QImage Class Reference
^{[QtGui module]}

Image Transformations

QImage supports a number of functions for creating a new image that is a transformed version of the original: The createAlphaMask() function builds and returns a 1-bpp mask from the alpha buffer in this image, and the createHeuristicMask() function creates and returns a 1-bpp heuristic mask for this image. The latter function works by selecting a color from one of the corners, then chipping away pixels of that color starting at all the edges.

The mirrored() function returns a mirror of the image in the desired direction, the scaled() returns a copy of the image scaled to a rectangle of the desired measures, the rgbSwapped() fucntion constructs a BGR image from a RGB image, and the alphaChannel() function constructs an image from this image's alpha channel.

The scaledToWidth() and scaledToHeight() functions return scaled copies of the image.

The transformed() function returns a copy of the image that is transformed with the given transformation matrix and transformation mode: Internally, the transformation matrix is adjusted to compensate for unwanted translation, i.e. transformed() returns the smallest image containing all transformed points of the original image. The static trueMatrix() function returns the actual matrix used for transforming the image.

There are also functions for changing attributes of an image in-place:

See also QImageReader, QImageWriter, and QPixmap.

Member Type Documentation

enum QImage::Format

The following image formats are available in all versions of Qt:

The following image format is specific to Qtopia Core:

See also format() and convertToFormat().

enum QImage::InvertMode

This enum type is used to describe how pixel values should be inverted in the invertPixels() function.

See also invertPixels().

Member Function Documentation

QImage::QImage ()

Constructs a null image.

See also isNull().

QImage::QImage ( const QSize & size, Format format )

Constructs an image with the given size and format.

QImage::QImage ( int width, int height, Format format )

Constructs an image with the given width, height and format.

QImage::QImage ( uchar * data, int width, int height, Format format )

Constructs an image with the given width, height and format, that uses an existing memory buffer, data. The width and height must be specified in pixels, and that data must be 32-bit aligned.

The buffer must remain valid throughout the life of the QImage. The image does not delete the buffer at destruction.

If the image is in an indexed color format, set the color table for the image using setColorTable().

QImage::QImage ( const char * const[] xpm )

Constructs an image from the given xpm image.

Make sure that the image is a valid XPM image. Errors are silently ignored.

Note that it's possible to squeeze the XPM variable a little bit by using an unusual declaration:

    static const char * const start_xpm[] = {
        "16 15 8 1",
        "a c #cec6bd",
    ....

The extra const makes the entire definition read-only, which is slightly more efficient (e.g. when the code is in a shared library) and ROMable when the application is to be stored in ROM.

QImage::QImage ( const QString & fileName, const char * format = 0 )

Constructs an image and tries to load the image from the file with the given fileName.

The loader attempts to read the image using the specified format. If the format is not specified (which is the default), the loader probes the file for a header to guess the file format.

If the loading of the image failed, this object is a null image.

The file name can either refer to an actual file on disk or to one of the application's embedded resources. See the Resource System overview for details on how to embed images and other resource files in the application's executable.

See also isNull() and Reading and Writing Image Files.

QImage::QImage ( const char * fileName, const char * format = 0 )

Constructs an image and tries to load the image from the file with the given fileName.

The loader attempts to read the image using the specified format. If the format is not specified (which is the default), the loader probes the file for a header to guess the file format.

If the loading of the image failed, this object is a null image.

The file name can either refer to an actual file on disk or to one of the application's embedded resources. See the Resource System overview for details on how to embed images and other resource files in the application's executable.

You can disable this constructor by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful, for example, if you want to ensure that all user-visible strings go through QObject::tr().

See also QString::fromAscii(), isNull(), and Reading and Writing Image Files.

QImage::QImage ( const QImage & image )

Constructs a shallow copy of the given image.

For more information about shallow copies, see the Implicit Data Sharing documentation.

See also copy().

QImage::~QImage ()

Destroys the image and cleans up.

bool QImage::allGray () const

Returns true if all the colors in the image are shades of gray (i.e. their red, green and blue components are equal); otherwise false.

Note that this function is slow for images without color table.

See also isGrayscale().

QImage QImage::alphaChannel () const

Extracts the alpha channel from this image as an 8 bit gray scale image and returns it.

See also setAlphaChannel(), hasAlphaChannel(), and Image Transformations.

uchar * QImage::bits ()

Returns a pointer to the first pixel data. This is equivalent to scanLine(0).

See also scanLine() and numBytes().

const uchar * QImage::bits () const

This is an overloaded member function, provided for convenience.

int QImage::bytesPerLine () const

Returns the number of bytes per image scanline.

This is equivalent to numBytes()/ height().

See also scanLine().

QRgb QImage::color ( int i ) const

Returns the color in the color table at index i. The first color is at index 0.

The colors in an image's color table are specified as ARGB quadruplets (QRgb). Use the qAlpha(), qRed(), qGreen(), and qBlue() functions to get the color value components.

See also setColor(), pixelIndex(), and Pixel Manipulation.

QVector<QRgb> QImage::colorTable () const

Returns a list of the colors contained in the image's color table, or an empty list if the image does not have a color table

See also setColorTable(), numColors(), and color().

QImage QImage::convertToFormat ( Format format, Qt::ImageConversionFlags flags = Qt::AutoColor ) const

Returns a copy of the image in the given format.

The specified image conversion flags control how the image data is handled during the conversion process.

See also Image Format.

QImage QImage::convertToFormat ( Format format, const QVector<QRgb> & colorTable, Qt::ImageConversionFlags flags = Qt::AutoColor ) const

This is an overloaded member function, provided for convenience.

Returns a copy of the image converted to the given format, using the specified colorTable.

QImage QImage::copy ( const QRect & rectangle = QRect() ) const

Returns a sub-area of the image as a new image.

The returned image is copied from the position (rectangle.x(), rectangle.y()) in this image, and will always have the size of the given rectangle.

In areas beyond this image, pixels are set to 0. For 32-bit RGB images, this means black; for 32-bit ARGB images, this means transparent black; for 8-bit images, this means the color with index 0 in the color table which can be anything; for 1-bit images, this means Qt::color0.

If the given rectangle is a null rectangle the entire image is copied.

See also QImage().

QImage QImage::copy ( int x, int y, int width, int height ) const

This is an overloaded member function, provided for convenience.

The returned image is copied from the position (x, y) in this image, and will always have the given width and height. In areas beyond this image, pixels are set to 0.

QImage QImage::createAlphaMask ( Qt::ImageConversionFlags flags = Qt::AutoColor ) const

Builds and returns a 1-bpp mask from the alpha buffer in this image. Returns a null image if the image's format is QImage::Format_RGB32.

The flags argument is a bitwise-OR of the Qt::ImageConversionFlags, and controls the conversion process. Passing 0 for flags sets all the default options.

The returned image has little-endian bit order (i.e. the image's format is QImage::Format_MonoLSB), which you can convert to big-endian (QImage::Format_Mono) using the convertToFormat() function.

See also createHeuristicMask() and Image Transformations.

QImage QImage::createHeuristicMask ( bool clipTight = true ) const

Creates and returns a 1-bpp heuristic mask for this image.

The function works by selecting a color from one of the corners, then chipping away pixels of that color starting at all the edges. The four corners vote for which color is to be masked away. In case of a draw (this generally means that this function is not applicable to the image), the result is arbitrary.

The returned image has little-endian bit order (i.e. the image's format is QImage::Format_MonoLSB), which you can convert to big-endian (QImage::Format_Mono) using the convertToFormat() function.

If clipTight is true (the default) the mask is just large enough to cover the pixels; otherwise, the mask is larger than the data pixels.

Note that this function disregards the alpha buffer.

See also createAlphaMask() and Image Transformations.

int QImage::depth () const

Returns the depth of the image.

The image depth is the number of bits used to encode a single pixel, also called bits per pixel (bpp) or bit planes of an image.

The supported depths are 1, 8 and 32.

See also convertToFormat(), Image Formats, and Image Information.

int QImage::dotsPerMeterX () const

Returns the number of pixels that fit horizontally in a physical meter. Together with dotsPerMeterY(), this number defines the intended scale and aspect ratio of the image.

See also setDotsPerMeterX() and Image Information.

int QImage::dotsPerMeterY () const

Returns the number of pixels that fit vertically in a physical meter. Together with dotsPerMeterX(), this number defines the intended scale and aspect ratio of the image.

See also setDotsPerMeterY() and Image Information.

void QImage::fill ( uint pixelValue )

Fills the entire image with the given pixelValue.

If the depth of this image is 1, only the lowest bit is used. If you say fill(0), fill(2), etc., the image is filled with 0s. If you say fill(1), fill(3), etc., the image is filled with 1s. If the depth is 8, the lowest 8 bits are used and if the depth is 16 the lowest 16 bits are used.

Note: QImage::pixel() returns the color of the pixel at the given coordinates while QColor::pixel() returns the pixel value of the underlying window system (essentially an index value), so normally you will want to use QImage::pixel() to use a color from an existing image or QColor::rgb() to use a specific color.

See also depth() and Image Transformations.

Format QImage::format () const

Returns the format of the image.

See also Image Formats.

QImage QImage::fromData ( const uchar * data, int size, const char * format = 0 )   [static]

Constructs a QImage from the first size bytes of the given binary data. the loader attempts to read the image using the specified format. If the format is not specified (which is the default), the loader probes the file for a header to guess the file format.

If the loading of the image failed, this object is a null image.

See also load(), save(), and Reading and Writing Image Files.

QImage QImage::fromData ( const QByteArray & data, const char * format = 0 )   [static]

This is an overloaded member function, provided for convenience.

Loads an image from the given QByteArray data.

bool QImage::hasAlphaChannel () const

Returns true if the image has a format that respects the alpha channel, otherwise returns false.

See also alphaChannel() and Image Information.

int QImage::height () const

Returns the height of the image.

See also Image Information.

void QImage::invertPixels ( InvertMode mode = InvertRgb )

Inverts all pixel values in the image.

The given invert mode only have a meaning when the image's depth is 32. The default mode is InvertRgb, which leaves the alpha channel unchanged. If the mode is InvertRgba, the alpha bits are also inverted.

Inverting an 8-bit image means to replace all pixels using color index i with a pixel using color index 255 minus i. The same is the case for a 1-bit image. Note that the color table is not changed.

See also Image Transformations.

bool QImage::isGrayscale () const

For 32-bit images, this function is equivalent to allGray().

For 8-bpp images, this function returns true if color(i) is QRgb(i, i, i) for all indexes of the color table; otherwise returns false.

See also allGray() and Image Formats.

bool QImage::isNull () const

Returns true if it is a null image, otherwise returns false.

A null image has all parameters set to zero and no allocated data.

bool QImage::load ( const QString & fileName, const char * format = 0 )

Loads an image from the file with the given fileName. Returns true if the image was successfully loaded; otherwise returns false.

The loader attempts to read the image using the specified format. If the format is not specified (which is the default), the loader probes the file for a header to guess the file format.

The file name can either refer to an actual file on disk or to one of the application's embedded resources. See the Resource System overview for details on how to embed images and other resource files in the application's executable.

See also Reading and Writing Image Files.

bool QImage::load ( QIODevice * device, const char * format )

This is an overloaded member function, provided for convenience.

This function reads a QImage from the given device. This can, for example, be used to load an image directly into a QByteArray.

bool QImage::loadFromData ( const uchar * data, int len, const char * format = 0 )

Loads an image from the first len bytes of the given binary data. Returns true if the image was successfully loaded; otherwise returns false.

The loader attempts to read the image using the specified format. If the format is not specified (which is the default), the loader probes the file for a header to guess the file format.

See also Reading and Writing Image Files.

bool QImage::loadFromData ( const QByteArray & data, const char * format = 0 )

This is an overloaded member function, provided for convenience.

Loads an image from the given QByteArray data.

QImage QImage::mirrored ( bool horizontal = false, bool vertical = true ) const

Returns a mirror of the image, mirrored in the horizontal and/or the vertical direction depending on whether horizontal and vertical are set to true or false.

Note that the original image is not changed.

See also Image Transformations.

int QImage::numBytes () const

Returns the number of bytes occupied by the image data.

See also bytesPerLine(), bits(), and Image Information.

int QImage::numColors () const

Returns the size of the color table for the image.

Notice that numColors() returns 0 for 32-bpp images because these images do not use color tables, but instead encode pixel values as ARGB quadruplets.

See also setNumColors() and Image Information.

QPoint QImage::offset () const

Returns the number of pixels by which the image is intended to be offset by when positioning relative to other images.

See also setOffset() and Image Information.

QRgb QImage::pixel ( int x, int y ) const

Returns the color of the pixel at the given coordinates.

If (x, y) is not valid, the results are undefined.

See also setPixel(), valid(), and Pixel Manipulation.

int QImage::pixelIndex ( int x, int y ) const

Returns the pixel index at the given coordinates.

If (x, y) is not valid, or if the image is not a paletted image (depth() > 8), the results are undefined.

See also valid(), depth(), and Pixel Manipulation.

QRect QImage::rect () const

Returns the enclosing rectangle (0, 0, width(), height()) of the image.

See also Image Information.

QImage QImage::rgbSwapped () const

Returns a QImage in which the values of the red and blue components of all pixels have been swapped, effectively converting an RGB image to an BGR image.

The original QImage is not changed.

See also Image Transformations.

bool QImage::save ( const QString & fileName, const char * format, int quality = -1 ) const

Saves the image to the file with the given fileName, using the given image file format and quality factor.

The quality factor must be in the range 0 to 100 or -1. Specify 0 to obtain small compressed files, 100 for large uncompressed files, and -1 (the default) to use the default settings.

Returns true if the image was successfully saved; otherwise returns false.

See also Reading and Writing Image Files.

bool QImage::save ( QIODevice * device, const char * format, int quality = -1 ) const

This is an overloaded member function, provided for convenience.

This function writes a QImage to the given device.

This can, for example, be used to save an image directly into a QByteArray:

            QImage image;
            QByteArray ba;
            QBuffer buffer(&ba);
            buffer.open(QIODevice::WriteOnly);
            image.save(&buffer, "PNG"); // writes image into ba in PNG format

QImage QImage::scaled ( const QSize & size, Qt::AspectRatioMode aspectRatioMode = Qt::IgnoreAspectRatio, Qt::TransformationMode transformMode = Qt::FastTransformation ) const

Returns a copy of the image scaled to a rectangle defined by the given size according to the given aspectRatioMode and transformMode.

• If aspectRatioMode is Qt::IgnoreAspectRatio, the image is scaled to size.
• If aspectRatioMode is Qt::KeepAspectRatio, the image is scaled to a rectangle as large as possible inside size, preserving the aspect ratio.
• If aspectRatioMode is Qt::KeepAspectRatioByExpanding, the image is scaled to a rectangle as small as possible outside size, preserving the aspect ratio.

If the given size is empty, this function returns a null image.

See also isNull() and Image Transformations.

QImage QImage::scaled ( int width, int height, Qt::AspectRatioMode aspectRatioMode = Qt::IgnoreAspectRatio, Qt::TransformationMode transformMode = Qt::FastTransformation ) const

This is an overloaded member function, provided for convenience.

Returns a copy of the image scaled to a rectangle with the given width and height according to the given aspectRatioMode and transformMode.

If either the width or the height is zero or negative, this function returns a null image.

QImage QImage::scaledToHeight ( int height, Qt::TransformationMode mode = Qt::FastTransformation ) const

Returns a scaled copy of the image. The returned image is scaled to the given height using the specified transformation mode.

This function automatically calculates the width of the image so that the ratio of the image is preserved.

If the given height is 0 or negative, a null image is returned.

See also Image Transformations.

QImage QImage::scaledToWidth ( int width, Qt::TransformationMode mode = Qt::FastTransformation ) const

Returns a scaled copy of the image. The returned image is scaled to the given width using the specified transformation mode.

This function automatically calculates the height of the image so that its aspect ratio is preserved.

If the given width is 0 or negative, a null image is returned.

See also Image Transformations.

uchar * QImage::scanLine ( int i )

Returns a pointer to the pixel data at the scanline with index i. The first scanline is at index 0.

The scanline data is aligned on a 32-bit boundary.

Warning: If you are accessing 32-bpp image data, cast the returned pointer to QRgb* (QRgb has a 32-bit size) and use it to read/write the pixel value. You cannot use the uchar* pointer directly, because the pixel format depends on the byte order on the underlying platform. Use qRed(), qGreen(), qBlue(), and qAlpha() to access the pixels.

See also bytesPerLine(), bits(), and Pixel Manipulation.

const uchar * QImage::scanLine ( int i ) const

This is an overloaded member function, provided for convenience.

int QImage::serialNumber () const

Returns a number that uniquely identifies the contents of this QImage object.

This means that multiple QImage objects can only have the same serial number as long as they refer to the same contents. A null image has always a serial number of 0.

The serial number is useful is when caching QImages.

See also Image Information.

void QImage::setAlphaChannel ( const QImage & alphaChannel )

Sets the alpha channel of this image to the given alphaChannel.

If alphaChannel is an 8 bit grayscale image, the intensity values are written into this buffer directly. Otherwise, alphaChannel is converted to 32 bit and the intensity of the RGB pixel values is used.

Note that the image will be converted to the Format_ARGB32_Premultiplied format if the function succeeds.

See also alphaChannel(), Image Transformations, and Image Formats.

void QImage::setColor ( int index, QRgb colorValue )

Sets the color at the given index in the color table, to the given to colorValue.

The color value is an ARGB quadruplet.

See also color(), setColorTable(), and Pixel Manipulation.

void QImage::setColorTable ( const QVector<QRgb> colors )

Sets the color table used to translate color indexes to QRgb values, to the specified colors.

See also colorTable(), setColor(), and Image Transformations.

void QImage::setDotsPerMeterX ( int x )

Sets the number of pixels that fit horizontally in a physical meter, to x.

Together with dotsPerMeterY(), this number defines the intended scale and aspect ratio of the image.

See also dotsPerMeterX() and Image Information.

void QImage::setDotsPerMeterY ( int y )

Sets the number of pixels that fit vertically in a physical meter, to y.

Together with dotsPerMeterX(), this number defines the intended scale and aspect ratio of the image.

See also dotsPerMeterY() and Image Information.

void QImage::setNumColors ( int numColors )

Resizes the color table to contain numColors entries.

If the color table is expanded, all the extra colors will be set to transparent (i.e qRgba(0, 0, 0, 0)).

See also numColors(), colorTable(), and Image Transformations.

void QImage::setOffset ( const QPoint & offset )

Sets the the number of pixels by which the image is intended to be offset by when positioning relative to other images, to offset.

See also offset() and Image Information.

void QImage::setPixel ( int x, int y, uint index_or_rgb )

Sets the pixel index or color at the coordinates (x, y) to index_or_rgb.

If the image's format is either monochrome or 8-bit, the given index_or_rgb value must be an index in the image's color table, otherwise the parameter must be a QRgb value.

If (x, y) is not valid coordinates in the image, or if index_or_rgb >= numColors() in the case of monochrome and 8-bit images, the result is undefined.

See also pixel() and Pixel Manipulation.

void QImage::setText ( const QString & key, const QString & text )

Sets the image text to the given text and associate it with the given key.

If you just want to store a single text block (i.e., a "comment" or just a description), you can either pass an empty key, or use a generic key like "Description".

The image text is embedded into the image data when you call save() or QImageWriter::write().

See also text() and textKeys().

QSize QImage::size () const

Returns the size of the image, i.e. its width() and height().

See also Image Information.

QString QImage::text ( const QString & key = QString() ) const

Returns the image text associated with the given key. If the specified key is an empty string, the whole image text is returned, with each key-text pair separated by a newline.

See also setText() and textKeys().

QStringList QImage::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.

See also text().

QImage QImage::transformed ( const QMatrix & matrix, Qt::TransformationMode mode = Qt::FastTransformation ) const

Returns a copy of the image that is transformed using the given transformation matrix and transformation mode.

The transformation matrix is internally adjusted to compensate for unwanted translation; i.e. the image produced is the smallest image that contains all the transformed points of the original image. Use the trueMatrix() function to retrieve the actual matrix used for transforming an image

See also trueMatrix() and Image Transformations.

QMatrix QImage::trueMatrix ( const QMatrix & matrix, int width, int height )   [static]

Returns the actual matrix used for transforming an image with the given width, height and matrix.

When transforming an image using the transformed() function, the transformation matrix is internally adjusted to compensate for unwanted translation, i.e. transformed() returns the smallest image containing all transformed points of the original image. This function returns the modified matrix, which maps points correctly from the original image into the new image.

See also transformed() and Image Transformations.

bool QImage::valid ( int x, int y ) const

Returns true if (x, y) is valid coordinates within the image; otherwise returns false.

int QImage::width () const

Returns the width of the image.

See also Image Information.

QImage::operator QVariant () const

Returns the image as a QVariant.

bool QImage::operator!= ( const QImage & image ) const

Returns true if this image and the given image have different contents; otherwise returns false.

The comparison can be slow, unless there is some obvious difference, such as different widths, in which case the function will return quickly.

See also operator=().

QImage & QImage::operator= ( const QImage & image )

Assigns a shallow copy of the given image to this image and returns a reference to this image.

For more information about shallow copies, see the Implicit Data Sharing documentation.

See also copy() and QImage().

bool QImage::operator== ( const QImage & image ) const

Returns true if this image and the given image have the same contents; otherwise returns false.

The comparison can be slow, unless there is some obvious difference (e.g. different size or format), in which case the function will return quickly.

See also operator=().

Related Non-Members

QDataStream & operator<< ( QDataStream & stream, const QImage & image )

This is an overloaded member function, provided for convenience.

Writes the given image to the given stream as a PNG image, or as a BMP image if the stream's version is 1. Note that writing the stream to a file will not produce a valid image file.

See also QImage::save() and Format of the QDataStream Operators.

QDataStream & operator>> ( QDataStream & stream, QImage & image )

This is an overloaded member function, provided for convenience.

Reads an image from the given stream and stores it in the given image.

See also QImage::load() and Format of the QDataStream Operators.