Moving a QPixmap to the corner of a QGraphicsScene

Question

I have a QGraphicsView and a QGraphicsScene connected like this:

graphicsScene->setSceneRect(this->graphicsView->rect());
graphicsView->setScene(this->Scene);

Then I load an image and add it to the scene:

  QPixmap pixmap;
  pixmap.load(fileName);

  pixmap = pixmap.scaled(this->graphicsView->size());
  QGraphicsPixmapItem* item = this->Scene->addPixmap(pixmap);

Now, as described in the documentation, the image corner is at (0,0), which is not the corner of the graphicsScene. I know I can position the resulting pixmap by doing:

item->setPos(this->Scene->sceneRect().x(), this->Scene->sceneRect().y());

However, I can't seem to make sense of the coordinates of the rect's of the scene or the view. Can anyone explain how I would move the pixmap to the corner of the scene/view?

Thanks,

David

EDIT: Here is the full form constructor. The QGraphicsView was created in Qt Designer and is inside of a GridLayout:

Form::Form(QWidget *parent)
    : QWidget(parent)
{
  setupUi(this);

  QGraphicsScene* scene = new QGraphicsScene;
  scene->setSceneRect(this->graphicsView->rect());
  this->graphicsView->setScene(scene);

  QPixmap pixmap;
  pixmap.load("image.png");
  pixmap = pixmap.scaled(this->graphicsView->size());

  scene->addPixmap(pixmap);

}

I also tried this:

  QGraphicsScene* scene = new QGraphicsScene;
  this->graphicsView->setScene(scene);

  QPixmap pixmap;
  pixmap.load("/home/doriad/glasses.jpg");

  QGraphicsPixmapItem * item = scene->addPixmap(pixmap);
  this->graphicsView->fitInView (item);

but the image appears tiny, rather than filling up the view like I would expect. Can anyone explain this?

The full project and image are available here: daviddoria.com/Uploads/qt/QPixmapPosition

Answer 1

Don't worry about scaling the pixmap yourself or even translating it, let the view do it for you.

Use graphicsView->fitInView(pixmap); but you should read the documentation for :

Qt's Graphics View Framework

void QGraphicsView::setSceneRect (QRectF )

void QGraphicsView::translate ( qreal dx, qreal dy )

void QGraphicsView::fitInView ( const QGraphicsItem * item, ... )

The way that QGraphicsScene and QGraphicsView interact is that you can have a single scene with at least one or more views.

A good example I like to think of is a zoomed in view of part of a map with a mini view of the entire map in the corner. There are two views, one of part of the map and one of the entire map, with one scene, the map itself.

So you put items in your scene and all the items in the scene are drawn relative in size to each other. The "scene rect" of your view, by default, scales to fit the items in the view until one unit in the scene is one pixel in the view or until it needs to zoom out to fit all the items in your scene.

If you call fitInView(someItem) it should scale your view of the scene so that the item specified fills it up and translates the view so that it is centered. If you need to translate or scale it more use the translate or scale functions in QGraphicsView.

When you are jumping between coordinate systems of your scene and view with your QRect's or QPoint's, use the helper functions: mapToScene and mapFromScene from QGraphicsView.

Answer 2

Try this:

QGraphicsScene* scene = new QGraphicsScene;
scene->setSceneRect(graphicsView->sceneRect());
QPixmap pixmap;
pixmap.load("/home/doriad/glasses.jpg");
pixmap=pixmap.scaledToWidth(this->graphicsView->width());
QGraphicsPixmapItem * item = scene->addPixmap(pixmap);
graphicsView->setScene(scene);

By default, the pixmap will be at (0,0) in the scene, and the scene will be at (0,0) in the view. QWidgets are sized by pixels. If you had a QGraphicsView the size of the screen and the resolution is 1440 x 900, you can position objects in that view from (0,0) or the top left corner of the screen, to (1440,900) the bottom right corner of the screen. Most QGraphicsItems are placed with reference to their top left corner. So placing a pixmap at (0,0) aligns the top left corner of the pixmap with the top left corner of the scene it's placed in. If your pixmap 'hangs off the bottom' of your view, try using:

pixmap=pixmap.scaledToHeight(this->graphicsView->height());

If you use the function:

this->graphicsView->fitInView (item);

The graphicsView will only scroll to the point where your item fits in the view.

Answer 3

I downloaded David's code and ran it two ways. Click the links to see the results.

1) with pixmap=pixmap.scaledToHeight(this->graphicsView->height());

2) with pixmap=pixmap.scaledToHeight(200);

I don't know enough to explain why this is happening, but I thought it would be a useful data point.

Answer 4

It turns out the problem was that the GraphicsView was in a layout. In my example, the resizing of the image was done in the Form constructor. Apparently this is before the layout takes its shape? I moved the code to a pushButton and when I click it the image is sized how I would expect.

I got the image to stay sized to the GraphicsView in the Layout by subclassing QGraphicsView and reimplementing :

class CustomGraphicsView: public QGraphicsView
{
  Q_OBJECT
    {
          void resizeEvent ( QResizeEvent * event )
          {
            emit resized();
          }

        signals:
          void resized();
    }

Then I connect this resized() signal to a slot that simply calls this->View->fitInView (this->ImageToTraceItem);

David