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QImageWriter Class Reference
[QtGui module]

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

 #include <QImageWriter>

Note: All functions in this class are reentrant.


Public Types

enum ImageWriterError { DeviceError, UnsupportedFormatError, 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
ImageWriterError error () const
QString errorString () const
QString fileName () const
QByteArray format () const
float gamma () 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 setQuality ( int quality )
void setText ( const QString & key, const QString & text )
bool supportsOption ( QImageIOHandler::ImageOption option ) const
bool write ( const QImage & image )

Static Public Members

QList<QByteArray> supportedImageFormats ()

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::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().

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().

int QImageWriter::quality () const

Returns the quality level of the image.

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::setQuality ( int quality )

This is an image format specific function that sets the quality level of the image to quality. For image formats that do not support setting the quality, 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 quality, high compression) to 100 (high quality, low compression).

See also quality().

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().

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

Returns the list of image formats supported by QImageWriter.

By default, Qt can write the following formats:

FormatDescription
BMPWindows Bitmap
JPGJoint Photographic Experts Group
JPEGJoint Photographic Experts Group
PNGPortable Network Graphics
PPMPortable Pixmap
TIFFTagged Image File Format
XBMX11 Bitmap
XPMX11 Pixmap

Reading and writing SVG files is supported through Qt's SVG Module.

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

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().

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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Qt 4.6.3