GraphicsView fitInView() very pixelated result when downshrinking

Question

I have searched everywhere and i cannot find any solution after 2 days of trying.

The Problem:

I'm doing an image Viewer with "Fit Image to View" feature. I load a picture of say 3000+ pixels in my GraphicsView (which is a lot smaller ofcourse), scrollbars appear that's good. When i click my btnFitView and executed:

ui->graphicsView->fitInView(scene->sceneRect(),Qt::KeepAspectRatio);

This is down scaling right? After fitInView() all lines are pixelated. It looks like a saw went over the lines on the image.

For example: image of a car has lines, image of a texbook (letters become in very bad quality).

My code sample:

// select file, load image in view
QString strFilePath = QFileDialog::getOpenFileName(
            this,
            tr("Open File"),
             "/home",
            tr("Images (*.png *.jpg)"));

imageObject = new QImage();
imageObject->load(strFilePath);
image = QPixmap::fromImage(*imageObject);

scene = new QGraphicsScene(this);
scene->addPixmap(image);
scene->setSceneRect(image.rect());
ui->graphicsView->setScene(scene);


// on_btnFitView_Clicked() :
ui->graphicsView->fitInView(scene->sceneRect(),Qt::KeepAspectRatio);

Just before fitInView(), sizes are:

qDebug()<<"sceneRect = "<< scene->sceneRect();
qDebug()<<"viewRect = " << ui->graphicsView->rect();

sceneRect =  QRectF(0,0 1000x750)
viewRect =   QRect(0,0 733x415)  

If it is necessary i can upload screenshots of original loaded image and fitted in view ?

Am i doing this right? It seems all examples on the Web work with fitInView for auto-fitting. Should i use some other operations on the pixmap perhaps?

SOLUTION

// LOAD IMAGE
bool ImgViewer::loadImage(const QString &strImagePath)
{
  m_image = new QImage(strImagePath);
  if(m_image->isNull()){
    return false;
  }
  clearView();

  m_pixmap = QPixmap::fromImage(*m_image);     
  m_pixmapItem = m_scene->addPixmap(m_pixmap); 
  m_scene->setSceneRect(m_pixmap.rect());      
  this->centerOn(m_pixmapItem);

  // preserve fitView if active
  if(m_IsFitInView)
      fitView();

  return true;
}

// TOGGLED FUNCTIONS
void ImgViewer::fitView()
{
    if(m_image->isNull())
      return;

    this->resetTransform();
    QPixmap px = m_pixmap; // use local pixmap (not original) otherwise image is blurred after scaling the same image multiple times
    px = px.scaled(QSize(this->width(),this->height()),Qt::KeepAspectRatio,Qt::SmoothTransformation);
    m_pixmapItem->setPixmap(px);
    m_scene->setSceneRect(px.rect());
}
void ImgViewer::originalSize()
{
    if(m_image->isNull())
       return;

    this->resetTransform();
    m_pixmap = m_pixmap.scaled(QSize(m_image.width(),m_image.height()),Qt::KeepAspectRatio,Qt::SmoothTransformation); 
    m_pixmapItem->setPixmap(m_pixmap);
    m_scene->setSceneRect(m_pixmap.rect());
    this->centerOn(m_pixmapItem); //ensure item is centered in the view.
}

On downshrink this produces good quality. Here are some stats after calling these 2 functions:

//    "originalSize()"  : IMAGE SIZE =   (1152, 2048)
//    "originalSize()"  : PIXMAP SIZE =  (1152, 2048)
//    "originalSize()"  : VIEW SIZE =    (698, 499)
//    "originalSize()"  : SCENE SIZE =   (1152, 2048)

//    "fitView()"   : IMAGE SIZE =   (1152, 2048)
//    "fitView()"   : PIXMAP SIZE =  (1152, 2048)
//    "fitView()"   : VIEW SIZE =    (698, 499) 
//    "fitView()"   : SCENE SIZE =   (280, 499)

There is a problem now, after call to fitView() look the size of scene? Much smaller.

And if fitView() is activated, and I now scale the image on wheelEvent (zoomIn/zoomOut), with the views scale function: scale(factor,factor); ..produces terrible result. This doesn't happen with originalSize() where scene size is equal to image size.

Answer 1

Think of the view as a window into the scene.

Moving the view large amounts, either zooming in or out, will likely create images that don't look great. Rather than the image being scaled as you would expect, the view is just moving away from the scene and doing its best to render the image, but the image has not been scaled, just transformed in the scene.

Rather than using QGraphicsView::fitInView, keep the main image in memory and create a scaled version of the image with QPixamp::scaled, each time FitInView is selected, or the user zooms in / out. Then set this QPixmap on the QGraphicsPixmapItem with setPixmap.

You may also want to think about dropping the scroll bars and allowing the user to drag the image around the screen, which provides a better user interface, in my opinion; though of-course it depends on your requirements.