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


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 reserved for alpha channel support in a future version of Qt.

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 alpha channel.

    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

Examples: qmag/qmag.cpp


Member Function Documentation

QImage::QImage ( const QImage &image)

Constructs a shallow copy of image.

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 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().

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.

int QImage::height () const

Returns the height of the image.

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

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.

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.

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::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().


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