Qt – QLabel showing an image inside a QScrollArea

imageqlabelqscrollareaqtscrollbar

I've successfully implemented a Image Viewer (for DICOM) in Qt.
I can see the image and I can zoom in and out correctly.

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

I've used the UI. I placed a QScrollArea. Inside, the QLabel.
The verticalScrollBarPolicy is ScrollBarAsNeeded.
The horizontalScrollBarPolicy is ScrollBarAsNeeded.

The problem is: it doesn't work. I zoom in, but no scrollbar appears.

Second try: using a layout inside the QScrollArea.

So now there's a QWidget between the QScrollArea and the QLabel: a horizontal layout.
Opened the same image, now I can see a vertical scroll bar on the right. The image is streched from left to right. When I zoom the image gets its correct proportion.

BUT… I zoom out and the scroll bar is the same, even if I can see the whole image. The horizontal scroll bar never appears.

Resizing the QLabel doesn't seem to affect. But if I resize the QScrollArea (resizing the main window) the horizontal scroll bar appears.

I've been checking some numbers:

In the QScrollArea

  • Its size changes: below 599 width (why this number? I can't see it anywhere) the horizontal bar appears.
  • sizeHint() always returns the same values: 33×41

In the QLabel

  • The dimensions change, but that doesn't affect.
  • sizeHint() always returns the same values: 560×1558

Here is the code in XML from the UI 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? Thank you.

Best Solution

I know this is an old post - but in case you or anyone is still having a problem, it might help to set QScrollArea::widgetResizable to false.

At least, when I tried a similar thing, my vertical scrollbar was always disabled (even though I set the size of the scrollable widget to have a height larger than the viewport) until I set this to false.

When it's true, I think it updates the size of the scrollable widget, thus the scrollbars should not be needed. This allows you to do what it does in the example I guess, and implement a stretch-to-fit function. (actually what I'm trying to do is fit to width, with just a vertical scroll bar)

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