内容

• 公共类型
• 公共函数
• 静态公共成员
• 详细描述
• 格式
• 图像的高分辨率版本

QImageReader 类

QImageReader 类提供用于从文件或其它设备读取图像的格式独立接口。 更多...

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

• 所有成员列表，包括继承成员

注意： 此类的所有函数 可重入 .

公共类型

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

公共函数

	QImageReader (const QString & fileName , const QByteArray & format = QByteArray())
	QImageReader (QIODevice * device , const QByteArray & format = QByteArray())
	QImageReader ()
	~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

静态公共成员

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

详细描述

读取图像的最常见方式是透过 QImage and QPixmap 的构造函数，或通过调用 QImage::load () 和 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.

若出现任何错误当读取图像时， read () 将返回 null QImage . You can then call error () 以查找出现的错误类型，或 errorString () 以获得人类可读的什么错误描述。

注意： 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.

格式

调用 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 ().

图像的高分辨率版本

It is possible to provide high resolution versions of images should a scaling between 设备像素 and 设备无关像素 be in effect.

The high resolution version is marked by the suffix @2x on the base name. The image read will have its 设备像素比率 set to a value of 2.

This can be disabled by setting the environment variable QT_HIGHDPI_DISABLE_2X_IMAGE_LOADING .

另请参阅 QImageWriter , QImageIOHandler , QImageIOPlugin , QMimeDatabase , QColorSpace , QImage::devicePixelRatio (), QPixmap::devicePixelRatio (), QIcon , QPainter::drawPixmap ()，和 QPainter::drawImage ().

成员类型文档编制

enum QImageReader:: ImageReaderError

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

常量	值	描述
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 不支持请求的图像格式。
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 .

成员函数文档编制

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

构造 QImageReader 对象采用文件名 fileName 和图像格式 format .

另请参阅 setFileName ().

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

构造 QImageReader 对象采用设备 device 和图像格式 format .

QImageReader:: QImageReader ()

构造空的 QImageReader 对象。 在读取图像之前，请致电xx setDevice () 或 setFileName ().

QImageReader:: ~QImageReader ()

销毁 QImageReader 对象。

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

返回当前分配限制 (以 MB 为单位)。

该函数在 Qt 6.0 引入。

另请参阅 setAllocationLimit ().

bool QImageReader:: autoDetectImageFormat () const

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

另请参阅 setAutoDetectImageFormat ().

[since 5.5] bool QImageReader:: autoTransform () const

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

该函数在 Qt 5.5 引入。

另请参阅 setAutoTransform (), transformation ()，和 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.

另请参阅 setBackgroundColor () 和 read ().

bool QImageReader:: canRead () const

返回 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.

注意： 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.

另请参阅 read (), supportedImageFormats ()，和 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 被返回。

另请参阅 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.

此函数返回 -1，若发生错误。

另请参阅 supportsAnimation (), QImageIOHandler::currentImageNumber ()，和 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.

另请参阅 supportsAnimation () 和 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.

另请参阅 setDecideFormatFromContent ().

QIODevice *QImageReader:: device () const

返回的设备目前被赋值给 QImageReader ，或 nullptr 若没有设备被赋值。

另请参阅 setDevice ().

QImageReader::ImageReaderError QImageReader:: error () const

返回最后出现的错误类型。

另请参阅 ImageReaderError and errorString ().

QString QImageReader:: errorString () const

返回最近发生错误的人类可读描述。

另请参阅 error ().

QString QImageReader:: fileName () const

若目前赋值设备是 QFile ，或者若 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 被返回。

另请参阅 setFileName () 和 setDevice ().

QByteArray QImageReader:: format () const

返回格式 QImageReader 用于读取图像。

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().

另请参阅 setFormat () 和 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.

此函数返回 -1，若发生错误。

另请参阅 supportsAnimation (), QImageIOHandler::imageCount ()，和 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.

另请参阅 QImageIOHandler::ImageOption , QImageIOHandler::option ()，和 QImageIOHandler::supportsOption ().

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

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

另请参阅 QImageReader::autoDetectImageFormat ().

[static, since 5.12] QList < QByteArray > QImageReader:: imageFormatsForMimeType (const QByteArray & mimeType )

Returns the list of image formats corresponding to mimeType .

注意， QGuiApplication 实例必须被创建在调用此函数之前。

该函数在 Qt 5.12 引入。

另请参阅 supportedImageFormats () 和 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.

另请参阅 jumpToNextImage () 和 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.

默认实现调用 read (), then discards the resulting image, but the image handler may have a more efficient way of implementing this operation.

另请参阅 jumpToImage () 和 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 () 将返回 false。

另请参阅 supportsAnimation (), QImageIOHandler::loopCount ()，和 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.

此函数返回 -1，若发生错误。

另请参阅 supportsAnimation (), QImageIOHandler::nextImageDelay ()，和 canRead ().

int QImageReader:: quality () const

返回图像格式的品质设置。

另请参阅 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 () 以查找出现的错误类型，或 errorString () 以获取人类可读错误描述。

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.

另请参阅 canRead (), supportedImageFormats (), supportsAnimation ()，和 QMovie .

bool QImageReader:: read ( QImage * image )

这是重载函数。

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

若 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.

另请参阅 canRead (), supportedImageFormats (), supportsAnimation ()，和 QMovie .

QRect QImageReader:: scaledClipRect () const

返回图像的比例缩放裁剪矩形。

另请参阅 setScaledClipRect ().

QSize QImageReader:: scaledSize () const

返回图像的比例缩放尺寸。

另请参阅 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.

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.

该函数在 Qt 6.0 引入。

另请参阅 allocationLimit ().

void QImageReader:: setAutoDetectImageFormat ( bool enabled )

若 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).

另请参阅 autoDetectImageFormat (), QImageIOHandler::canRead ()，和 QImageIOPlugin::capabilities ().

[since 5.5] void QImageReader:: setAutoTransform ( bool enabled )

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

该函数在 Qt 5.5 引入。

另请参阅 autoTransform (), transformation ()，和 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.

另请参阅 backgroundColor () 和 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 ().

另请参阅 clipRect (), setScaledSize ()，和 setScaledClipRect ().

void QImageReader:: setDecideFormatFromContent ( bool ignored )

若 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.

这还禁用自动检测图像格式。

另请参阅 decideFormatFromContent ().

void QImageReader:: setDevice ( QIODevice * device )

设置 QImageReader 的设备到 device . If a device has already been set, the old device is removed from QImageReader and is otherwise left unchanged.

若设备尚未打开， 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.

另请参阅 device () 和 setFileName ().

void QImageReader:: setFileName (const QString & fileName )

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

若 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.

另请参阅 fileName (), setDevice ()，和 supportedImageFormats ().

void QImageReader:: setFormat (const QByteArray & format )

设置格式 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");
					

可以调用 supportedImageFormats () 了解完整格式列表 QImageReader 支持。

另请参阅 format ().

void QImageReader:: setQuality ( int quality )

将图像格式的品质设置设为 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).

另请参阅 quality () 和 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.

另请参阅 scaledClipRect () 和 setScaledSize ().

void QImageReader:: setScaledSize (const QSize & size )

将图像的比例缩放尺寸设为 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.

另请参阅 scaledSize (), setClipRect ()，和 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.

另请参阅 QImageIOHandler::ImageOption , QImageIOHandler::option ()，和 QImageIOHandler::supportsOption ().

[since 5.4] QByteArray QImageReader:: subType () const

返回图像的子类型。

该函数在 Qt 5.4 引入。

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

返回图像格式列表支持通过 QImageReader .

默认情况下，Qt 可以读取以下格式：

格式	MIME 类型	描述
BMP	image/bmp	Windows 位图
GIF	image/gif	GIF (图形互换格式) 可选
JPG	image/jpeg	JPEG (联合摄影专家组)
PNG	image/png	PNG (便携式网络图形)
PBM	image/x-portable-bitmap	PBM (便携式位图)
PGM	image/x-portable-graymap	PGM (便携式灰度图)
PPM	image/x-portable-pixmap	Portable Pixmap (便携式像素图)
XBM	image/x-xbitmap	X11 Bitmap (X11 位图)
XPM	image/x-xpixmap	X11 Pixmap (X11 像素图)
SVG	image/svg+xml	SVG (可伸缩向量图形)

读写 SVG 文件的支持是透过 Qt SVG 模块。 Qt Image Formats 模块为额外图像格式提供支持。

注意， QApplication 实例必须被创建在调用此函数之前。

另请参阅 setFormat (), QImageWriter::supportedImageFormats ()，和 QImageIOPlugin .

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

返回 MIME (多用途 Internet 邮件扩展) 类型列表支持通过 QImageReader .

注意， QApplication 实例必须被创建在调用此函数之前。

另请参阅 supportedImageFormats () 和 QImageWriter::supportedMimeTypes ().

[since 5.4] QList < QByteArray > QImageReader:: supportedSubTypes () const

返回由图像所支持的子类型列表。

该函数在 Qt 5.4 引入。

bool QImageReader:: supportsAnimation () const

返回 true 若图像格式支持动画；否则，返回 false。

另请参阅 QMovie::supportedFormats ().

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

返回 true 若读取器支持 option ；否则返回 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();
					

另请参阅 QImageWriter::supportsOption ().

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

返回的图像文本关联 key .

支持此选项的实现是透过 QImageIOHandler::Description .

另请参阅 textKeys () 和 QImageWriter::setText ().

QStringList QImageReader:: textKeys () const

Returns the text keys for this image. You can use these keys with text () 列表某个键的图像文本。

支持此选项的实现是透过 QImageIOHandler::Description .

另请参阅 text (), QImageWriter::setText ()，和 QImage::textKeys ().

[since 5.5] 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 被返回。

该函数在 Qt 5.5 引入。

另请参阅 setAutoTransform () 和 autoTransform ().