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.

The direct pixel access functionality of QImage makes it very suitable for image processing and for pixmap archiving.

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 and QString.

See also: QImageIO, QPixmap and Shared Classes


Member Function Documentation

QImage::QImage ( const QImage &image)

Constructs a shallow copy of image.

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

Constructs an image from loading fileName and an optional format.

See also: load().

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

Constructs an image from xpm, which must be a valid XPM image.

Errors are silently ignored.

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 ()

Constructs a null image.

See also: isNull().

QImage::~QImage ()

Destroys the image and cleans up.

QImage::Endian QImage::bitOrder() const

Returns the bit order for the image.

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

If it is not a 1-bit 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 ( QImage::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 setBitOrder().

QImage QImage::convertDepth ( int depth) 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 or 32.

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::copy () const

Returns a deep copy of the image.

bool QImage::create ( int width, int height, int depth, int numColors=0, QImage::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 filled with the pixel value 0.

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().

QImage QImage::createAlphaMask ( bool dither=FALSE) 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.

If dither is TRUE, this function uses the Floyd-Steinberg dithering algorithm to create the mask. It gives accurate results but is somewhat slow. If dither is FALSE, this function avoids dithering and instead interprets all alpha values from 127 and below as transparent and values from 128 and above as opaque. It is much faster than dithering and usually sufficient when reading images with transparent colors.

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

See also: setAlphaBuffer().

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 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.

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 char *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 supported image formats.

See also: load() and save().

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.

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 char *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.

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 32-bpp images, since these images do not use color tables, but instead encode pixel values as RGB triplets.

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().

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.

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

Saves the image to the file fileName, using the image file format format. 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 uint* 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 qRgb() and friends (qcolor.h) to access the pixels.

See also: bits().

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().

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().

QSize QImage::size () const

Returns the size of the image.

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

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).

int QImage::width () const

Returns the width of the image.

See also: heigth(), size() and rect().


Related Functions

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

Writes an image to the stream as a BMP image.

See also: QImage::save().

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

Reads an image from the stream.

See also: QImage::load().


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