QImageWriter Class

The QImageWriter class provides a format independent interface for writing images to files or other devices. More...

Header: #include <QImageWriter>
qmake: QT += gui

Note: All functions in this class are reentrant.

Public Types

enum ImageWriterError { DeviceError, UnsupportedFormatError, InvalidImageError, UnknownError }

Public Functions

QImageWriter()
QImageWriter(QIODevice *device, const QByteArray &format)
QImageWriter(const QString &fileName, const QByteArray &format = QByteArray())
~QImageWriter()
bool canWrite() const
int compression() const
QIODevice *device() const
QImageWriter::ImageWriterError error() const
QString errorString() const
QString fileName() const
QByteArray format() const
float gamma() const
bool optimizedWrite() const
bool progressiveScanWrite() const
int quality() const
void setCompression(int compression)
void setDevice(QIODevice *device)
void setFileName(const QString &fileName)
void setFormat(const QByteArray &format)
void setGamma(float gamma)
void setOptimizedWrite(bool optimize)
void setProgressiveScanWrite(bool progressive)
void setQuality(int quality)
void setSubType(const QByteArray &type)
void setText(const QString &key, const QString &text)
void setTransformation(QImageIOHandler::Transformations transform)
QByteArray subType() const
QList<QByteArray> supportedSubTypes() const
bool supportsOption(QImageIOHandler::ImageOption option) const
QImageIOHandler::Transformations transformation() const
bool write(const QImage &image)

Static Public Members

QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType)
QList<QByteArray> supportedImageFormats()
QList<QByteArray> supportedMimeTypes()

Detailed Description

The QImageWriter class provides a format independent interface for writing images to files or other devices.

QImageWriter supports setting format specific options, such as the gamma level, compression level and quality, prior to storing the image. If you do not need such options, you can use QImage::save() or QPixmap::save() instead.

To store an image, you start by constructing a QImageWriter object. Pass either a file name or a device pointer, and the image format to QImageWriter's constructor. You can then set several options, such as the gamma level (by calling setGamma()) and quality (by calling setQuality()). canWrite() returns true if QImageWriter can write the image (i.e., the image format is supported and the device is open for writing). Call write() to write the image to the device.

If any error occurs when writing the image, write() will return false. 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.

Call supportedImageFormats() for a list of formats that QImageWriter can write. QImageWriter supports all built-in image formats, in addition to any image format plugins that support writing.

See also QImageReader, QImageIOHandler, and QImageIOPlugin.

Member Type Documentation

enum QImageWriter::ImageWriterError

This enum describes errors that can occur when writing images with QImageWriter.

ConstantValueDescription
QImageWriter::DeviceError1QImageWriter encountered a device error when writing the image data. Consult your device for more details on what went wrong.
QImageWriter::UnsupportedFormatError2Qt does not support the requested image format.
QImageWriter::InvalidImageError3An attempt was made to write an invalid QImage. An example of an invalid image would be a null QImage.
QImageWriter::UnknownError0An unknown error occurred. If you get this value after calling write(), it is most likely caused by a bug in QImageWriter.

Member Function Documentation

QImageWriter::QImageWriter()

Constructs an empty QImageWriter object. Before writing, you must call setFormat() to set an image format, then setDevice() or setFileName().

QImageWriter::QImageWriter(QIODevice *device, const QByteArray &format)

Constructs a QImageWriter object using the device device and image format format.

QImageWriter::QImageWriter(const QString &fileName, const QByteArray &format = QByteArray())

Constructs a QImageWriter objects that will write to a file with the name fileName, using the image format format. If format is not provided, QImageWriter will detect the image format by inspecting the extension of fileName.

QImageWriter::~QImageWriter()

Destructs the QImageWriter object.

bool QImageWriter::canWrite() const

Returns true if QImageWriter can write the image; i.e., the image format is supported and the assigned device is open for reading.

See also write(), setDevice(), and setFormat().

int QImageWriter::compression() const

Returns the compression of the image.

See also setCompression().

QIODevice *QImageWriter::device() const

Returns the device currently assigned to QImageWriter, or 0 if no device has been assigned.

See also setDevice().

QImageWriter::ImageWriterError QImageWriter::error() const

Returns the type of error that last occurred.

See also ImageWriterError and errorString().

QString QImageWriter::errorString() const

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

See also error().

QString QImageWriter::fileName() const

If the currently assigned device is a QFile, or if setFileName() has been called, this function returns the name of the file QImageWriter writes to. 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 QImageWriter::format() const

Returns the format QImageWriter uses for writing images.

See also setFormat().

float QImageWriter::gamma() const

Returns the gamma level of the image.

See also setGamma().

[static] QList<QByteArray> QImageWriter::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.

This function was introduced in Qt 5.12.

See also supportedImageFormats() and supportedMimeTypes().

bool QImageWriter::optimizedWrite() const

Returns whether optimization has been turned on for writing the image.

This function was introduced in Qt 5.5.

See also setOptimizedWrite().

bool QImageWriter::progressiveScanWrite() const

Returns whether the image should be written as a progressive image.

This function was introduced in Qt 5.5.

See also setProgressiveScanWrite().

int QImageWriter::quality() const

Returns the quality setting of the image format.

See also setQuality().

void QImageWriter::setCompression(int compression)

This is an image format specific function that set the compression of an image. For image formats that do not support setting the compression, this value is ignored.

The value range of compression depends on the image format. For example, the "tiff" format supports two values, 0(no compression) and 1(LZW-compression).

See also compression().

void QImageWriter::setDevice(QIODevice *device)

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

If the device is not already open, QImageWriter will attempt to open the device in QIODevice::WriteOnly 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 QImageWriter::setFileName(const QString &fileName)

Sets the file name of QImageWriter to fileName. Internally, QImageWriter will create a QFile and open it in QIODevice::WriteOnly mode, and use this file when writing images.

See also fileName() and setDevice().

void QImageWriter::setFormat(const QByteArray &format)

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

QImageWriter writer;
writer.setFormat("png"); // same as writer.setFormat("PNG");

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

See also format().

void QImageWriter::setGamma(float gamma)

This is an image format specific function that sets the gamma level of the image to gamma. For image formats that do not support setting the gamma level, this value is ignored.

The value range of gamma depends on the image format. For example, the "png" format supports a gamma range from 0.0 to 1.0.

See also gamma() and quality().

void QImageWriter::setOptimizedWrite(bool optimize)

This is an image format-specific function which sets the optimize flags when writing images. For image formats that do not support setting an optimize flag, this value is ignored.

The default is false.

This function was introduced in Qt 5.5.

See also optimizedWrite().

void QImageWriter::setProgressiveScanWrite(bool progressive)

This is an image format-specific function which turns on progressive scanning when writing images. For image formats that do not support setting a progressive scan flag, this value is ignored.

The default is false.

This function was introduced in Qt 5.5.

See also progressiveScanWrite().

void QImageWriter::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) encoding execution time and compression level. This function sets the level of that tradeoff for image formats that support it. For other formats, this value is ignored.

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

See also quality().

void QImageWriter::setSubType(const QByteArray &type)

This is an image format specific function that sets the subtype of the image to type. Subtype can be used by a handler to determine which format it should use while saving the image.

For example, saving an image in DDS format with A8R8G8R8 subtype:

QImageWriter writer("some/image.dds");
if (writer.supportsOption(QImageIOHandler::SubType))
    writer.setSubType("A8R8G8B8");
writer.write(image);

This function was introduced in Qt 5.4.

See also subType().

void QImageWriter::setText(const QString &key, const QString &text)

Sets the image text associated with the key key to text. This is useful for storing copyright information or other information about the image. Example:

QImage image("some/image.jpeg");
QImageWriter writer("images/outimage.png", "png");
writer.setText("Author", "John Smith");
writer.write(image);

If you want to store a single block of data (e.g., a comment), you can pass an empty key, or use a generic key like "Description".

The key and text will be embedded into the image data after calling write().

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

This function was introduced in Qt 4.1.

See also QImage::setText() and QImageReader::text().

void QImageWriter::setTransformation(QImageIOHandler::Transformations transform)

Sets the image transformations metadata including orientation to transform.

If transformation metadata is not supported by the image format, the transform is applied before writing.

This function was introduced in Qt 5.5.

See also transformation() and write().

QByteArray QImageWriter::subType() const

Returns the subtype of the image.

This function was introduced in Qt 5.4.

See also setSubType().

[static] QList<QByteArray> QImageWriter::supportedImageFormats()

Returns the list of image formats supported by QImageWriter.

By default, Qt can write the following formats:

FormatMIME typeDescription
BMPimage/bmpWindows Bitmap
JPGimage/jpegJoint Photographic Experts Group
PNGimage/pngPortable Network Graphics
PBMimage/x-portable-bitmapPortable Bitmap
PGMimage/x-portable-graymapPortable Graymap
PPMimage/x-portable-pixmapPortable Pixmap
XBMimage/x-xbitmapX11 Bitmap
XPMimage/x-xpixmapX11 Pixmap

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 QApplication instance must be created before this function is called.

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

[static] QList<QByteArray> QImageWriter::supportedMimeTypes()

Returns the list of MIME types supported by QImageWriter.

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

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

QList<QByteArray> QImageWriter::supportedSubTypes() const

Returns the list of subtypes supported by an image.

This function was introduced in Qt 5.4.

bool QImageWriter::supportsOption(QImageIOHandler::ImageOption option) const

Returns true if the writer 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()).

QImageWriter writer(fileName);
if (writer.supportsOption(QImageIOHandler::Description))
    writer.setText("Author", "John Smith");

Options can be tested after the writer has been associated with a format.

This function was introduced in Qt 4.2.

See also QImageReader::supportsOption() and setFormat().

QImageIOHandler::Transformations QImageWriter::transformation() const

Returns the transformation and orientation the image has been set to written with.

This function was introduced in Qt 5.5.

See also setTransformation().

bool QImageWriter::write(const QImage &image)

Writes the image image to the assigned device or file name. Returns true on success; otherwise returns false. If the operation fails, you can call error() to find the type of error that occurred, or errorString() to get a human readable description of the error.

See also canWrite(), error(), and errorString().

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