documentviewer: Add simple image viewer

Since qtbase/b8f588bea74aae0a890e1af18b936b0bfbf8c237
removed the QtWidgets image viewer example, there
is no way to exercise the image plugins.
Re-add the code as documentviewer plugin.

Pick-to: 6.9
Change-Id: I0eb26d02a9c11cdb0fce315ffc6f7c662e98c924
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

examples/demos/documentviewer/doc/src/documentviewer.qdoc

@@ -142,6 +142,23 @@
     It supports editing text files, copy/cut and paste, printing, and
     saving changes.
 
+    \section1 ImageViewer class
+
+    \c ImageViewer displays images as supported by QImageReader, using
+    a QLabel.
+
+    In the constructor, we increase the allocation limit of QImageReader to
+    allow for larger photos:
+
+    \snippet demos/documentviewer/plugins/imageviewer/imageviewer.cpp init
+
+    In the openFile() function, we load the image and determine its size.
+    If it is larger than the screen, we downscale it to screen size,
+    maintaining the aspect ratio. This calculation has to be done in native
+    pixels, and the device pixel ratio needs to be set on the resulting pixmap
+    for it to appear crisp:
+
+    \snippet demos/documentviewer/plugins/imageviewer/imageviewer.cpp open
 
     \section1 JsonViewer class
 

examples/demos/documentviewer/plugins/CMakeLists.txt

@@ -1,6 +1,7 @@
 # Copyright (C) 2023 The Qt Company Ltd.
 # SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
+add_subdirectory(imageviewer)
 add_subdirectory(jsonviewer)
 add_subdirectory(txtviewer)
 

examples/demos/documentviewer/plugins/imageviewer/CMakeLists.txt

@@ -0,0 +1,41 @@
+# Copyright (C) 2023 The Qt Company Ltd.
+# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
+
+find_package(Qt6 REQUIRED COMPONENTS Core Gui Widgets
+                 OPTIONAL_COMPONENTS PrintSupport)
+
+qt_add_plugin(imageviewer
+    CLASS_NAME ImageViewer
+    imageviewer.cpp imageviewer.h
+)
+
+set_target_properties(imageviewer PROPERTIES
+    LIBRARY_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}/app"
+)
+
+target_include_directories(imageviewer PRIVATE
+    ../../app
+)
+
+target_link_libraries(imageviewer PRIVATE
+    Qt6::Core
+    Qt6::Gui
+    Qt6::Widgets
+    abstractviewer
+)
+
+if(TARGET Qt6::PrintSupport)
+    target_link_libraries(imageviewer PRIVATE Qt6::PrintSupport)
+endif()
+
+if(WIN32)
+    set(install_destination "${CMAKE_INSTALL_BINDIR}/app")
+elseif(APPLE)
+    set(install_destination ".")
+else()
+    set(install_destination "${CMAKE_INSTALL_BINDIR}")
+endif()
+install(TARGETS imageviewer
+    RUNTIME DESTINATION "${install_destination}"
+    LIBRARY DESTINATION "${install_destination}"
+)

examples/demos/documentviewer/plugins/imageviewer/imageviewer.cpp

@@ -0,0 +1,207 @@
+// Copyright (C) 2025 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
+
+#include "imageviewer.h"
+
+#include <QFileDialog>
+#include <QLabel>
+#include <QMainWindow>
+#include <QToolBar>
+
+#include <QColorSpace>
+#include <QGuiApplication>
+#include <QIcon>
+#include <QImageReader>
+#include <QKeySequence>
+#include <QPainter>
+#include <QPixmap>
+#include <QScreen>
+
+#include <QDir>
+
+#ifdef QT_DOCUMENTVIEWER_PRINTSUPPORT
+#  include <QPrinter>
+#endif
+
+using namespace Qt::StringLiterals;
+
+static QStringList imageFormats()
+{
+    QStringList result;
+    const QByteArrayList &allFormats = QImageReader::supportedImageFormats();
+    for (const auto &format : allFormats) {
+        if (format != "tif" && format != "cur") // duplicate/non-existent
+            result.append("image/"_L1 + QLatin1StringView(format));
+    }
+    return result;
+}
+
+static QString msgOpen(const QString &name, const QImage &image)
+{
+    const QString description = image.colorSpace().isValid()
+    ? image.colorSpace().description() : ImageViewer::tr("unknown");
+    return ImageViewer::tr("Opened \"%1\", %2x%3, Depth: %4 (%5)")
+                           .arg(QDir::toNativeSeparators(name))
+                           .arg(image.width()).arg(image.height())
+                           .arg(image.depth()).arg(description);
+}
+
+//! [init]
+ImageViewer::ImageViewer() : m_formats(imageFormats())
+{
+    connect(this, &AbstractViewer::uiInitialized, this, &ImageViewer::setupImageUi);
+    QImageReader::setAllocationLimit(1024); // MB
+}
+//! [init]
+
+ImageViewer::~ImageViewer() = default;
+
+void ImageViewer::init(QFile *file, QWidget *parent, QMainWindow *mainWindow)
+{
+    m_imageLabel = new QLabel(parent);
+    m_imageLabel->setFrameShape(QFrame::Box);
+    m_imageLabel->setAlignment(Qt::AlignCenter);
+    m_imageLabel->setScaledContents(true);
+
+    AbstractViewer::init(file, m_imageLabel, mainWindow);
+
+    m_toolBar = addToolBar(tr("Images"));
+
+    m_zoomInAct = m_toolBar->addAction(tr("Zoom &In"), this, &ImageViewer::zoomIn);
+    m_zoomInAct->setIcon(QIcon::fromTheme(QIcon::ThemeIcon::ZoomIn));
+    m_zoomInAct->setShortcut(QKeySequence::ZoomIn);
+
+    m_zoomOutAct = m_toolBar->addAction(tr("Zoom &Out"), this, &ImageViewer::zoomOut);
+    m_zoomOutAct->setIcon(QIcon::fromTheme(QIcon::ThemeIcon::ZoomOut));
+    m_zoomOutAct->setShortcut(QKeySequence::ZoomOut);
+
+    m_resetZoomAct = m_toolBar->addAction(tr("Reset Zoom"), this, &ImageViewer::resetZoom);
+    m_resetZoomAct->setIcon(QIcon::fromTheme(QIcon::ThemeIcon::ZoomFitBest));
+    m_resetZoomAct->setShortcut(QKeySequence(Qt::ControlModifier | Qt::Key_0));
+}
+
+QStringList ImageViewer::supportedMimeTypes() const
+{
+    return m_formats;
+}
+
+void ImageViewer::clear()
+{
+    m_imageLabel->setPixmap({});
+    m_maxScaleFactor = m_minScaleFactor = m_initialScaleFactor = m_scaleFactor = 1;
+}
+
+void ImageViewer::setupImageUi()
+{
+    openFile();
+}
+
+//! [open]
+void ImageViewer::openFile()
+{
+#if QT_CONFIG(cursor)
+    QGuiApplication::setOverrideCursor(Qt::WaitCursor);
+#endif
+    const QString name = m_file->fileName();
+    QImageReader reader(name);
+    const QImage origImage = reader.read();
+    if (origImage.isNull()) {
+        statusMessage(tr("Cannot read file %1:\n%2.")
+                      .arg(QDir::toNativeSeparators(name),
+                           reader.errorString()), tr("open"));
+        disablePrinting();
+#if QT_CONFIG(cursor)
+        QGuiApplication::restoreOverrideCursor();
+#endif
+        return;
+    }
+
+    clear();
+
+    QImage image = origImage.colorSpace().isValid()
+        ? origImage.convertedToColorSpace(QColorSpace::SRgb)
+        : origImage;
+
+    const auto devicePixelRatio = m_imageLabel->devicePixelRatioF();
+    m_imageSize = QSizeF(image.size()) / devicePixelRatio;
+
+    QPixmap pixmap = QPixmap::fromImage(image);
+    pixmap.setDevicePixelRatio(devicePixelRatio);
+    m_imageLabel->setPixmap(pixmap);
+
+    const QSizeF targetSize = m_imageLabel->parentWidget()->size();
+    if (m_imageSize.width() > targetSize.width()
+        || m_imageSize.height() > targetSize.height()) {
+        m_initialScaleFactor = qMin(targetSize.width() / m_imageSize.width(),
+                                    targetSize.height() / m_imageSize.height());
+    }
+    m_maxScaleFactor = 3 * m_initialScaleFactor;
+    m_minScaleFactor = m_initialScaleFactor / 3;
+    doSetScaleFactor(m_initialScaleFactor);
+
+    statusMessage(msgOpen(name, origImage));
+#if QT_CONFIG(cursor)
+    QGuiApplication::restoreOverrideCursor();
+#endif
+
+    maybeEnablePrinting();
+}
+//! [open]
+
+void ImageViewer::setScaleFactor(qreal scaleFactor)
+{
+    if (!qFuzzyCompare(m_scaleFactor, scaleFactor))
+        doSetScaleFactor(scaleFactor);
+}
+
+void ImageViewer::doSetScaleFactor(qreal scaleFactor)
+{
+    m_scaleFactor = scaleFactor;
+    const QSize labelSize = (m_imageSize * m_scaleFactor).toSize();
+    m_imageLabel->setFixedSize(labelSize);
+    enableZoomActions();
+}
+
+void ImageViewer::zoomIn()
+{
+    setScaleFactor(m_scaleFactor * 1.25);
+}
+
+void ImageViewer::zoomOut()
+{
+    setScaleFactor(m_scaleFactor * 0.8);
+}
+
+void ImageViewer::resetZoom()
+{
+    setScaleFactor(m_initialScaleFactor);
+}
+
+bool ImageViewer::hasContent() const
+{
+    return !m_imageLabel->pixmap().isNull();
+}
+
+void ImageViewer::enableZoomActions()
+{
+    m_resetZoomAct->setEnabled(!qFuzzyCompare(m_scaleFactor, m_initialScaleFactor));
+    m_zoomInAct->setEnabled(m_scaleFactor < m_maxScaleFactor);
+    m_zoomOutAct->setEnabled(m_scaleFactor > m_minScaleFactor);
+}
+
+#ifdef QT_DOCUMENTVIEWER_PRINTSUPPORT
+void ImageViewer::printDocument(QPrinter *printer) const
+{
+    if (!hasContent())
+        return;
+
+    QPainter painter(printer);
+    QPixmap pixmap = m_imageLabel->pixmap();
+    QRect rect = painter.viewport();
+    QSize size = pixmap.size();
+    size.scale(rect.size(), Qt::KeepAspectRatio);
+    painter.setViewport(rect.x(), rect.y(), size.width(), size.height());
+    painter.setWindow(pixmap.rect());
+    painter.drawPixmap(0, 0, pixmap);
+}
+#endif // QT_DOCUMENTVIEWER_PRINTSUPPORT

examples/demos/documentviewer/plugins/imageviewer/imageviewer.h

@@ -0,0 +1,63 @@
+// Copyright (C) 2025 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
+
+#ifndef IMAGEVIEWER_H
+#define IMAGEVIEWER_H
+
+#include "viewerinterfaces.h"
+
+#include <QSizeF>
+
+QT_FORWARD_DECLARE_CLASS(QLabel)
+
+class ImageViewer : public ViewerInterface
+{
+    Q_OBJECT
+    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.Examples.DocumentViewer.ViewerInterface" FILE "imageviewer.json")
+    Q_INTERFACES(ViewerInterface)
+public:
+    Q_DISABLE_COPY_MOVE(ImageViewer)
+
+    ImageViewer();
+    ~ImageViewer() override;
+
+    void init(QFile *file, QWidget *parent, QMainWindow *mainWindow) override;
+    QString viewerName() const override { return QLatin1StringView(staticMetaObject.className()); };
+    QStringList supportedMimeTypes() const override;
+    bool hasContent() const override;
+    QByteArray saveState() const override { return {}; }
+    bool restoreState(QByteArray &) override { return true; }
+    bool supportsOverview() const override { return false; }
+
+#ifdef QT_DOCUMENTVIEWER_PRINTSUPPORT
+protected:
+    void printDocument(QPrinter *printer) const override;
+#endif // QT_DOCUMENTVIEWER_PRINTSUPPORT
+
+private slots:
+    void setupImageUi();
+    void clear();
+    void zoomIn();
+    void zoomOut();
+    void resetZoom();
+
+private:
+    void openFile();
+    void setScaleFactor(qreal scaleFactor);
+    void doSetScaleFactor(qreal scaleFactor);
+    void enableZoomActions();
+
+    QLabel *m_imageLabel{};
+    QToolBar *m_toolBar{};
+    QAction *m_zoomInAct{};
+    QAction *m_zoomOutAct{};
+    QAction *m_resetZoomAct{};
+    QStringList m_formats;
+    qreal m_scaleFactor = 1;
+    qreal m_initialScaleFactor = 1;
+    qreal m_minScaleFactor = 1;
+    qreal m_maxScaleFactor = 1;
+    QSizeF m_imageSize;
+};
+
+#endif //IMAGEVIEWER_H

examples/demos/documentviewer/plugins/imageviewer/imageviewer.json

@@ -0,0 +1 @@
+{ "Keys": [ "imageviewer" ] }