QImageReader Class Reference

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

 #include <QImageReader>

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
QColor backgroundColor () const
bool canRead () const
QRect clipRect () const
int currentImageNumber () const
QRect currentImageRect () const
bool decideFormatFromContent () const
QIODevice * device () const
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 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
bool supportsAnimation () const
bool supportsOption ( QImageIOHandler::ImageOption option ) const
QString text ( const QString & key ) const
QStringList textKeys () const

Static Public Members

QByteArray imageFormat ( const QString & fileName )
QByteArray imageFormat ( QIODevice * device )
QList<QByteArray> supportedImageFormats ()

Detailed Description

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

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.

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.

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().

See also QImageWriter, QImageIOHandler, and QImageIOPlugin.

Member Type Documentation

enum QImageReader::ImageReaderError

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

ConstantValueDescription
QImageReader::FileNotFoundError1QImageReader 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::DeviceError2QImageReader encountered a device error when reading the image. You can consult your particular device for more details on what went wrong.
QImageReader::UnsupportedFormatError3Qt does not support the requested image format.
QImageReader::InvalidDataError4The image data was invalid, and QImageReader was unable to read an image from it. The can happen if the image file is damaged.
QImageReader::UnknownError0An 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().

QImageReader::QImageReader ( QIODevice * device, const QByteArray & format = QByteArray() )

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

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().

QImageReader::~QImageReader ()

Destructs the QImageReader object.

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().

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.

This function was introduced in Qt 4.1.

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.

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

See also read() and supportedImageFormats().

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 0 if no device has been assigned.

See also setDevice().

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.

This function was introduced in Qt 4.5.

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

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

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

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

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

See also QImageReader::autoDetectImageFormat().

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 level of the image.

This function was introduced in Qt 4.2.

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 )

This is an overloaded function.

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.

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().

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::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.

This function was introduced in Qt 4.1.

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 QIODevice::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 QIODevice::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 )

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).

This function was introduced in Qt 4.2.

See also quality().

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.

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().

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

Returns the list of image formats supported by QImageReader.

By default, Qt can read the following formats:

FormatDescription
BMPWindows Bitmap
GIFGraphic Interchange Format (optional)
JPGJoint Photographic Experts Group
JPEGJoint Photographic Experts Group
MNGMultiple-image Network Graphics
PNGPortable Network Graphics
PBMPortable Bitmap
PGMPortable Graymap
PPMPortable Pixmap
TIFFTagged Image File Format
XBMX11 Bitmap
XPMX11 Pixmap
SVGScalable Vector Graphics
TGATarga Image Format

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

TGA support only extends to reading non-RLE compressed files. In particular calls to capabilities for the tga plugin returns only QImageIOPlugin::CanRead, not QImageIOPlugin::CanWrite.

To configure Qt with GIF support, pass -qt-gif to the configure script or check the appropriate option in the graphical installer.

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

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

bool QImageReader::supportsAnimation () const

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

This function was introduced in Qt 4.1.

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();

This function was introduced in Qt 4.2.

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.

This function was introduced in Qt 4.1.

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.

This function was introduced in Qt 4.1.

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

Notes provided by the Qt Community

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