitialized_B)
{
int InfoSizeUnitMargin_i=15;
int InfoPosUnitMargin_i=1;

  //get the cursor position in the scene
  int x = mouseEvent->scenePos().x();
  int y = mouseEvent->scenePos().y();


  //check if position is in the scene rectangle : todo
  //if(m_pGraphPixmap_O->contains(mouseEvent->scenePos()))   // tient compte de l'alpha value
  //if(sceneRect().contains(mouseEvent->scenePos()))   
  QRectF rectTemp = m_pGraphPixmap_O->boundingRect();
  QPointF pointftemp=mouseEvent->scenePos();

   //Returns true if the point (x, y) is inside or on the edge of the rectangle; otherwise returns false. ==> 
  if((x>=0)&&(y>=0)&&(x<=width()-1)&&(y<=height()-1))  
  {
     m_pGraphPixmap_O->setPos(0,0);

     //set the lines positions 
     m_pHLine_O->setPos(0.5, y+0.5); // a line has no width so, +0.5 to paint the line on the pixel and not at the border of two pixels
     m_pVLine_O->setPos(x+0.5,0.5);
     
     //pixel access is done with QImage
     QImage Img = m_pGraphPixmap_O->pixmap().toImage();
     QRgb RgbPix=Img.pixel(x,y);

     int a=qAlpha(RgbPix);
     int r=qRed(RgbPix);
     int g=qGreen(RgbPix);
     int b=qBlue(RgbPix);

     /*
        Set the text position.
        Be careful to print the text only inside the existing scene size. 
        Otherwise, the scene size will augment.
     */
     m_pPixelInfo_O->setPlainText(QString("POS(%1,%2)\nRGBA(%3,%4,%5,%6)").arg(x).arg(y).arg(r).arg(g).arg(b).arg(a));  
     //m_pRectInfo_O = addRect(QRectF(0, 0, m_pPixlInfo_O->boundingRect().width()+InfoSizeUnitMargin_i, m_pPixlInfo_O->boundingRect().height()+2),QPen(Qt::black), QBrush(Qt::black));

     qDebug()<< QString(" m_pPixlInfo_O (%1, %2)").arg(m_pPixelInfo_O->boundingRect().width()).arg(m_pPixelInfo_O->boundingRect().height());

     bool InfoAtCursorRight_B=(x+m_pPixelInfo_O->boundingRect().width()+InfoPosUnitMargin_i+InfoSizeUnitMargin_i <width())?true:false;
     bool InfoAtCursorBottom_B=(y+m_pPixelInfo_O->boundingRect().height()+InfoPosUnitMargin_i+InfoSizeUnitMargin_i <height())?true:false;

     if(InfoAtCursorRight_B && InfoAtCursorBottom_B)
     {
        m_pPixelInfo_O->setPos(x+InfoPosUnitMargin_i,y+InfoPosUnitMargin_i);
     }
     else if(!InfoAtCursorRight_B && InfoAtCursorBottom_B)
     {
        m_pPixelInfo_O->setPos(x-m_pPixelInfo_O->boundingRect().width()-(InfoPosUnitMargin_i+InfoSizeUnitMargin_i),y+InfoPosUnitMargin_i);
     }
     else if(InfoAtCursorRight_B && !InfoAtCursorBottom_B)
     {
        m_pPixelInfo_O->setPos(x+InfoPosUnitMargin_i,y-m_pPixelInfo_O->boundingRect().height()-InfoPosUnitMargin_i);
     }
     else
     {
        m_pPixelInfo_O->setPos(x-m_pPixelInfo_O->boundingRect().width()-(InfoPosUnitMargin_i+InfoSizeUnitMargin_i),y-m_pPixelInfo_O->boundingRect().height()-InfoPosUnitMargin_i);
     }

     m_pRectInfo_O->setPos(m_pPixelInfo_O->x(), m_pPixelInfo_O->y());

     qDebug()<< QString(" (%1, %2) :r %3 g %4 b %5 a %6").arg(x).arg(y).arg(r).arg(g).arg(b).arg(a);

     // the image update s  not perfect : there is a problem with lines drawing
     //QPixmap Pixmap_O=m_pGraphPixmap_O->pixmap();
     //m_pGraphPixmap_O->update();//otherwise, there is a problem with lines drawing

     update(); // otherwise, there are some strange behaviors in the painting

  }
  else
  {

  }

}

}

@

[lugemononeurone]

Mistake : lines do not ignore the transformation.