QImageReader Class | Qt GUI | Qt 6.11.0
Description: The QImageReader class provides a format independent interface for reading images from files or other devices.

- Qt 6.11
- Qt GUI
- C++ Classes
- QImageReader
- Qt 6.11.0 Reference Documentation

# QImageReader Class

The QImageReader class provides a format independent interface for reading images from files or other devices. More...

Header: | `#include <QImageReader>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Gui)` `target_link_libraries(mytarget PRIVATE Qt6::Gui)`
qmake: | `QT += gui`

- List of all members, including inherited members
- QImageReader is part of Painting Classes.

Note: All functions in this class are reentrant.

## Public Types

enum | ImageReaderError { FileNotFoundError, DeviceError, UnsupportedFormatError, InvalidDataError, UnknownError }

## Public Functions

| QImageReader()
| QImageReader(QIODevice *device, const QByteArray &format = QByteArray())
| QImageReader(const QString &fileName, const QByteArray &format = QByteArray())
| ~QImageReader()
bool | autoDetectImageFormat() const
bool | autoTransform() const
QColor | backgroundColor() const
bool | canRead() const
QRect | clipRect() const
int | currentImageNumber() const
QRect | currentImageRect() const
bool | decideFormatFromContent() const
QIODevice * | device() const
QImageReader::ImageReaderError | error() const
QString | errorString() const
QString | fileName() const
QByteArray | format() const
int | imageCount() const
QImage::Format | imageFormat() const
bool | jumpToImage(int imageNumber)
bool | jumpToNextImage()
int | loopCount() const
int | nextImageDelay() const
int | quality() const
QImage | read()
bool | read(QImage *image)
QRect | scaledClipRect() const
QSize | scaledSize() const
void | setAutoDetectImageFormat(bool enabled)
void | setAutoTransform(bool enabled)
void | setBackgroundColor(const QColor &color)
void | setClipRect(const QRect &rect)
void | setDecideFormatFromContent(bool ignored)
void | setDevice(QIODevice *device)
void | setFileName(const QString &fileName)
void | setFormat(const QByteArray &format)
void | setQuality(int quality)
void | setScaledClipRect(const QRect &rect)
void | setScaledSize(const QSize &size)
QSize | size() const
QByteArray | subType() const
QList<QByteArray> | supportedSubTypes() const
bool | supportsAnimation() const
bool | supportsOption(QImageIOHandler::ImageOption option) const
QString | text(const QString &key) const
QStringList | textKeys() const
QImageIOHandler::Transformations | transformation() const

## Static Public Members

`(since 6.0)` int | allocationLimit()
QByteArray | imageFormat(QIODevice *device)
QByteArray | imageFormat(const QString &fileName)
QList<QByteArray> | imageFormatsForMimeType(const QByteArray &mimeType)
`(since 6.0)` void | setAllocationLimit(int mbLimit)
QList<QByteArray> | supportedImageFormats()
QList<QByteArray> | supportedMimeTypes()

## Detailed Description

The most common way to read images is through QImage and QPixmap's constructors, or by calling QImage::load() and QPixmap::load(). QImageReader is a specialized class which gives you more control when reading images. For example, you can read an image into a specific size by calling setScaledSize(), and you can select a clip rect, effectively loading only parts of an image, by calling setClipRect(). Depending on the underlying support in the image format, this can save memory and speed up loading of images.

To read an image, you start by constructing a QImageReader object. Pass either a file name or a device pointer, and the image format to QImageReader's constructor. You can then set several options, such as the clip rect (by calling setClipRect()) and scaled size (by calling setScaledSize()). canRead() returns the image if the QImageReader can read the image (i.e., the image format is supported and the device is open for reading). Call read() to read the image.

If any error occurs when reading the image, read() will return a null QImage. You can then call error() to find the type of error that occurred, or errorString() to get a human readable description of what went wrong.

Note: QImageReader assumes exclusive control over the file or device that is assigned. Any attempts to modify the assigned file or device during the lifetime of the QImageReader object will yield undefined results.

### Formats

Call supportedImageFormats() for a list of formats that QImageReader can read. QImageReader supports all built-in image formats, in addition to any image format plugins that support reading. Call supportedMimeTypes() to obtain a list of supported MIME types, which for example can be passed to QFileDialog::setMimeTypeFilters().

QImageReader autodetects the image format by default, by looking at the provided (optional) format string, the file name suffix, and the data stream contents. You can enable or disable this feature, by calling setAutoDetectImageFormat().

### High Resolution Versions of Images

It is possible to provide high resolution versions of images should a scaling between device pixels and device independent pixels be in effect.

The high resolution version is marked by the suffix `@2x` on the base name. The image read will have its device pixel ratio set to a value of 2.

This can be disabled by setting the environment variable `QT_HIGHDPI_DISABLE_2X_IMAGE_LOADING`.

See also QImageWriter, QImageIOHandler, QImageIOPlugin, QMimeDatabase, QColorSpace, QImage::devicePixelRatio(), QPixmap::devicePixelRatio(), QIcon, QPainter::drawPixmap(), and QPainter::drawImage().

## Member Type Documentation

### enum QImageReader::ImageReaderError

This enum describes the different types of errors that can occur when reading images with QImageReader.

Constant | Value | Description
--- | --- | ---
`QImageReader::FileNotFoundError` | `1` | QImageReader was used with a file name, but not file was found with that name. This can also happen if the file name contained no extension, and the file with the correct extension is not supported by Qt.
`QImageReader::DeviceError` | `2` | QImageReader encountered a device error when reading the image. You can consult your particular device for more details on what went wrong.
`QImageReader::UnsupportedFormatError` | `3` | Qt does not support the requested image format.
`QImageReader::InvalidDataError` | `4` | The image data was invalid, and QImageReader was unable to read an image from it. The can happen if the image file is damaged.
`QImageReader::UnknownError` | `0` | An unknown error occurred. If you get this value after calling read(), it is most likely caused by a bug in QImageReader.

## Member Function Documentation

### QImageReader::QImageReader()

Constructs an empty QImageReader object. Before reading an image, call setDevice() or setFileName().

### `[explicit]` QImageReader::QImageReader(QIODevice *device, const QByteArray &format = QByteArray())

Constructs a QImageReader object with the device device and the image format format.

### `[explicit]` QImageReader::QImageReader(const QString &fileName, const QByteArray &format = QByteArray())

Constructs a QImageReader object with the file name fileName and the image format format.

See also setFileName().

### `[noexcept]` QImageReader::~QImageReader()

Destructs the QImageReader object.

### `[static, since 6.0]` int QImageReader::allocationLimit()

Returns the current allocation limit, in megabytes.

This function was introduced in Qt 6.0.

See also setAllocationLimit().

### bool QImageReader::autoDetectImageFormat() const

Returns `true` if image format autodetection is enabled on this image reader; otherwise returns `false`. By default, autodetection is enabled.

See also setAutoDetectImageFormat().

### bool QImageReader::autoTransform() const

Returns `true` if the image handler will apply transformation metadata on read().

See also setAutoTransform(), transformation(), and read().

### QColor QImageReader::backgroundColor() const

Returns the background color that's used when reading an image. If the image format does not support setting the background color an invalid color is returned.

See also setBackgroundColor() and read().

### bool QImageReader::canRead() const

Returns `true` if an image can be read for the device (i.e., the image format is supported, and the device seems to contain valid data); otherwise returns `false`.

canRead() is a lightweight function that only does a quick test to see if the image data is valid. read() may still return false after canRead() returns `true`, if the image data is corrupt.

Note: A QMimeDatabase lookup is normally a better approach than this function for identifying potentially non-image files or data.

For images that support animation, canRead() returns `false` when all frames have been read.

See also read(), supportedImageFormats(), and QMimeDatabase.

### QRect QImageReader::clipRect() const

Returns the clip rect (also known as the ROI, or Region Of Interest) of the image. If no clip rect has been set, an invalid QRect is returned.

See also setClipRect().

### int QImageReader::currentImageNumber() const

For image formats that support animation, this function returns the sequence number of the current frame. If the image format doesn't support animation, 0 is returned.

This function returns -1 if an error occurred.

See also supportsAnimation(), QImageIOHandler::currentImageNumber(), and canRead().

### QRect QImageReader::currentImageRect() const

For image formats that support animation, this function returns the rect for the current frame. Otherwise, a null rect is returned.

See also supportsAnimation() and QImageIOHandler::currentImageRect().

### bool QImageReader::decideFormatFromContent() const

Returns whether the image reader should decide which plugin to use only based on the contents of the datastream rather than on the file extension.

See also setDecideFormatFromContent().

### QIODevice *QImageReader::device() const

Returns the device currently assigned to QImageReader, or `nullptr` if no device has been assigned.

See also setDevice().

### QImageReader::ImageReaderError QImageReader::error() const

Returns the type of error that occurred last.

See also ImageReaderError and errorString().

### QString QImageReader::errorString() const

Returns a human readable description of the last error that occurred.

See also error().

### QString QImageReader::fileName() const

If the currently assigned device is a QFile, or if setFileName() has been called, this function returns the name of the file QImageReader reads from. Otherwise (i.e., if no device has been assigned or the device is not a QFile), an empty QString is returned.

See also setFileName() and setDevice().

### QByteArray QImageReader::format() const

Returns the format QImageReader uses for reading images.

You can call this function after assigning a device to the reader to determine the format of the device. For example:

     QImageReader reader("image.png");
     // reader.format() == "png"

If the reader cannot read any image from the device (e.g., there is no image there, or the image has already been read), or if the format is unsupported, this function returns an empty QByteArray().

See also setFormat() and supportedImageFormats().

### int QImageReader::imageCount() const

For image formats that support animation, this function returns the total number of images in the animation. If the format does not support animation, 0 is returned.

This function returns -1 if an error occurred.

See also supportsAnimation(), QImageIOHandler::imageCount(), and canRead().

### QImage::Format QImageReader::imageFormat() const

Returns the format of the image, without actually reading the image contents. The format describes the image format QImageReader::read() returns, not the format of the actual image.

If the image format does not support this feature, this function returns an invalid format.

See also QImageIOHandler::ImageOption, QImageIOHandler::option(), and QImageIOHandler::supportsOption().

### `[static]` QByteArray QImageReader::imageFormat(QIODevice *device)

If supported, this function returns the image format of the device device. Otherwise, an empty string is returned.

See also QImageReader::autoDetectImageFormat().

### `[static]` QByteArray QImageReader::imageFormat(const QString &fileName)

If supported, this function returns the image format of the file fileName. Otherwise, an empty string is returned.

### `[static]` QList<QByteArray> QImageReader::imageFormatsForMimeType(const QByteArray &mimeType)

Returns the list of image formats corresponding to mimeType.

Note that the QGuiApplication instance must be created before this function is called.

See also supportedImageFormats() and supportedMimeTypes().

### bool QImageReader::jumpToImage(int imageNumber)

For image formats that support animation, this function skips to the image whose sequence number is imageNumber, returning true if successful or false if the corresponding image cannot be found.

The next call to read() will attempt to read this image.

See also jumpToNextImage() and QImageIOHandler::jumpToImage().

### bool QImageReader::jumpToNextImage()

For image formats that support animation, this function steps over the current image, returning true if successful or false if there is no following image in the animation.

The default implementation calls read(), then discards the resulting image, but the image handler may have a more efficient way of implementing this operation.

See also jumpToImage() and QImageIOHandler::jumpToNextImage().

### int QImageReader::loopCount() const

For image formats that support animation, this function returns the number of times the animation should loop. If this function returns -1, it can either mean the animation should loop forever, or that an error occurred. If an error occurred, canRead() will return false.

See also supportsAnimation(), QImageIOHandler::loopCount(), and canRead().

### int QImageReader::nextImageDelay() const

For image formats that support animation, this function returns the number of milliseconds to wait until displaying the next frame in the animation. If the image format doesn't support animation, 0 is returned.

This function returns -1 if an error occurred.

See also supportsAnimation(), QImageIOHandler::nextImageDelay(), and canRead().

### int QImageReader::quality() const

Returns the quality setting of the image format.

See also setQuality().

### QImage QImageReader::read()

Reads an image from the device. On success, the image that was read is returned; otherwise, a null QImage is returned. You can then call error() to find the type of error that occurred, or errorString() to get a human readable description of the error.

For image formats that support animation, calling read() repeatedly will return the next frame. When all frames have been read, a null image will be returned.

See also canRead(), supportedImageFormats(), supportsAnimation(), and QMovie.

### bool QImageReader::read(QImage *image)

Reads an image from the device into image, which must point to a QImage. Returns `true` on success; otherwise, returns `false`.

If image has same format and size as the image data that is about to be read, this function may not need to allocate a new image before reading. Because of this, it can be faster than the other read() overload, which always constructs a new image; especially when reading several images with the same format and size.

     QImage icon(64, 64, QImage::Format_RGB32);
     QImageReader reader("icon_64x64.bmp");
     if (reader.read(&icon)) {
         // Display icon
     }

For image formats that support animation, calling read() repeatedly will return the next frame. When all frames have been read, a null image will be returned.

This is an overloaded function.

See also canRead(), supportedImageFormats(), supportsAnimation(), and QMovie.

### QRect QImageReader::scaledClipRect() const

Returns the scaled clip rect of the image.

See also setScaledClipRect().

### QSize QImageReader::scaledSize() const

Returns the scaled size of the image.

See also setScaledSize().

### `[static, since 6.0]` void QImageReader::setAllocationLimit(int mbLimit)

Sets the allocation limit to mbLimit megabytes. Images that would require a QImage memory allocation above this limit will be rejected. If mbLimit is 0, the allocation size check will be disabled.

This limit helps applications avoid unexpectedly large memory usage from loading corrupt image files. It is normally not needed to change it. The default limit is large enough for all commonly used image sizes.

At runtime, this value may be overridden by the environment variable `QT_IMAGEIO_MAXALLOC`.

Note: The memory requirements are calculated for a minimum of 32 bits per pixel, since Qt will typically convert an image to that depth when it is used in GUI. This means that the effective allocation limit is significantly smaller than mbLimit when reading 1 bpp and 8 bpp images.

This function was introduced in Qt 6.0.

See also allocationLimit().

### void QImageReader::setAutoDetectImageFormat(bool enabled)

If enabled is true, image format autodetection is enabled; otherwise, it is disabled. By default, autodetection is enabled.

QImageReader uses an extensive approach to detecting the image format; firstly, if you pass a file name to QImageReader, it will attempt to detect the file extension if the given file name does not point to an existing file, by appending supported default extensions to the given file name, one at a time. It then uses the following approach to detect the image format:

- Image plugins are queried first, based on either the optional format string, or the file name suffix (if the source device is a file). No content detection is done at this stage. QImageReader will choose the first plugin that supports reading for this format.
- If no plugin supports the image format, Qt's built-in handlers are checked based on either the optional format string, or the file name suffix.
- If no capable plugins or built-in handlers are found, each plugin is tested by inspecting the content of the data stream.
- If no plugins could detect the image format based on data contents, each built-in image handler is tested by inspecting the contents.
- Finally, if all above approaches fail, QImageReader will report failure when trying to read the image.

By disabling image format autodetection, QImageReader will only query the plugins and built-in handlers based on the format string (i.e., no file name extensions are tested).

See also autoDetectImageFormat(), QImageIOHandler::canRead(), and QImageIOPlugin::capabilities().

### void QImageReader::setAutoTransform(bool enabled)

Determines that images returned by read() should have transformation metadata automatically applied if enabled is `true`.

See also autoTransform(), transformation(), and read().

### void QImageReader::setBackgroundColor(const QColor &color)

Sets the background color to color. Image formats that support this operation are expected to initialize the background to color before reading an image.

See also backgroundColor() and read().

### void QImageReader::setClipRect(const QRect &rect)

Sets the image clip rect (also known as the ROI, or Region Of Interest) to rect. The coordinates of rect are relative to the untransformed image size, as returned by size().

See also clipRect(), setScaledSize(), and setScaledClipRect().

### void QImageReader::setDecideFormatFromContent(bool ignored)

If ignored is set to true, then the image reader will ignore specified formats or file extensions and decide which plugin to use only based on the contents in the datastream.

Setting this flag means that all image plugins gets loaded. Each plugin will read the first bytes in the image data and decide if the plugin is compatible or not.

This also disables auto detecting the image format.

See also decideFormatFromContent().

### void QImageReader::setDevice(QIODevice *device)

Sets QImageReader's device to device. If a device has already been set, the old device is removed from QImageReader and is otherwise left unchanged.

If the device is not already open, QImageReader will attempt to open the device in ReadOnly mode by calling open(). Note that this does not work for certain devices, such as QProcess, QTcpSocket and QUdpSocket, where more logic is required to open the device.

See also device() and setFileName().

### void QImageReader::setFileName(const QString &fileName)

Sets the file name of QImageReader to fileName. Internally, QImageReader will create a QFile object and open it in ReadOnly mode, and use this when reading images.

If fileName does not include a file extension (e.g., .png or .bmp), QImageReader will cycle through all supported extensions until it finds a matching file.

See also fileName(), setDevice(), and supportedImageFormats().

### void QImageReader::setFormat(const QByteArray &format)

Sets the format QImageReader will use when reading images, to format. format is a case insensitive text string. Example:

     QImageReader reader;
     reader.setFormat("png"); // same as reader.setFormat("PNG");

You can call supportedImageFormats() for the full list of formats QImageReader supports.

See also format().

### void QImageReader::setQuality(int quality)

Sets the quality setting of the image format to quality.

Some image formats, in particular lossy ones, entail a tradeoff between a) visual quality of the resulting image, and b) decoding execution time. This function sets the level of that tradeoff for image formats that support it.

In case of scaled image reading, the quality setting may also influence the tradeoff level between visual quality and execution speed of the scaling algorithm.

The value range of quality depends on the image format. For example, the "jpeg" format supports a quality range from 0 (low visual quality) to 100 (high visual quality).

See also quality() and setScaledSize().

### void QImageReader::setScaledClipRect(const QRect &rect)

Sets the scaled clip rect to rect. The scaled clip rect is the clip rect (also known as ROI, or Region Of Interest) that is applied after the image has been scaled.

See also scaledClipRect() and setScaledSize().

### void QImageReader::setScaledSize(const QSize &size)

Sets the scaled size of the image to size. The scaling is performed after the initial clip rect, but before the scaled clip rect is applied. The algorithm used for scaling depends on the image format. By default (i.e., if the image format does not support scaling), QImageReader will use QImage::scale() with Qt::SmoothScaling.

If only one dimension is set in size, the other one will be computed from the image's natural size so as to maintain the aspect ratio.

See also scaledSize(), setClipRect(), and setScaledClipRect().

### QSize QImageReader::size() const

Returns the size of the image, without actually reading the image contents.

If the image format does not support this feature, this function returns an invalid size. Qt's built-in image handlers all support this feature, but custom image format plugins are not required to do so.

See also QImageIOHandler::ImageOption, QImageIOHandler::option(), and QImageIOHandler::supportsOption().

### QByteArray QImageReader::subType() const

Returns the subtype of the image.

### `[static]` QList<QByteArray> QImageReader::supportedImageFormats()

Returns the list of image formats supported by QImageReader.

By default, Qt can read the following formats:

Format | MIME type | Description
--- | --- | ---
BMP | image/bmp | Windows Bitmap
GIF | image/gif | Graphic Interchange Format (optional)
JPG | image/jpeg | Joint Photographic Experts Group
PNG | image/png | Portable Network Graphics
PBM | image/x-portable-bitmap | Portable Bitmap
PGM | image/x-portable-graymap | Portable Graymap
PPM | image/x-portable-pixmap | Portable Pixmap
XBM | image/x-xbitmap | X11 Bitmap
XPM | image/x-xpixmap | X11 Pixmap
SVG | image/svg+xml | Scalable Vector Graphics

Reading and writing SVG files is supported through the Qt SVG module. The Qt Image Formats module provides support for additional image formats.

Note that the QCoreApplication instance must be created before this function is called.

See also setFormat(), QImageWriter::supportedImageFormats(), and QImageIOPlugin.

### `[static]` QList<QByteArray> QImageReader::supportedMimeTypes()

Returns the list of MIME types supported by QImageReader.

Note that the QApplication instance must be created before this function is called.

See also supportedImageFormats() and QImageWriter::supportedMimeTypes().

### QList<QByteArray> QImageReader::supportedSubTypes() const

Returns the list of subtypes supported by an image.

### bool QImageReader::supportsAnimation() const

Returns `true` if the image format supports animation; otherwise, false is returned.

See also QMovie::supportedFormats().

### bool QImageReader::supportsOption(QImageIOHandler::ImageOption option) const

Returns `true` if the reader supports option; otherwise returns false.

Different image formats support different options. Call this function to determine whether a certain option is supported by the current format. For example, the PNG format allows you to embed text into the image's metadata (see text()), and the BMP format allows you to determine the image's size without loading the whole image into memory (see size()).

     QImageReader reader(":/image.png");
     if (reader.supportsOption(QImageIOHandler::Size))
         qDebug() << "Size:" << reader.size();

See also QImageWriter::supportsOption().

### QString QImageReader::text(const QString &key) const

Returns the image text associated with key.

Support for this option is implemented through QImageIOHandler::Description.

See also textKeys() and QImageWriter::setText().

### QStringList QImageReader::textKeys() const

Returns the text keys for this image. You can use these keys with text() to list the image text for a certain key.

Support for this option is implemented through QImageIOHandler::Description.

See also text(), QImageWriter::setText(), and QImage::textKeys().

### QImageIOHandler::Transformations QImageReader::transformation() const

Returns the transformation metadata of the image, including image orientation. If the format does not support transformation metadata, QImageIOHandler::TransformationNone is returned.

See also setAutoTransform() and autoTransform().