Image resolution lowered on setting background of a QLabel through QPixmap

[Cupidvogel]

Hi, I have a QLabel where I need to fit an image. If I load the image using painter and then update the label, it comes up fine. However, I will need to zoom in/out the image (that I try to do by calling resize() on the QLabel) on click (zoom the image inline, not load a zoomed version of the same image!), which this method is not allowing me to do so. So I decided to do it through QPixmap. This is my code (kind of copied from Qt official ImageViewer example here:

#include <QtWidgets>    
#include "imageviewer.h"
 
ImageViewer::ImageViewer()
{
    imageLabel = new QLabel;
    imageLabel->setBackgroundRole(QPalette::Base);
    imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
    imageLabel->setScaledContents(true);
    setCentralWidget(imageLabel);
 
    createActions();
    createMenus();
 
    resize(570,357);
}
 
 
bool ImageViewer::loadFile(const QString &fileName)
{
    QImageReader reader(fileName);
    const QImage image = reader.read();
    if (image.isNull()) {
        QMessageBox::information(this, QGuiApplication::applicationDisplayName(),
                                 tr("Cannot load %1.").arg(QDir::toNativeSeparators(fileName)));
        setWindowFilePath(QString());
        imageLabel->setPixmap(QPixmap());
        imageLabel->adjustSize();
        return false;
    }
    imageLabel->setPixmap(QPixmap::fromImage(image).scaled(size(),Qt::IgnoreAspectRatio,Qt::SmoothTransformation));
    return true;
}
 
 
 
void ImageViewer::open()
{
    QStringList mimeTypeFilters;
    foreach (const QByteArray &mimeTypeName, QImageReader::supportedMimeTypes())
        mimeTypeFilters.append(mimeTypeName);
    mimeTypeFilters.sort();
    const QStringList picturesLocations = QStandardPaths::standardLocations(QStandardPaths::PicturesLocation);
    QFileDialog dialog(this, tr("Open File"),
                       picturesLocations.isEmpty() ? QDir::currentPath() : picturesLocations.last());
    dialog.setAcceptMode(QFileDialog::AcceptOpen);
    dialog.setMimeTypeFilters(mimeTypeFilters);
    dialog.selectMimeTypeFilter("image/jpeg");
 
    while (dialog.exec() == QDialog::Accepted && !loadFile(dialog.selectedFiles().first())) {}
}
 
void ImageViewer::createActions()
{
    openAct = new QAction(tr("&Open..."), this);
    openAct->setShortcut(tr("Ctrl+O"));
    connect(openAct, SIGNAL(triggered()), this, SLOT(open()));
 
}
 
void ImageViewer::createMenus()
{
    fileMenu = new QMenu(tr("&File"), this);
    fileMenu->addAction(openAct);
 
    menuBar()->addMenu(fileMenu);
 
}

#include <QtWidgets>    
#include "imageviewer.h"

ImageViewer::ImageViewer()
{
    imageLabel = new QLabel;
    imageLabel->setBackgroundRole(QPalette::Base);
    imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
    imageLabel->setScaledContents(true);
    setCentralWidget(imageLabel);

    createActions();
    createMenus();

    resize(570,357);
}


bool ImageViewer::loadFile(const QString &fileName)
{
    QImageReader reader(fileName);
    const QImage image = reader.read();
    if (image.isNull()) {
        QMessageBox::information(this, QGuiApplication::applicationDisplayName(),
                                 tr("Cannot load %1.").arg(QDir::toNativeSeparators(fileName)));
        setWindowFilePath(QString());
        imageLabel->setPixmap(QPixmap());
        imageLabel->adjustSize();
        return false;
    }
    imageLabel->setPixmap(QPixmap::fromImage(image).scaled(size(),Qt::IgnoreAspectRatio,Qt::SmoothTransformation));
    return true;
}



void ImageViewer::open()
{
    QStringList mimeTypeFilters;
    foreach (const QByteArray &mimeTypeName, QImageReader::supportedMimeTypes())
        mimeTypeFilters.append(mimeTypeName);
    mimeTypeFilters.sort();
    const QStringList picturesLocations = QStandardPaths::standardLocations(QStandardPaths::PicturesLocation);
    QFileDialog dialog(this, tr("Open File"),
                       picturesLocations.isEmpty() ? QDir::currentPath() : picturesLocations.last());
    dialog.setAcceptMode(QFileDialog::AcceptOpen);
    dialog.setMimeTypeFilters(mimeTypeFilters);
    dialog.selectMimeTypeFilter("image/jpeg");

    while (dialog.exec() == QDialog::Accepted && !loadFile(dialog.selectedFiles().first())) {}
}

void ImageViewer::createActions()
{
    openAct = new QAction(tr("&Open..."), this);
    openAct->setShortcut(tr("Ctrl+O"));
    connect(openAct, SIGNAL(triggered()), this, SLOT(open()));

}

void ImageViewer::createMenus()
{
    fileMenu = new QMenu(tr("&File"), this);
    fileMenu->addAction(openAct);

    menuBar()->addMenu(fileMenu);

}

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Header file:

#ifndef IMAGEVIEWER_H
#define IMAGEVIEWER_H
 
#include <QMainWindow>
 
class QAction;
class QLabel;
class QMenu;
class QScrollArea;
class QScrollBar;
 
class ImageViewer : public QMainWindow
{
    Q_OBJECT
 
public:
    ImageViewer();
    bool loadFile(const QString &);
 
private slots:
    void open();
 
private:
    void createActions();
    void createMenus();
 
    QLabel *imageLabel;
    QAction *openAct;
    QMenu *fileMenu;
};
 
#endif

#ifndef IMAGEVIEWER_H
#define IMAGEVIEWER_H

#include <QMainWindow>

class QAction;
class QLabel;
class QMenu;
class QScrollArea;
class QScrollBar;

class ImageViewer : public QMainWindow
{
    Q_OBJECT

public:
    ImageViewer();
    bool loadFile(const QString &);

private slots:
    void open();

private:
    void createActions();
    void createMenus();

    QLabel *imageLabel;
    QAction *openAct;
    QMenu *fileMenu;
};

#endif

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Main.cpp:

#include <QApplication>
#include <QCommandLineParser>
 
#include "imageviewer.h"
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGuiApplication::setApplicationDisplayName(ImageViewer::tr("Image Viewer"));
    ImageViewer imageViewer;
    imageViewer.show();
    return app.exec();
}

#include <QApplication>
#include <QCommandLineParser>

#include "imageviewer.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGuiApplication::setApplicationDisplayName(ImageViewer::tr("Image Viewer"));
    ImageViewer imageViewer;
    imageViewer.show();
    return app.exec();
}

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This is the image I am uploading to the app: link. As you can see, the viewport size here is 570 by 357. The image I am using is 2560 by 1600 (around 16 MB size), both have an aspect ratio of almost 1.6. However, on loading the image, it comes as rather blurred:

TH9uC.jpg

How do I make it look sharp as the original image, just as seen when using QPainter?

[ChrisW67]

Firstly, you are resizing the image to the size() of the QMainWindow rather than the label you will be putting the pixmap into. Secondly, you are telling Qt to ignore the aspect ratio when resizing so it probably Will stretch/shrink the image in one direction to fit the size you gave it.

[Cupidvogel]

Changed to imageLabel->setPixmap(QPixmap::fromImage(image).scaled(QSize( 570,377),Qt::KeepAspectRatio,Qt::SmoothTransformat ion)); Same result. I have already tried it, I was just experimenting with various options and put this version in the question.

[Cupidvogel]

Changed to a solution where I am caching a pixmap variable and updating it. That solves the problem. It is a known Qt bug on Retina Display Mac.