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The QImageReader class provides a format independent interface for reading images from files or other devices. More...
#include <QImageReader>
Note: All the functions in this class are reentrant.
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.
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. |
Constructs an empty QImageReader object. Before reading an image, call setDevice() or setFileName().
Constructs a QImageReader object with the device device and the image format format.
Constructs a QImageReader object with the file name fileName and the image format format.
See also setFileName().
Destructs the QImageReader object.
Returns true if image format autodetection is enabled on this image reader; otherwise returns false. By default, autodetection is enabled.
See also setAutoDetectImageFormat().
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().
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().
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().
For image formats that support animation, this function returns the sequence number of the current frame. Otherwise, -1 is returned.
See also supportsAnimation() and QImageIOHandler::currentImageNumber().
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().
Returns the device currently assigned to QImageReader, or 0 if no device has been assigned.
See also setDevice().
Returns the type of error that occurred last.
See also ImageReaderError and errorString().
Returns a human readable description of the last error that occurred.
See also error().
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().
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().
For image formats that support animation, this function returns the total number of images in the animation.
Certain animation formats do not support this feature, in which case 0 is returned.
See also supportsAnimation() and QImageIOHandler::imageCount().
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().
If supported, this function returns the image format of the file fileName. Otherwise, an empty string is returned.
If supported, this function returns the image format of the device device. Otherwise, an empty string is returned.
See also QImageReader::autoDetectImageFormat().
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().
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().
For image formats that support animation, this function returns the number of times the animation should loop. Otherwise, it returns -1.
See also supportsAnimation() and QImageIOHandler::loopCount().
For image formats that support animation, this function returns the number of milliseconds to wait until displaying the next frame in the animation. Otherwise, 0 is returned.
See also supportsAnimation() and QImageIOHandler::nextImageDelay().
Returns the quality level of the image.
This function was introduced in Qt 4.2.
See also setQuality().
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.
This is an overloaded member function, provided for convenience.
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.
Returns the scaled clip rect of the image.
See also setScaledClipRect().
Returns the scaled size of the image.
See also setScaledSize().
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:
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().
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().
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().
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().
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().
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().
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().
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().
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().
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().
Returns the list of image formats supported by QImageReader.
By default, Qt can read the following formats:
| Format | Description |
|---|---|
| BMP | Windows Bitmap |
| GIF | Graphic Interchange Format (optional) |
| JPG | Joint Photographic Experts Group |
| JPEG | Joint Photographic Experts Group |
| MNG | Multiple-image Network Graphics |
| PNG | Portable Network Graphics |
| PBM | Portable Bitmap |
| PGM | Portable Graymap |
| PPM | Portable Pixmap |
| TIFF | Tagged Image File Format |
| XBM | X11 Bitmap |
| XPM | X11 Pixmap |
Reading and writing SVG files is supported through Qt's SVG Module.
To configure Qt with GIF support, pass -qt-gif to the configure script or check the appropriate option in the graphical installer.
See also setFormat(), QImageWriter::supportedImageFormats(), and QImageIOPlugin.
Returns true if the image format supports animation; otherwise, false is returned.
This function was introduced in Qt 4.1.
See also QMovie::supportedFormats().
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().
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().
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().
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