QLabel shows image inside QScrollArea

Question

I have successfully implemented an image viewer (for DICOM) in Qt. I see an image, and I can zoom in and out.

Now I want to see scroll bars if the image is too large to show when I zoom in.

I used the user interface. I posted a QScrollArea . Inside QLabel . VerticalScrollBarPolicy - ScrollBarAsNeeded. ScrollBarPolicy - ScrollBarAsNeeded

The problem is that it does not work. I am zooming in but the scrollbar is not showing.

Second attempt: using layout inside QScrollArea.

So now there is a QWidget between QScrollArea and QLabel: horizontal layout. Opened the same image, now I see a vertical scroll bar on the right. The image is stretched from left to right. When I enlarge the image, it gets the correct proportion.

BUT ... I zoom out and the scrollbar is the same even if I see the whole image. A horizontal scrollbar never appears.

Resizing QLabel does not seem to be affected. But if I resize QScrollArea (resize the main window), a horizontal scrollbar will appear.

I checked several numbers:

In QScrollArea

• Its size changes: the width is below 599 (why is this number? I do not see it anywhere) a horizontal panel appears.
• sizeHint () always returns the same values: 33x41

In QLabel

• Resizing, but it does not affect.
• sizeHint () always returns the same values: 560x1558

Here is the XML code from the user interface designer:

<widget class="QWidget" name="centralWidget"> <property name="autoFillBackground"> <bool>false</bool> </property> <layout class="QVBoxLayout" name="verticalLayout"> <property name="margin"> <number>0</number> </property> <item> <widget class="QScrollArea" name="scrollArea"> <property name="widgetResizable"> <bool>true</bool> </property> <property name="alignment"> <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop</set> </property> <widget class="QWidget" name="scrollAreaWidgetContents"> <property name="geometry"> <rect> <x>0</x> <y>0</y> <width>637</width> <height>649</height> </rect> </property> <layout class="QHBoxLayout" name="horizontalLayout_2"> <item> <widget class="QLabel" name="miImagen"> <property name="sizePolicy"> <sizepolicy hsizetype="Preferred" vsizetype="Preferred"> <horstretch>0</horstretch> <verstretch>0</verstretch> </sizepolicy> </property> <property name="autoFillBackground"> <bool>true</bool> </property> <property name="scaledContents"> <bool>true</bool> </property> <property name="alignment"> <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop</set> </property> </widget> </item> </layout> </widget> </widget> </item> </layout> </widget> 

What am I missing? Thanks.

Answer 1

I know this is an old post, but if you or anyone else has a problem, this can help set QScrollArea::widgetResizable to false.

At least when I tried a similar thing, my vertical scrollbar was always turned off (although I set the size of the scrollable widget so that the height was larger than the viewport) until I set it to false.

When this is true, I think it updates the size of the scrollable widget, so scrollbars are not needed. This allows you to do what it does in the example that I am assuming and implement the stretch function. (in fact, what I'm trying to do matches the width, only with a vertical scrollbar)

Answer 2

Have you tried following the Qt scroll pane example ? If you use QLabel to display your image, then using QScrollArea is pretty much the standard way to achieve what you want. You use it like this (from the example):

  imageLabel = new QLabel; imageLabel->setBackgroundRole(QPalette::Base); imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored); imageLabel->setScaledContents(true); scrollArea = new QScrollArea; scrollArea->setBackgroundRole(QPalette::Dark); scrollArea->setWidget(imageLabel); setCentralWidget(scrollArea); 

Then scaling is performed as follows:

  void ImageViewer::zoomIn() { scaleImage(1.25); } void ImageViewer::zoomOut() { scaleImage(0.8); } void ImageViewer::scaleImage(double factor) { Q_ASSERT(imageLabel->pixmap()); scaleFactor *= factor; imageLabel->resize(scaleFactor * imageLabel->pixmap()->size()); adjustScrollBar(scrollArea->horizontalScrollBar(), factor); adjustScrollBar(scrollArea->verticalScrollBar(), factor); zoomInAct->setEnabled(scaleFactor < 3.0); zoomOutAct->setEnabled(scaleFactor > 0.333); } void ImageViewer::adjustScrollBar(QScrollBar *scrollBar, double factor) { scrollBar->setValue(int(factor * scrollBar->value() + ((factor - 1) * scrollBar->pageStep()/2))); } 

You can, of course, better understand what is happening by looking at an example, but its essence. I think maybe the adjustScrollBar () function might be most useful to you.

Your last comment on the original post is correct, QScrollArea doesn’t magically notice the resizing of its contents. See how this example uses adjustScrollBar () to compensate for this fact.