I know that this is a very specific C++ and Qt related question, but maybe someone can help me, anyway ...
See the code below: I want to display an image within a scroll area. The view port of the scroll area shall have a defined initial size. That means, if the image's size is bigger than the initial size of the view port, scroll bars will be visible, otherwise not.
// create label for displaying an image
QImage image( ":/test.png" );
QLabel *label = new QLabel( this );
label->setPixmap( image.toPixmap() );
// put label into scroll area
QScollArea *area = new QScrollArea( this );
area->setWidget( label );
// set the initial size of the view port
// NOTE: This is what I'd like to do, but this method does not exist :(
area->setViewPortSize( QSize( 300, 300 ) );
It shall be possible to resize the whole application so that the view port will get another size than the initial one.
Unfortunatelly I was not able to find out, how to set the size of the view port. Qt's layout mechanism seems to set a default size for the view port, but up to now I was not able to change it. Setting a new size with
area->setMinimumSize( QSize( 300, 300 ) );
will actually set the demanded size, but then the scroll area looses the ability to get resized to a size smaller than 300x300.
Any ideas?
I think that you are looking at the problem the wrong way. The QScrollArea is just a widget that you put in a frame or QMainWindow. The size of the widget is controlled by the layout of the widget that contains it.
Take a look at this example from Trolltech: Image Viewer Example
You can try:
class MyScrollArea : public QScrollArea
{
virtual QSize sizeHint() const { return QSize( 300, 300 ); }
};
// create label for displaying an image
QImage image( ":/test.png" );
Label *label = new QLabel;
label->setPixmap( image.toPixmap() );
// put label into scroll area
QScollArea *area = new MyScrollArea( this );
area->setWidget( label );
However layout and Qt is amazingly Voodoo. It is IMO its least functional part.
if that doesn't work, try calling QWidget::resize() on various widgets.
Is the scroll area the top level widget? If so, simply call
area->resize(300,300);
If it's inside a hierarchy you need to resize the toplevel appropriately (complex), or set the minimumSize of the area. You could also try to experiment with the LayoutPolicy - assuming the sizeHint is QSize(300,300) you can give it the appropriate size policy according to what's defined in https://doc.qt.io/qt-5/qsizepolicy.html#Policy-enum
I don't think you can do exactly that very easily, which is (if I'm reading correctly), size the widget so that the internal area is 300x300. You might be able to fudge it, however, since a scroll area is a type of frame, which inherits from QWidget. This means you could just call area->resize( 300 + fudge, 300 + fudge ), where your fudge values account for the extra bit taken up by the frame's drawing.
I'm not sure this would work in a dynamically resizable dialog, however. I haven't ever done anything quite like this.
If you're trying to display an image inside a scroll area, your best bet isn't going with a label.
You should try using a QGraphicsView/QGraphicsScene/QGraphicPixmapItem (instead of the Scroll Area and label). The performance is far better when displaying images. The scroll area and label will re-draw the image very poorly as you move around using the scroll bars.
For example, you have a ".ui" file with a QGraphicsView on the gui called "qgvImageView" and a QImage called "image"...
QGraphicsScene *scene = new QGraphicsScene(qgvImageView);
QPixmap pixTmp(QPixmap::fromImage(image));
QGraphicsPixmapItem * ppixItem = scene->addPixmap( pixTmp );
ppixItem->setPos(0,0);
Check out the QT Documentation. BTW: This was introduced in Qt 4.2
I'm not sure if this will specifically fix the problem, but there is a chance that the QGraphicsView will react better to what you're trying to do.
How about using
area->setGeometry(int x, int y, int w, int h);
Related
This sounds like a question that's been asked here before, but I can't seem to find what I"m looking for.
So, I've got a vertical layout containing a QGraphicsView. I'm trying to implement the ability for the user to resize the view given any size they want. Like in MSPaint, you can choose an exact size for the canvas.
When I run the program, this works perfectly the first time, and on the vertical layout, I get a QGraphicsView of the exact size I specified, however, when the function resizeCanvas is called at runtime, I don't get the same effect.
Instead, I the QGraphicsView will either look like it's the same size if the specified values were greater than the existing ones (Like setting an 800x600 QGraphicsScene being changed to a 400x300),
or if the opposite is true (a 400x300 QGraphicsScene being changed to an 800x600), I'll get scrollbars rather than a bigger QGraphicsView widget.
canvasScene = new QGraphicsScene(this);
resizeCanvas(canvasWidth, canvasHeight);
void MainWindow::resizeCanvas(int x, int y) {
canvasWidth = x;
canvasHeight = y;
canvasScene->setSceneRect(100, 100, canvasWidth, canvasHeight);
updateCanvas();
}
void MainWindow::updateCanvas() {
ui->canvas->setScene(canvasScene);
}
How can I make it resize the actual widget?
My solution was simply to set the geometry of the QGraphicsView to the new values given
I have the following, quite simple setup:
In a QWidget that is displayed as a window (no parent), there is a single QVBoxLayout.
In that QVBoxLayout, there is a single custom QGraphicsView.
The size policy of that custom QGraphicsView is set to Preferred/Preferred and setHeightForWidth set to true (and overwritten in the custom class) - I want to preserve aspect ratio.
The whole constructor of the widget here:
graphicsView = new CustomGraphicsView();
QVBoxLayout *layout = new QVBoxLayout();
layout->setMargin(0);
QSizePolicy sp(QSizePolicy::Preferred, QSizePolicy::Preferred);
sp.setHeightForWidth(true);
graphicsView->setSizePolicy(sp);
layout->addWidget(graphicsView);
setLayout(layout);
The setup works, and the aspect ratio is maintained when dragging the width of the window bigger (the height grows with it).
But as soon as I drag the window width smaller, the aspect ratio of the graphics view is maintained, but the height of the window itself won't shrink. The result being a small graphics view with lots of space above and below it that shouldn't be there.
Investigating it further I was trying to find out where things break, so I overloaded all the sizeHint(), minimumSize(), minimumHeight(), etc. functions of my custom graphics view.
Just to discover that not a single one of them is ever being called while I am manually resizing the window. The only thing called as expected is heightForWidth - which returns my calculated value - but its return value is not applied when the window is shrinking.
So, not only do I not know why the layout won't shrink on its own despite having the Preferred vertical policy (which explicity says "The widget can be expanded, but there is no advantage to it being larger than sizeHint()").
I also don't know how the layout gets the size from the widget to begin with. I assumed sizeHint() since all of the documentation permanently refers to it, but that is obviously wrong in this case.
What I already tried is to set the vertical policy to every possible value. None of them would cause the window to grow and shrink as it is supposed to.
My current workaround:
I have added the resizeEvent(...) function to the widget and inside that, I manually resize() the whole widget if its height exceeds the value returned by the heightForWidth() function of the custom graphics widget.
Okay as far as workarounds go, but it leads to heavy flickering (as usual when resizing inside a resizeEvent).
Any ideas on either problem?
I need to create a scrollable, owner-drawn widget that behaves a lot like QPlainTextEdit with word-wrapped text, in the sense that the height depends on the width - as the content width decreases, the content height increases.
What is the best approach to do it? I was thinking about putting my QWidget-derived class inside a QScrollArea, but QPlainTextEdit is derived from QScrollArea instead, should I go that route?
Also, I want to paint only the visible area in paintEvent(), it would be wasteful otherwise.
Right now I'm examining the code of QPlainTextEdit, but it is rather complex and not easy to read, so if anyone knows of a code example that's simpler on the web, you can give me a link, it would help a lot.
I'll post the solution I came up with. It's not the best, but it mostly works.
I did not derive from QAbstractScrollArea in the end, instead I simply embedded my widget in a QScrollArea with a vertical layout, which worked well-enough.
I implemented resizeEvent() (I saw this from QPlainTextEdit implementation), and each time the width changes, I recalculate the height, and I set the widget's minimum height to that. I set the minimum height because of how the layout works.
void MyWidget::resizeEvent(QResizeEvent *e)
{
// If the widget's width has changed, we recalculate the new height
// of our widget.
if (e->size().width() == e->oldSize().width()) {
return;
}
setMinimumHeight(calculateHeightFromWidth(e->size().width()));
}
For drawing only the visible area see Get visible area of QPainter
Is there a way to define an initial size for a child window widget (inherited from QMdiSubWindow)? I don't want to necessary limit the minimum size or prevent it from being resized, but just to show the window at the first time with a given size.
I've tried to reimplement sizeHint and to define different size policies, but even with those changes the autoAjust call seem to make the window very small (size 200 x 200) when it is first displayed. The window contains a widget with this hierarchy: QVBoxLayout -> QScrollArea -> QLabel. The QLabel is used to show an image with the size 512 x 512.
EDIT: Correct the class to QMdiSubWindow.
you can use resize(int w, int h).
It will not set the maximum and minimum size.
It will just change the initial size of the child window.
Well actually, what it really does is: it changes the "current" size of the child window. But the first current size is the "initial" size. So basically its the same.
To my knowledge, there's no such thing in Qt as a QSubWindow.
However, a call to setGeometry on a QWidget will set its size.
After much searching for an answer, and experimenting, I found this to work for me.
child->parentWidget()->resize(900, 700);
child->parentWidget()->updateGeometry();
child->show();
// child->showMaximized();
You can use showMaximized() in place of show() and the (900, 700) will still be used if the window is later changed to normal.
Ok, so I'd like to display an image with qt where the image resizes with the browser and maintains the aspect ratio while also remaining centered in the window. I can get the resizing with aspect ratio to work correctly, but when I align it with Qt::AlignCenter, the qwidget no longer resizes (remains a fixed size). So basically, I can get either option to work but not together.
A good example of what I'm trying to do would be the imshow() function in matlab. This resizes the image while maintaining the aspect ratio and also centering the image in the window. The code I have is soemthing like this:
void roilayout::resizeEvent(QResizeEvent *event)
{
QSize p(roiview->refimage->size());
p.scale(roiview->view->size(), Qt::KeepAspectRatio);
roiview->view->resize(p);
}
and in the constructor:
roiview = new roiwindow;
roiview->view->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
roiview->view->setCursor(Qt::CrossCursor);
roiview->view->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
roiview->view->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
QHBoxLayout *layoutContainer = new QHBoxLayout;
layoutContainer->addWidget(roiview->view);
setLayout(layoutContainer);
I searched google and couldnt find anything. Also asked a similar question a little while back but from the answers it appears I didn't ask the question clearly enough. Thanks.
A couple of things: First it would be helpful to know what type of control "view" is.
Also, I don't think you should need to resize the child control "view" (whatever type it is) within the parent's resizeEvent() callback.
A better solution might be to set the sizeHint policy on the child widget to automatically expand.