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QImage Class Reference


The QImage class provides a hardware-independent pixmap representation with direct access to the pixel data. More...

#include <qimage.h>

List of all member functions.

Public Members

Static Public Members

Related Functions

(Note that these are not member functions.)

Detailed Description

The QImage class provides a hardware-independent pixmap representation with direct access to the pixel data.

It is one of the two classes Qt provides for dealing with images, the other being QPixmap. QImage is designed and optimized for I/O and for direct pixel access/manipulation, QPixmap is designed and optimized for drawing. There are (slow) functions to convert between QImage and QPixmap; QPixmap::convertToImage() and QPixmap::convertFromImage().

An image has the parameters width, height and depth (bits per pixel, bpp), a color table and the actual pixels. QImage supports 1-bpp, 8-bpp and 32-bpp image data. 1-bpp and 8-bpp images use a color lookup table; the pixel value is a color table index.

32-bpp images encode an RGB value in 24 bits and ignore the color table. The most significant byte is used for the alpha buffer.

An entry in the color table is an RGB triplet encoded as uint. Use the qRed, qGreen and qBlue functions (qcolor.h) to access the components, and qRgb to make an RGB triplet (see the QColor class documentation).

1-bpp (monochrome) images have a color table with maximum 2 colors. There are two different formats; big endian (MSB first) or little endian (LSB first) bit order. To access a single bit, you will have to do some bitshifts:

    QImage image;
      // sets bit at (x,y) to 1
    if ( image.bitOrder() == QImage::LittleEndian )
        *(image.scanLine(y) + (x >> 3)) |= 1 << (x & 7);
    else
        *(image.scanLine(y) + (x >> 3)) |= 1 << (7 -(x & 7));

If this looks complicated, it might be a good idea to convert the 1-bpp image to an 8-bpp image using convertDepth().

8-bpp images are much easier to work with than 1-bpp images because they have a single byte per pixel:

    QImage image;
      // set entry 19 in the color table to yellow
    image.setColor( 19, qRgb(255,255,0) );
      // set 8 bit pixel at (x,y) to value yellow (in color table)
    *(image.scanLine(y) + x) = 19;

32-bpp images ignore the color table, instead each pixel contains the RGB triplet. 24 bits contain the RGB value and the most significant byte is reserved for the alpha buffer.

    QImage image;
      // sets 32 bit pixel at (x,y) to yellow.
    uint *p = (uint *)image.scanLine(y) + x;
    *p = qRgb(255,255,0);

The scanlines are 32-bit aligned for all depths.

The QImage class uses explicit sharing, similar to that of QArray.

See also QImageIO, QPixmap and Shared Classes

Examples: qtimage/qtimage.cpp qmag/qmag.cpp desktop/desktop.cpp


Member Type Documentation

QImage::Endian

This enum type is used to describe the endianness of the CPU and graphics hardware.

The current values are:


Member Function Documentation

QImage::QImage ()

Constructs a null image.

See also isNull().

QImage::QImage ( const QByteArray & array )

Constructs an image from array, which must be in a supported image format image.

See also loadFromData().

QImage::QImage ( const QSize & size, int depth, int numColors=0, Endian bitOrder=IgnoreEndian )

Constructs an image with size size pixels, depth depth bits, numColors and bitOrder endianness.

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

Constructs an image from loading fileName and an optional format.

See also load().

QImage::QImage ( const QImage & image )

Constructs a shallow copy of image.

QImage::QImage ( const char * xpm[] )

Constructs an image from xpm, which must be 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.

In order to use that sort of declaration, you must cast the variable back to const char ** when you create the QImage.

QImage::QImage ( int w, int h, int depth, int numColors=0, Endian bitOrder=IgnoreEndian )

Constructs an image with w width, h height, depth bits per pixel, numColors colors and bit order bitOrder.

Using this constructor is the same as first constructing a null image and then calling the create() function.

See also create().

QImage::QImage ( uchar * yourdata, int w, int h, int depth, QRgb * colortable, int numColors, Endian bitOrder )

Constructs an image that uses an existing memory buffer. The buffer must remain valid for the life of the QImage. The image will not delete the buffer at destruction.

If colortable is 0, a color table sufficient for numColors will be allocated (and destructed later).

QImage::~QImage ()

Destructs the image and cleans up.

bool QImage::allGray () const

Returns TRUE if all the colors in the image are shades of gray, that is their R, G, and B components are equal. This function is slow for large 16-bit and 32-bit images.

QImage::Endian QImage::bitOrder() const

Returns the bit order for the image.

If it is a 1-bpp image, this function returns either QImage::BigEndian or QImage::LittleEndian.

If it is not a 1-bpp image, this function returns QImage::IgnoreEndian.

See also depth().

uchar * QImage::bits () const

Returns a pointer to the first pixel data. Equivalent to scanLine(0).

See also scanLine().

int QImage::bytesPerLine () const

Returns the number of bytes per image scanline. This is equivalent to numBytes()/height().

QRgb QImage::color ( int i ) const

Returns the color in the color table at index i.

A color value is an RGB triplet. Use the qRed(), qGreen() and qBlue() functions (defined in qcolor.h) to get the color value components.

See also setColor() and QColor.

QRgb * QImage::colorTable () const

Returns a pointer to the color table.

QImage QImage::convertBitOrder ( Endian bitOrder ) const

Converts the bit order of the image to bitOrder and returns the converted image.

Returns *this if the bitOrder is equal to the image bit order, or a null image if this image cannot be converted.

See also bitOrder() and systemBitOrder().

QImage QImage::convertDepth ( int depth, int conversion_flags ) const

Converts the depth (bpp) of the image to depth and returns the converted image. The original image is left undisturbed.

The depth argument must be 1, 8, 16 or 32.

See QPixmap::convertFromImage for a description of the conversion_flags argument.

Returns *this if depth is equal to the image depth, or a null image if this image cannot be converted.

See also depth() and isNull().

QImage QImage::convertDepth ( int depth ) const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

QImage QImage::convertDepthWithPalette ( int d, QRgb * palette, int palette_count, int conversion_flags=0 ) const

Note: currently no closest-color search is made. If colors are found that are not in the palette, the palette may not be used at all. This result should not be considered valid, as it may change in future implementations.

Currently inefficient for non 32-bit images.

QImage QImage::copy () const

Returns a deep copy of the image.

QImage QImage::copy ( int x, int y, int w, int h, int conversion_flags=0 ) const

Returns a deep copy of a sub-area of the image.

The returned image is always w by h pixels is size. If the area is beyond this image, the pixels are filled with pixel 0.

See also bitBlt().

QImage QImage::copy ( const QRect & r ) const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

bool QImage::create ( int width, int height, int depth, int numColors=0, Endian bitOrder=IgnoreEndian )

Sets the image width, height, depth, number of colors and bit order. Returns TRUE if successful, or FALSE if the parameters are incorrect or if memory cannot be allocated.

The width and height is limited to 32767. depth must be 1, 8 or 32. If depth is 1, then bitOrder must be set to either QImage::LittleEndian or QImage::BigEndian. For other depths, bitOrder must be QImage::IgnoreEndian.

This function allocates a color table and a buffer for the image data. The image data is not initialized.

The image buffer is allocated as a single block that consists of a table of scanline pointers (jumpTable()) and the image data (bits()).

See also width(), height(), depth(), numColors(), bitOrder(), jumpTable(), scanLine(), bits(), bytesPerLine() and numBytes().

bool QImage::create ( const QSize &, int depth, int numColors=0, Endian bitOrder=IgnoreEndian )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

QImage QImage::createAlphaMask ( int conversion_flags=0 ) const

Builds and returns a 1-bpp mask from the alpha buffer in this image. Returns a null image if alpha buffer mode is disabled.

See QPixmap::convertFromImage for a description of the conversion_flags argument.

The returned image has little-endian bit order, which you can convert to big-endianness using convertBitOrder().

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

Creates and returns a 1-bpp heuristic mask for this image. It 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 over 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 voting results are undocumented.

The returned image has little-endian bit order, which you can convert to big-endianness using convertBitOrder().

This function disregards the alpha buffer.

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, 16 and 32.

void QImage::detach ()

Detaches from shared image data and makes sure that this image is the only one referring the data.

If multiple images share common data, this image makes a copy of the data and detaches itself from the sharing mechanism. Nothing is done if there is just a single reference.

int QImage::dotsPerMeterX () const

Returns the number of pixels that fit horizontally in a physical meter. This and dotsPerMeterY() define the intended scale and aspect ratio of the image.

See also setDotsPerMeterX().

int QImage::dotsPerMeterY () const

Returns the number of pixels that fit vertically in a physical meter. This and dotsPerMeterX() define the intended scale and aspect ratio of the image.

See also setDotsPerMeterY().

void QImage::fill ( uint pixel )

Fills the entire image with the pixel value pixel.

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.

If the depth is 32 and the image has no alpha buffer, the pixel value is written to each pixel in the image. If the image has an alpha buffer, only the 24 RGB bits are set and the upper 8 bits (alpha value) are left unchanged.

bool QImage::hasAlphaBuffer () const

Returns TRUE if alpha buffer mode is enabled, otherwise FALSE.

See also setAlphaBuffer().

int QImage::height () const

Returns the height of the image.

See also width(), size() and rect().

const char* QImage::imageFormat ( const QString & fileName ) [static]

Returns a string that specifies the image format of the file fileName, or null if the file cannot be read or if the format cannot be recognized.

The QImageIO documentation lists the guaranteed supported image formats, or use the QImage::inputFormats() QImage::outputFormats() to get lists that include installed formats.

See also load() and save().

QStringList QImage::inputFormatList () [static]

Returns a list of image formats which are supported for image input.

QStrList QImage::inputFormats () [static]

Returns a list of image formats which are supported for image input.

See also inputFormatList().

void QImage::invertPixels ( bool invertAlpha = TRUE )

Inverts all pixel values in the image.

For 32 bit (24 RGB + alpha buffer) images, set invertAlpha to FALSE if you want the alpha bits to be unchanged, otherwise they are inverted too.

bool QImage::isGrayscale () const

Returns TRUE if the image is allGray(), and if the image is 32-bpp or a 256-color 8-bpp image for which color(i) is QRgb(i,i,i).

bool QImage::isNull () const

Returns TRUE if it is a null image.

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

Examples: qtimage/qtimage.cpp

uchar ** QImage::jumpTable () const

Returns a pointer to the scanline pointer table.

This is the beginning of the data block for the image.

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

Loads an image from the file fileName. Returns TRUE if successful, or FALSE if the image could not be loaded.

If format is specified, the loader attempts to read the image using the specified format. If format is not specified (default), the loader reads a few bytes from the header to guess the file format.

The QImageIO documentation lists the supported image formats and explains how to add extra formats.

See also loadFromData(), save(), imageFormat(), QPixmap::load() and QImageIO.

bool QImage::loadFromData ( const uchar * buf, uint len, const char * format=0 )

Loads an image from the binary data in buf (len bytes). Returns TRUE if successful, or FALSE if the image could not be loaded.

If format is specified, the loader attempts to read the image using the specified format. If format is not specified (default), the loader reads a few bytes from the header to guess the file format.

The QImageIO documentation lists the supported image formats and explains how to add extra formats.

See also load(), save(), imageFormat(), QPixmap::loadFromData() and QImageIO.

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

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Loads from a QByteArray.

QImage QImage::mirror () const

Returns a QImage which is a vertically mirrored copy of this image. The original QImage is left unchanged.

QImage QImage::mirror ( bool horizontal, bool vertical ) const

Returns the image mirrored in the horizontal and/or vertical direction.

See also smoothScale().

int QImage::numBytes () const

Returns the number of bytes occupied by the image data.

See also bytesPerLine().

int QImage::numColors () const

Returns the size of the color table for the image.

Notice that numColors() returns 0 for 16-bpp and 32-bpp images, since these images do not use color tables, but instead encode pixel values as RGB triplets.

QPoint QImage::offset () const

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

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

Returns TRUE if this image and i have different contents, and FALSE if they they have the same. This can be slow. Of course, this function returns quickly if e.g. the two images' widths are different.

See also operator=().

QImage & QImage::operator= ( const QPixmap & pixmap )

Sets the image bits to the pixmap contents and returns a reference to the image.

If the image shares data with other images, it will first dereference the shared data.

Makes a call to QPixmap::convertToImage().

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

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

See also copy().

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

Returns TRUE if this image and i have the same contents, and FALSE if they differ. This can be slow. Of course, this function returns quickly if e.g. the two images' widths are different.

See also operator=().

QStringList QImage::outputFormatList () [static]

Returns a list of image formats which are supported for image output.

QStrList QImage::outputFormats () [static]

Returns a list of image formats which are supported for image output.

See also QImage::outputFormatList().

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

Returns the actual color of the pixel at the given coordinates.

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

See also setPixel(), qRed(), qGreen() and qBlue().

Examples: qmag/qmag.cpp

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.

QRect QImage::rect () const

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

See also width(), height() and size().

void QImage::reset ()

Resets all image parameters and deallocates the image data.

Examples: qtimage/qtimage.cpp

bool QImage::save ( const QString & fileName, const char * format ) const

Saves the image to the file fileName, using the image file format format and default quality settings. Returns TRUE if successful, or FALSE if the image could not be saved.

See also load(), loadFromData(), imageFormat(), QPixmap::save() and QImageIO.

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

Saves the image to the file fileName, using the image file format format and a quality factor quality. quality must be in the range [0,100] or -1. Specify 0 to obtain small compressed files, 100 for large uncompressed files and -1 to use the default settings. Returns TRUE if successful, or FALSE if the image could not be saved.

See also load(), loadFromData(), imageFormat(), QPixmap::save() and QImageIO.

uchar * QImage::scanLine ( int i ) const

Returns a pointer to the pixel data at the i'th scanline.

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. Hint: use qRed() and friends (qcolor.h) to access the pixels.

Warning: If you are accessing 16-bpp image data, you have to handle endianness yourself for now.

See also bits().

Examples: desktop/desktop.cpp

void QImage::setAlphaBuffer ( bool enable )

Enables alpha buffer mode if enable is TRUE, otherwise disables it. The default setting is disabled.

An 8-bpp image has 8 bit pixels. A pixel is an index into the color table, which contains 32-bit color values. In a 32-bpp image, the 32 bit pixels are the color values.

This 32 bit value is encoded as follows: The lower 24 bits are used for the red, green and blue components. The upper 8 bits contain the alpha component.

The alpha component specifies the transparency of a pixel. 0 means completely transparent and 255 means opaque. The alpha component is ignored if you do not enable alpha buffer mode.

The alpha buffer is used to set a mask when a QImage is translated to a QPixmap.

See also hasAlphaBuffer() and createAlphaMask().

void QImage::setColor ( int i, QRgb c )

Sets a color in the color table at index i to c.

A color value is an RGB triplet. Use the qRgb function (defined in qcolor.h) to make RGB triplets.

See also color().

Examples: desktop/desktop.cpp

void QImage::setDotsPerMeterX ( int x )

Sets the value returned by dotsPerMeterX() to x.

void QImage::setDotsPerMeterY ( int y )

Sets the value returned by dotsPerMeterY() to y.

void QImage::setNumColors ( int numColors )

Resizes the color table to numColors colors.

If the color table is expanded, then all new colors will be set to black (RGB 0,0,0).

See also color() and setColor().

void QImage::setOffset ( const QPoint & p )

Sets the value returned by offset() to p.

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

Sets the pixel index or color at the given coordinates.

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

See also qRgb() and qRgba().

void QImage::setText ( const char * key, const char * lang, const QString & s )

Records s for the keyword key. The key should be a portable keyword recognizable by other software - some suggested values can be found in the PNG specification. s can be any text. lang should specify the language code (see RFC 1766) or 0.

Note that no QImageIO handlers currently read or write this data. In an upcoming Qt version, PNG I/O will do so.

QSize QImage::size () const

Returns the size of the image.

See also width(), height() and rect().

QImage QImage::smoothScale ( int width, int height ) const

Returns a copy of the image smoothly scaled to width by height pixels. For 32-bpp images, and 1-bpp/8-bpp color images, the result will be 32-bpp, while all-gray images (including black-and-white 1-bpp) will produce 8-bit grayscale images with the palette spanning 256 grays from black to white.

This function uses code based on pnmscale.c by Jef Poskanzer.

pnmscale.c - read a portable anymap and scale it

Copyright (C) 1989, 1991 by Jef Poskanzer.

Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. This software is provided "as is" without express or implied warranty.

See also mirror().

QImage QImage::swapRGB () const

Returns a QImage where the values of the red and blue components of all pixels have been swapped, effectively converting a RGB image to a BGR image. The original QImage is left unchanged.

QImage::Endian QImage::systemBitOrder() [static]

Determines the bit order of the display hardware. Returns QImage::LittleEndian (LSB first) or QImage::BigEndian (MSB first).

QImage::Endian QImage::systemByteOrder() [static]

Determines the host computer byte order. Returns QImage::LittleEndian (LSB first) or QImage::BigEndian (MSB first).

QString QImage::text ( const QImageTextKeyLang & kl ) const

Returns the string recorded for the keyword and language kl, or in a default language if lang is 0.

QString QImage::text ( const char * key, const char * lang=0 ) const

Returns the string recorded for the keyword key in language lang, or in a default language if lang is 0.

QStringList QImage::textKeys () const

Returns the keywords for which some texts are recorded.

See also textList(), text(), setText() and textLanguages().

QStringList QImage::textLanguages () const

Returns the language identifiers for which some texts are recorded.

See also textList(), text(), setText() and textKeys().

QValueList<QImageTextKeyLang> QImage::textList () const

Returns a list of QImageTextKeyLang objects which enumerate all the texts key/languaage pairs set by setText() for this image.

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

Tests if the ( x, y ) is a valid coordinate in the image.

Examples: qmag/qmag.cpp

int QImage::width () const

Returns the width of the image.

See also height(), size() and rect().


Related Functions

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

Writes an image to the stream as a PNG image.

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

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

Reads an image from the stream.

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


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