The Image Viewer example shows how to combine QLabel and QScrollArea to display an image. QLabel is typically used for displaying a text, but it can also display an image. QScrollArea provides a scrolling view around another widget. If the child widget exceeds the size of the frame, QScrollArea automatically provides scroll bars.
The example demonstrates how QLabel's ability to scale its contents (QLabel.scaledContents), and QScrollArea's ability to automatically resize its contents (QScrollArea.widgetResizable), can be used to implement zooming and scaling features.
In addition the example shows how to use QPainter to print an image.
""" class ImageViewer(QMainWindow): def __init__(self, parent=None): super().__init__(parent) self._scale_factor = 1.0 self._first_file_dialog = True self._image_label = QLabel() self._image_label.setBackgroundRole(QPalette.Base) self._image_label.setSizePolicy(QSizePolicy.Ignored, QSizePolicy.Ignored) self._image_label.setScaledContents(True) self._scroll_area = QScrollArea() self._scroll_area.setBackgroundRole(QPalette.Dark) self._scroll_area.setWidget(self._image_label) self._scroll_area.setVisible(False) self.setCentralWidget(self._scroll_area) self._create_actions() self.resize(QGuiApplication.primaryScreen().availableSize() * 3 / 5) def load_file(self, fileName): reader = QImageReader(fileName) reader.setAutoTransform(True) new_image = reader.read() native_filename = QDir.toNativeSeparators(fileName) if new_image.isNull(): error = reader.errorString() QMessageBox.information(self, QGuiApplication.applicationDisplayName(), f"Cannot load {native_filename}: {error}") return False self._set_image(new_image) self.setWindowFilePath(fileName) w = self._image.width() h = self._image.height() d = self._image.depth() color_space = self._image.colorSpace() description = color_space.description() if color_space.isValid() else 'unknown' message = f'Opened "{native_filename}", {w}x{h}, Depth: {d} ({description})' self.statusBar().showMessage(message) return True def _set_image(self, new_image): self._image = new_image if self._image.colorSpace().isValid(): self._image.convertToColorSpace(QColorSpace.SRgb) self._image_label.setPixmap(QPixmap.fromImage(self._image)) self._scale_factor = 1.0 self._scroll_area.setVisible(True) self._print_act.setEnabled(True) self._fit_to_window_act.setEnabled(True) self._update_actions() if not self._fit_to_window_act.isChecked(): self._image_label.adjustSize() def _save_file(self, fileName): writer = QImageWriter(fileName) native_filename = QDir.toNativeSeparators(fileName) if not writer.write(self._image): error = writer.errorString() message = f"Cannot write {native_filename}: {error}" QMessageBox.information(self, QGuiApplication.applicationDisplayName(), message) return False self.statusBar().showMessage(f'Wrote "{native_filename}"') return True @Slot() def _open(self): dialog = QFileDialog(self, "Open File") self._initialize_image_filedialog(dialog, QFileDialog.AcceptOpen) while (dialog.exec() == QDialog.Accepted and not self.load_file(dialog.selectedFiles()[0])): pass @Slot() def _save_as(self): dialog = QFileDialog(self, "Save File As") self._initialize_image_filedialog(dialog, QFileDialog.AcceptSave) while (dialog.exec() == QDialog.Accepted and not self._save_file(dialog.selectedFiles()[0])): pass @Slot() def _print_(self): printer = QPrinter() dialog = QPrintDialog(printer, self) if dialog.exec() == QDialog.Accepted: with QPainter(printer) as painter: pixmap = self._image_label.pixmap() rect = painter.viewport() 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) @Slot() def _copy(self): QGuiApplication.clipboard().setImage(self._image) @Slot() def _paste(self): new_image = QGuiApplication.clipboard().image() if new_image.isNull(): self.statusBar().showMessage("No image in clipboard") else: self._set_image(new_image) self.setWindowFilePath('') w = new_image.width() h = new_image.height() d = new_image.depth() message = f"Obtained image from clipboard, {w}x{h}, Depth: {d}" self.statusBar().showMessage(message) @Slot() def _zoom_in(self): self._scale_image(1.25) @Slot() def _zoom_out(self): self._scale_image(0.8) @Slot() def _normal_size(self): self._image_label.adjustSize() self._scale_factor = 1.0 @Slot() def _fit_to_window(self): fit_to_window = self._fit_to_window_act.isChecked() self._scroll_area.setWidgetResizable(fit_to_window) if not fit_to_window: self._normal_size() self._update_actions() @Slot() def _about(self): QMessageBox.about(self, "About Image Viewer", ABOUT) def _create_actions(self): file_menu = self.menuBar().addMenu("&File") self._open_act = file_menu.addAction("&Open...") self._open_act.triggered.connect(self._open) self._open_act.setShortcut(QKeySequence.Open) self._save_as_act = file_menu.addAction("&Save As...") self._save_as_act.triggered.connect(self._save_as) self._save_as_act.setEnabled(False) self._print_act = file_menu.addAction("&Print...") self._print_act.triggered.connect(self._print_) self._print_act.setShortcut(QKeySequence.Print) self._print_act.setEnabled(False) file_menu.addSeparator() self._exit_act = file_menu.addAction("E&xit") self._exit_act.triggered.connect(self.close) self._exit_act.setShortcut("Ctrl+Q") edit_menu = self.menuBar().addMenu("&Edit") self._copy_act = edit_menu.addAction("&Copy") self._copy_act.triggered.connect(self._copy) self._copy_act.setShortcut(QKeySequence.Copy) self._copy_act.setEnabled(False) self._paste_act = edit_menu.addAction("&Paste") self._paste_act.triggered.connect(self._paste) self._paste_act.setShortcut(QKeySequence.Paste) view_menu = self.menuBar().addMenu("&View") self._zoom_in_act = view_menu.addAction("Zoom &In (25%)") self._zoom_in_act.setShortcut(QKeySequence.ZoomIn) self._zoom_in_act.triggered.connect(self._zoom_in) self._zoom_in_act.setEnabled(False) self._zoom_out_act = view_menu.addAction("Zoom &Out (25%)") self._zoom_out_act.triggered.connect(self._zoom_out) self._zoom_out_act.setShortcut(QKeySequence.ZoomOut) self._zoom_out_act.setEnabled(False) self._normal_size_act = view_menu.addAction("&Normal Size") self._normal_size_act.triggered.connect(self._normal_size) self._normal_size_act.setShortcut("Ctrl+S") self._normal_size_act.setEnabled(False) view_menu.addSeparator() self._fit_to_window_act = view_menu.addAction("&Fit to Window") self._fit_to_window_act.triggered.connect(self._fit_to_window) self._fit_to_window_act.setEnabled(False) self._fit_to_window_act.setCheckable(True) self._fit_to_window_act.setShortcut("Ctrl+F") help_menu = self.menuBar().addMenu("&Help") about_act = help_menu.addAction("&About") about_act.triggered.connect(self._about) about_qt_act = help_menu.addAction("About &Qt") about_qt_act.triggered.connect(QApplication.aboutQt) def _update_actions(self): has_image = not self._image.isNull() self._save_as_act.setEnabled(has_image) self._copy_act.setEnabled(has_image) enable_zoom = not self._fit_to_window_act.isChecked() self._zoom_in_act.setEnabled(enable_zoom) self._zoom_out_act.setEnabled(enable_zoom) self._normal_size_act.setEnabled(enable_zoom) def _scale_image(self, factor): self._scale_factor *= factor new_size = self._scale_factor * self._image_label.pixmap().size() self._image_label.resize(new_size) self._adjust_scrollbar(self._scroll_area.horizontalScrollBar(), factor) self._adjust_scrollbar(self._scroll_area.verticalScrollBar(), factor) self._zoom_in_act.setEnabled(self._scale_factor < 3.0) self._zoom_out_act.setEnabled(self._scale_factor > 0.333) def _adjust_scrollbar(self, scrollBar, factor): pos = int(factor * scrollBar.value() + ((factor - 1) * scrollBar.pageStep() / 2)) scrollBar.setValue(pos) def _initialize_image_filedialog(self, dialog, acceptMode): if self._first_file_dialog: self._first_file_dialog = False locations = QStandardPaths.standardLocations(QStandardPaths.PicturesLocation) directory = locations[-1] if locations else QDir.currentPath() dialog.setDirectory(directory) mime_types = [m.data().decode('utf-8') for m in QImageWriter.supportedMimeTypes()] mime_types.sort() dialog.setMimeTypeFilters(mime_types) dialog.selectMimeTypeFilter("image/jpeg") dialog.setAcceptMode(acceptMode) if acceptMode == QFileDialog.AcceptSave: dialog.setDefaultSuffix("jpg")