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QColor Class Reference


The QColor class provides colors based on RGB. More...

#include <qcolor.h>

List of all member functions.

Public Members

Static Public Members

Related Functions

(Note that these are not member functions.)

Detailed Description

The QColor class provides colors based on RGB.

A color is normally specified in terms of RGB (red,green and blue) components, but it is also possible to specify HSV (hue,saturation and value) or set a color name (the names are copied from from the X11 color database).

In addition to the RGB value, a QColor also has a pixel value. This value is used by the underlying window system to refer to a color. It can be thought of as an index into the display hardware's color table.

There are 19 predefined QColor objects: black, white, darkGray, gray, lightGray, red, green, blue, cyan, magenta, yellow, darkRed, darkGreen, darkBlue, darkCyan, darkMagenta, darkYellow, color0 and color1.

The colors color0 (zero pixel value) and color1 (non-zero pixel value) are special colors for drawing in bitmaps.

The QColor class has an efficient, dynamic color allocation strategy. A color is normally allocated the first time it is used (lazy allocation), that is, whenever the pixel() function is called:

  1. Is the pixel value valid? If it is, just return it, otherwise, allocate a pixel value.
  2. Check an internal hash table to see if we allocated an equal RGB value earlier. If we did, set the pixel value and return.
  3. Try to allocate the RGB value. If we succeed, we get a pixel value which we save in the internal table with the RGB value. Return the pixel value.
  4. The color could not be allocated. Find the closest matching color and save it in the internal table.

Since many people don't know the HSV color model very well, we'll cover it briefly here.

The RGB model is hardware-oriented. Its representation is close to what most monitors show. In contrast, HSV represents color in a way more suited to traditional human perception of color. For example, the relationships "stronger than", "darker than", "the opposite of" are easily expressed in HSV, but are much harder to express in RGB.

HSV, like RGB, has three components. They are:

Here are some examples: Pure red is H=0, S=255, V=255. A dark red, moving slightly towards the magenta, could be H=350 (equvalent to -10), S=255, V=180. A grayish light red could have H about 0 (say 350-359 or 0-10), S about 50-100, and S=255.

See also: QPalette, QColorGroup, QApplication::setColorSpec() and Color FAQ.

Examples: grapher/grapher.cpp drawlines/connect.cpp drawdemo/drawdemo.cpp progress/progress.cpp desktop/desktop.cpp picture/picture.cpp


Member Function Documentation

QColor::QColor()

Constructs an invalid color with the RGB value (0,0,0). An invalid color is a color that is not properly set up for the underlying window system.

See also: isValid().

QColor::QColor(QRgbrgb, uintpixel=0xffffffff)

Constructs a color with a RGB value and a custom pixel value.

If the pixel = 0xffffffff, then the color uses the RGB value in a standard way. If pixel is something else, then the pixel value will be set directly to pixel (skips the normal allocation procedure).

QColor::QColor(constQString&name)

Constructs a named color in the same way as setNamedColor().

See also: setNamedColor().

QColor::QColor(constQColor&c)

Constructs a color that is a copy of c.

QColor::QColor(constchar*name)

Constructs a named color in the same way as setNamedColor().

See also: setNamedColor().

QColor::QColor(intr, intg, intb)

Constructs a color with the RGB value (r,g,b). r, g and b must be in the range 0..255.

See also: setRgb().

QColor::QColor(intx, inty, intz, SpeccolorSpec)

Constructs a color with the RGB or HSV value (x,y,z).

The (x,y,z) triplet defines an RGB value if colorSpec == QColor::Rgb. x (red), y (green) and z (blue) must be in the range 0..255.

The (x,y,z) triplet defines a HSV value if colorSpec == QColor::Hsv. x (hue) must be in the range -1..360 (-1 means achromatic), and y (saturation) and z (value) must be in the range 0..255.

See also: setRgb() and setHsv().

uintQColor::alloc()

Allocates the RGB color and returns the pixel value.

Allocating a color means to obtain a pixel value from the RGB specification. The pixel value is an index into the global color table, but should be considered an arbitrary platform-dependent value.

The pixel() function calls alloc() if necessary, so in general you don't need to call this function.

See also: setLazyAlloc() and enterAllocContext().

intQColor::blue()const

Returns the B (blue) component of the RGB value.

voidQColor::cleanup() [static]

Internal clean up required for QColor. This function is called from the QApplication destructor.

See also: initialize().

intQColor::currentAllocContext() [static]

Returns the current color allocation context.

The default context is 0.

See also: enterAllocContext() and leaveAllocContext().

QColorQColor::dark(intfactor=200)const

Returns a darker (or lighter) color.

Returns a darker color if factor is greater than 100. Setting factor to 300 returns a color that has one third the brightness.

Returns a lighter color if factor is less than 100, equal to light(10000 / factor).

This function converts the current RGB color to HSV, divides V by factor and converts back to RGB.

See also: light().

Examples: desktop/desktop.cpp

voidQColor::destroyAllocContext(intcontext) [static]

Destroys a color allocation context, context.

This function deallocates all colors that were allocated in the specified context. If context == -1, it frees up all colors that the application has allocated. If context == -2, it frees up all colors that the application has allocated, except those in the default context.

The function does nothing for true color displays.

See also: enterAllocContext() and alloc().

intQColor::enterAllocContext() [static]

Enters a color allocation context and returns a nonzero unique identifier.

Color allocation contexts are useful for programs that need to allocate many colors and throw them away later, like image viewers. The allocation context functions work for true color displays as well as colormap display, except that QColor::destroyAllocContext() does nothing for true color.

Example:

    QPixmap loadPixmap( QString fileName )
    {
        static int alloc_context = 0;
        if ( alloc_context )
            QColor::destroyAllocContext( alloc_context );
        alloc_context = QColor::enterAllocContext();
        QPixmap pm( fileName );
        QColor::leaveAllocContext();
        return pm;
    }

The example code loads a pixmap from file. It frees up all colors that were allocated the last time loadPixmap() was called.

The initial/default context is 0. Qt keeps a list of colors associated with their allocation contexts. You can call destroyAllocContext() to get rid of all colors that were allocated in a specific context.

Calling enterAllocContext() enters an allocation context. The allocation context lasts until you call leaveAllocContext(). QColor has an internal stack of allocation contexts. Each call to enterAllocContex() must have a corresponding leaveAllocContext().

      // context 0 active
    int c1 = QColor::enterAllocContext();       // enter context c1
      // context c1 active
    int c2 = QColor::enterAllocContext();       // enter context c2
      // context c2 active
    QColor::leaveAllocContext();                // leave context c2
      // context c1 active
    QColor::leaveAllocContext();                // leave context c1
      // context 0 active
      // Now, free all colors that were allocated in context c2
    QColor::destroyAllocContext( c2 );

You may also want to set the application's color specification. See QApplication::setColorSpec() for more information.

See also: leaveAllocContext(), currentAllocContext(), destroyAllocContext() and QApplication::setColorSpec().

voidQColor::getHsv(int&h, int&s, int&v)const

This function is obsolete. It is provided to keep old programs working. We strongly advise against using it in new code.

Returns the current RGB value as HSV.

Arguments:

The hue defines the color. Its range is 0..359 if the color is chromatic and -1 if the color is achromatic. The saturation and value both vary between 0 and 255 inclusive.

See also: setHsv() and rgb().

intQColor::green()const

Returns the G (green) component of the RGB value.

voidQColor::hsv(int*h, int*s, int*v)const

This function is obsolete. It is provided to keep old programs working. We strongly advise against using it in new code.

Returns the current RGB value as HSV.

Arguments:

The hue defines the color. Its range is 0..359 if the color is chromatic and -1 if the color is achromatic. The saturation and value both vary between 0 and 255 inclusive.

See also: setHsv() and rgb().

voidQColor::initialize() [static]

Internal initialization required for QColor. This function is called from the QApplication constructor.

See also: cleanup().

boolQColor::isDirty()const

Returns TRUE if the color is dirty, i.e. lazy allocation is enabled and an RGB/HSV value has been set but not allocated.

See also: setLazyAlloc(), alloc() and pixel().

boolQColor::isValid()const

Returns FALSE if the color is invalid, i.e. it was constructed using the default constructor.

boolQColor::lazyAlloc() [static]

Returns TRUE if lazy color allocation is enabled (on-demand allocation), or FALSE if it is disabled (immediate allocation).

See also: setLazyAlloc().

voidQColor::leaveAllocContext() [static]

Leaves a color allocation context.

See enterAllocContext() for a detailed explanation.

See also: enterAllocContext() and currentAllocContext().

QColorQColor::light(intfactor=150)const

Returns a lighter (or darker) color.

Returns a lighter color if factor is greater than 100. Setting factor to 150 returns a color that is 50% brighter.

Returns a darker color if factor is less than 100, equal to dark(10000 / factor).

This function converts the current RGB color to HSV, multiplies V with factor and converts back to RGB.

See also: dark().

Examples: desktop/desktop.cpp

intQColor::maxColors() [static]

Returns the maximum number of colors supported by the underlying window system.

intQColor::numBitPlanes() [static]

Returns the number of color bit planes for the underlying window system.

The returned values is equal to the default pixmap depth;

See also: QPixmap::defaultDepth().

boolQColor::operator!=(constQColor&c)const

Returns TRUE if this color has different RGB value from c, or FALSE if they have equal RGB values.

QColor&QColor::operator=(constQColor&c)

Assigns a copy of the color and returns a reference to this color.

boolQColor::operator==(constQColor&c)const

Returns TRUE if this color has the same RGB value as c, or FALSE if they have different RGB values.

uintQColor::pixel()const

Returns the pixel value.

This value is used by the underlying window system to refer to a color. It can be thought of as an index into the display hardware's color table, but the value is an arbitrary 32-bit value.

See also: setLazyAlloc() and alloc().

intQColor::red()const

Returns the R (red) component of the RGB value.

QRgbQColor::rgb()const

Returns the RGB value.

The return type QRgb is equivalent to unsigned int.

See also: setRgb(), hsv(), qRed(), qBlue() and qGreen().

voidQColor::rgb(int*r, int*g, int*b)const

Returns the red, green and blue components of the RGB value in *r, *g and *b. The value range for a component is 0..255.

See also: setRgb() and hsv().

voidQColor::setHsv(inth, ints, intv)

Sets a HSV color value.

Arguments:

See also: getHsv() and setRgb().

Examples: grapher/grapher.cpp drawdemo/drawdemo.cpp progress/progress.cpp

voidQColor::setLazyAlloc(boolenable) [static]

Enables or disables lazy color allocation.

If lazy allocation is enabled, colors are allocated the first time they are used (upon calling the pixel() function). If lazy allocation is disabled, colors are allocated when they are constructed or when either setRgb() or setHsv() is called.

Lazy color allocation is enabled by default.

See also: lazyAlloc(), pixel() and alloc().

voidQColor::setNamedColor(constQString&name)

Sets the RGB value to that of the named color.

The color name may be in one of five formats: