Qt resizing a QLabel containing a QPixmap while keeping its aspect ratio

Question

I use a QLabel to display the content of a bigger  dynamically changing QPixmap to the user  It would be nice to make this label smaller larger depending on the space available  The screen size is not always as big as the QPixmap   How can I modify the QSizePolicy and sizeHint   of the QLabel to resize the QPixmap while keeping the aspect ratio of the original QPixmap   I can t modify sizeHint   of the QLabel  setting the minimumSize   to zero does not help  Setting hasScaledContents   on the QLabel allows growing  but breaks the aspect ratio thingy     Subclassing QLabel did help  but this solution adds too much code for just a simple problem     Any smart hints how to accomplish this without subclassing

User · Answer

I tried using phyatt s AspectRatioPixmapLabel class  but experienced a few problems    Sometimes my app entered an infinite loop of resize events  I traced this back to the call of QLabel  setPixmap      inside the resizeEvent method  because QLabel actually calls updateGeometry inside setPixmap  which may trigger resize events    heightForWidth seemed to be ignored by the containing widget  a QScrollArea in my case  until I started setting a size policy for the label  explicitly calling policy setHeightForWidth true  I want the label to never grow more than the original pixmap size QLabel s implementation of minimumSizeHint   does some magic for labels containing text  but always resets the size policy to the default one  so I had to overwrite it   That said  here is my solution  I found that I could just use setScaledContents true  and let QLabel handle the resizing  Of course  this depends on the containing widget   layout honoring the heightForWidth   aspectratiopixmaplabel h   ifndef ASPECTRATIOPIXMAPLABEL H  define ASPECTRATIOPIXMAPLABEL H   include  lt QLabel gt   include  lt QPixmap gt   class AspectRatioPixmapLabel   public QLabel       Q OBJECT public      explicit AspectRatioPixmapLabel const QPixmap  amp pixmap  QWidget  parent   0       virtual int heightForWidth int width  const      virtual bool hasHeightForWidth     return true        virtual QSize sizeHint   const   return pixmap  - gt size          virtual QSize minimumSizeHint   const   return QSize 0  0          endif    ASPECTRATIOPIXMAPLABEL H   aspectratiopixmaplabel cpp   include  aspectratiopixmaplabel h   AspectRatioPixmapLabel  AspectRatioPixmapLabel const QPixmap  amp pixmap  QWidget  parent        QLabel parent        QLabel  setPixmap pixmap       setScaledContents true       QSizePolicy policy QSizePolicy  Maximum  QSizePolicy  Maximum       policy setHeightForWidth true       this- gt setSizePolicy policy      int AspectRatioPixmapLabel  heightForWidth int width  const       if  width  gt  pixmap  - gt width              return pixmap  - gt height          else           return   qreal pixmap  - gt height   width  pixmap  - gt width

User · Answer

Adapted from Timmmm to PYQT5  from PyQt5 QtGui import QPixmap from PyQt5 QtGui import QResizeEvent from PyQt5 QtWidgets import QLabel   class Label QLabel        def   init   self           super Label  self    init             self pixmap width  int   1         self pixmapHeight  int   1      def setPixmap self  pm  QPixmap  - gt  None          self pixmap width   pm width           self pixmapHeight   pm height            self updateMargins           super Label  self  setPixmap pm       def resizeEvent self  a0  QResizeEvent  - gt  None          self updateMargins           super Label  self  resizeEvent a0       def updateMargins self           if self pixmap   is None              return         pixmapWidth   self pixmap   width           pixmapHeight   self pixmap   height           if pixmapWidth  lt   0 or pixmapHeight  lt   0              return         w  h   self width    self height           if w  lt   0 or h  lt   0              return          if w   pixmapHeight  gt  h   pixmapWidth              m   int  w -  pixmapWidth   h   pixmapHeight     2              self setContentsMargins m  0  m  0          else              m   int  h -  pixmapHeight   w   pixmapWidth     2              self setContentsMargins 0  m  0  m

User · Answer

I have polished this missing subclass of QLabel   It is awesome and works well   aspectratiopixmaplabel h   ifndef ASPECTRATIOPIXMAPLABEL H  define ASPECTRATIOPIXMAPLABEL H   include  lt QLabel gt   include  lt QPixmap gt   include  lt QResizeEvent gt   class AspectRatioPixmapLabel   public QLabel       Q OBJECT public      explicit AspectRatioPixmapLabel QWidget  parent   0       virtual int heightForWidth  int width   const      virtual QSize sizeHint   const      QPixmap scaledPixmap   const  public slots      void setPixmap   const QPixmap  amp         void resizeEvent QResizeEvent     private      QPixmap pix       endif    ASPECTRATIOPIXMAPLABEL H   aspectratiopixmaplabel cpp   include  aspectratiopixmaplabel h     include  lt QDebug gt   AspectRatioPixmapLabel  AspectRatioPixmapLabel QWidget  parent        QLabel parent        this- gt setMinimumSize 1 1       setScaledContents false      void AspectRatioPixmapLabel  setPixmap   const QPixmap  amp  p        pix   p      QLabel  setPixmap scaledPixmap        int AspectRatioPixmapLabel  heightForWidth  int width   const       return pix isNull     this- gt height       qreal pix height   width  pix width       QSize AspectRatioPixmapLabel  sizeHint   const       int w   this- gt width        return QSize  w  heightForWidth w        QPixmap AspectRatioPixmapLabel  scaledPixmap   const       return pix scaled this- gt size    Qt  KeepAspectRatio  Qt  SmoothTransformation      void AspectRatioPixmapLabel  resizeEvent QResizeEvent   e        if  pix isNull            QLabel  setPixmap scaledPixmap         Hope that helps   Updated resizeEvent  per  dmzl s answer

User · Answer

The Qt documentations has an Image Viewer example which demonstrates handling resizing images inside a QLabel  The basic idea is to use QScrollArea as a container for the QLabel and if needed use label setScaledContents bool  and scrollarea setWidgetResizable bool  to fill available space and or ensure QLabel inside is resizable  Additionally  to resize QLabel while honoring aspect ratio use  label setPixmap pixmap scaled width  height  Qt  KeepAspectRatio  Qt  FastTransformation     The width and height can be set based on scrollarea width   and scrollarea height    In this way there is no need to subclass QLabel

User · Answer

In order to change the label size you can select an appropriate size policy for the label like expanding or minimum expanding    You can scale the pixmap by keeping its aspect ratio every time it changes   QPixmap p     load pixmap    get label dimensions int w   label- gt width    int h   label- gt height        set a scaled pixmap to a w x h window keeping its aspect ratio  label- gt setPixmap p scaled w h Qt  KeepAspectRatio      There are two places where you should add this code    When the pixmap is updated In the resizeEvent of the widget that contains the label

User · Answer

I just use contentsMargin to fix the aspect ratio    pragma once   include  lt QLabel gt   class AspectRatioLabel   public QLabel   public      explicit AspectRatioLabel QWidget  parent   nullptr  Qt  WindowFlags f   Qt  WindowFlags          AspectRatioLabel     public slots      void setPixmap const QPixmap amp  pm    protected      void resizeEvent QResizeEvent  event  override   private      void updateMargins         int pixmapWidth   0      int pixmapHeight   0          include  AspectRatioLabel h   AspectRatioLabel  AspectRatioLabel QWidget  parent  Qt  WindowFlags f    QLabel parent  f       AspectRatioLabel   AspectRatioLabel        void AspectRatioLabel  setPixmap const QPixmap amp  pm        pixmapWidth   pm width        pixmapHeight   pm height         updateMargins        QLabel  setPixmap pm      void AspectRatioLabel  resizeEvent QResizeEvent  event        updateMargins        QLabel  resizeEvent event      void AspectRatioLabel  updateMargins         if  pixmapWidth  lt   0    pixmapHeight  lt   0          return       int w   this- gt width        int h   this- gt height         if  w  lt   0    h  lt   0          return       if  w   pixmapHeight  gt  h   pixmapWidth                int m    w -  pixmapWidth   h   pixmapHeight     2          setContentsMargins m  0  m  0             else               int m    h -  pixmapHeight   w   pixmapWidth     2          setContentsMargins 0  m  0  m             Works perfectly for me so far  You re welcome

[c++] Qt: resizing a QLabel containing a QPixmap while keeping its aspect ratio

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