QImageReader Class

Call https://() for a list of formats that https:// can read. https:// supports all built-in image formats, in addition to any image format plugins that support reading. Call https://() to obtain a list of supported MIME types, which for example can be passed to https://().

https:// 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 https://().

It is possible to provide high resolution versions of images should a scaling between device pixels and device independent pixels be in effect.

The high resolution version is marked by the suffix @2x on the base name. The image read will have its device pixel ratio set to a value of 2.

This can be disabled by setting the environment variable QT_HIGHDPI_DISABLE_2X_IMAGE_LOADING.

See also https://, https://, https://, https://, https://, QImage::devicePixelRatio(), https://(), https://, https://(), https://()

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

Constructs an empty https:// object. Before reading an image, call https://() or https://().

Constructs a https:// object with the device device and the image format format.

Constructs a https:// object with the file name fileName and the image format format.

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Destructs the https:// object.

Returns the current allocation limit, in megabytes.

This function was introduced in Qt 6.0.

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

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Returns true if the image handler will apply transformation metadata on https://().

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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.

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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. https://() 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.

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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 https:// is returned.

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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.

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For image formats that support animation, this function returns the rect for the current frame. Otherwise, a null rect is returned.

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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.

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Returns the device currently assigned to https://, or nullptr if no device has been assigned.

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Returns the type of error that occurred last.

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

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If the currently assigned device is a https://, or if https://() has been called, this function returns the name of the file https:// reads from. Otherwise (i.e., if no device has been assigned or the device is not a https://), an empty https:// is returned.

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Returns the format https:// 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().

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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.

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Returns the format of the image, without actually reading the image contents. The format describes the image format https://() returns, not the format of the actual image.

If the image format does not support this feature, this function returns an invalid format.

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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.

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Returns the list of image formats corresponding to mimeType.

Note that the https:// instance must be created before this function is called.

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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 https://() will attempt to read this image.

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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 https://(), then discards the resulting image, but the image handler may have a more efficient way of implementing this operation.

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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, https://() will return false.

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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.

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Returns the quality setting of the image format.

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Reads an image from the device. On success, the image that was read is returned; otherwise, a null https:// is returned. You can then call https://() to find the type of error that occurred, or https://() 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.

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This is an overloaded function.

Reads an image from the device into image, which must point to a https://. 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 https://() 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 https://() repeatedly will return the next frame. When all frames have been read, a null image will be returned.

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Returns the scaled clip rect of the image.

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Returns the scaled size of the image.

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Sets the allocation limit to mbLimit megabytes. Images that would require a https:// memory allocation above this limit will be rejected. If mbLimit is 0, the allocation size check will be disabled.

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.

At runtime, this value may be overridden by the environment variable QT_IMAGEIO_MAXALLOC.

This function was introduced in Qt 6.0.

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If enabled is true, image format autodetection is enabled; otherwise, it is disabled. By default, autodetection is enabled.

https:// uses an extensive approach to detecting the image format; firstly, if you pass a file name to https://, 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. https:// 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, https:// will report failure when trying to read the image.

By disabling image format autodetection, https:// will only query the plugins and built-in handlers based on the format string (i.e., no file name extensions are tested).

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

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Sets the background color to color. Image formats that support this operation are expected to initialize the background to color before reading an image.

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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 https://().

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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.

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

If the device is not already open, https:// will attempt to open the device in https:// mode by calling open(). Note that this does not work for certain devices, such as https://, https:// and https://, where more logic is required to open the device.

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Sets the file name of https:// to fileName. Internally, https:// will create a https:// object and open it in https:// mode, and use this when reading images.

If fileName does not include a file extension (e.g., .png or .bmp), https:// will cycle through all supported extensions until it finds a matching file.

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Sets the format https:// 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 https://() for the full list of formats https:// supports.

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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) 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).

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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.

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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), https:// will use QImage::scale() with Qt::SmoothScaling.

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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.

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Returns the subtype of the image.

Returns the list of image formats supported by https://.

By default, Qt can read the following formats:

Reading and writing SVG files is supported through the https:// module. The https:// module provides support for additional image formats.

Note that the https:// instance must be created before this function is called.

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Returns the list of MIME types supported by https://.

Note that the https:// instance must be created before this function is called.

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Returns the list of subtypes supported by an image.

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

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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 https://()), and the BMP format allows you to determine the image's size without loading the whole image into memory (see https://()).

QImageReader reader(":/image.png");
if (reader.supportsOption(QImageIOHandler::Size))
    qDebug() << "Size:" << reader.size();

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Returns the image text associated with key.

Support for this option is implemented through https://.

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Returns the text keys for this image. You can use these keys with https://() to list the image text for a certain key.

Support for this option is implemented through https://.

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Returns the transformation metadata of the image, including image orientation. If the format does not support transformation metadata, https:// is returned.

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