QColor Class

Qt 3.0.5

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QColor Class Reference

The QColor class provides colors based on RGB or HSV values. More...

#include <qcolor.h>

List of all member functions.

Public Members

  • enum Spec { Rgb, Hsv }
  • QColor ()
  • QColor ( int r, int g, int b )
  • QColor ( int x, int y, int z, Spec colorSpec )
  • QColor ( QRgb rgb, uint pixel = 0xffffffff )
  • QColor ( const QString & name )
  • QColor ( const char * name )
  • QColor ( const QColor & c )
  • QColor & operator= ( const QColor & c )
  • bool isValid () const
  • QString name () const
  • void setNamedColor ( const QString & name )
  • void rgb ( int * r, int * g, int * b ) const
  • QRgb rgb () const
  • void setRgb ( int r, int g, int b )
  • void setRgb ( QRgb rgb )
  • int red () const
  • int green () const
  • int blue () const
  • void hsv ( int * h, int * s, int * v ) const
  • void getHsv ( int & h, int & s, int & v ) const  (obsolete)
  • void setHsv ( int h, int s, int v )
  • QColor light ( int factor = 150 ) const
  • QColor dark ( int factor = 200 ) const
  • bool operator== ( const QColor & c ) const
  • bool operator!= ( const QColor & c ) const
  • uint alloc ()
  • uint pixel () const

Static Public Members

Related Functions

  • QDataStream & operator<< ( QDataStream & s, const QColor & c )
  • QDataStream & operator>> ( QDataStream & s, QColor & c )
  • int qRed ( QRgb rgb )
  • int qGreen ( QRgb rgb )
  • int qBlue ( QRgb rgb )
  • int qAlpha ( QRgb rgba )
  • QRgb qRgb ( int r, int g, int b )
  • QRgb qRgba ( int r, int g, int b, int a )
  • int qGray ( int r, int g, int b )
  • int qGray ( qRgb rgb )

Detailed Description

The QColor class provides colors based on RGB or HSV values.

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 and a validity. The pixel 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.

The validity (isValid()) indicates whether the color is legal at all. For example, a RGB color with RGB values out of range is illegal. For performance reasons, QColor mostly disregards illegal colors. The result of using an invalid color is unspecified and will usually be surprising.

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

Qt Colors

The colors color0 (zero pixel value) and color1 (non-zero pixel value) are special colors for drawing in bitmaps. Painting with color0 sets the bitmap bits to 0 (transparent, i.e. background), and painting with color1 sets the bits to 1 (opaque, i.e. foreground).

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 that 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.

A color can be set by passing setNamedColor() an RGB string like "#112233", or a color name, e.g. "blue". The names are taken from X11's rgb.txt database but can also be used under Windows. To get a lighter or darker color use light() and dark() respectively. Colors can also be set using setRgb() and setHsv(). The color components can be accessed in one go with rgb() and hsv(), or individually with red(), green() and blue().

Use maxColors() and numBitPlanes() to determine the maximum number of colors and the number of bit planes supported by the underlying window system,

If you need to allocate many colors temporarily, for example in an image viewer application, enterAllocContext(), leaveAllocContext() and destroyAllocContext() will prove useful.

HSV Colors

Because 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 the human perception of color. For example, the relationships "stronger than", "darker than" and "the opposite of" are easily expressed in HSV but are much harder to express in RGB.

HSV, like RGB, has three components:

  • H, for hue, is either 0-359 if the color is chromatic (not gray), or meaningless if it is gray. It represents degrees on the color wheel familiar to most people. Red is 0 (degrees), green is 120 and blue is 240.

  • S, for saturation, is 0-255, and the bigger it is, the stronger the color is. Grayish colors have saturation near 0; very strong colors have saturation near 255.

  • V, for value, is 0-255 and represents lightness or brightness of the color. 0 is black; 255 is as far from black as possible.

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 (equivalent 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.

Qt returns a hue value of -1 for achromatic colors. If you pass a too-big hue value, Qt forces it into range. Hue 360 or 720 is treated as 0; hue 540 is treated as 180.

See also QPalette, QColorGroup, QApplication::setColorSpec(), Color FAQ, Widget Appearance and Style, Graphics Classes and Image Processing Classes.

Member Type Documentation

QColor::Spec

The type of color specified, either RGB or HSV, e.g. in the QColor::QColor( x, y, z, colorSpec) constructor.

  • QColor::Rgb
  • QColor::Hsv

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.

The alpha value of an invalid color is unspecified.

See also isValid().

QColor::QColor ( int r, int g, int b )

Constructs a color with the RGB value r, g, b, in the same way as setRgb().

The color is left invalid if any or the arguments are illegal.

See also setRgb().

QColor::QColor ( int x, int y, int z, Spec colorSpec )

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

The arguments are an RGB value if colorSpec is QColor::Rgb. x (red), y (green), and z (blue). All of them must be in the range 0-255.

The arguments are an HSV value if colorSpec is QColor::Hsv. x (hue) must be -1 for achromatic colors and 0-359 for chromatic colors; y (saturation) and z (value) must both be in the range 0-255.

See also setRgb() and setHsv().

QColor::QColor ( QRgb rgb, uint pixel = 0xffffffff )

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

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

QColor::QColor ( const QString & name )

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

The color is left invalid if name cannot be parsed.

See also setNamedColor().

QColor::QColor ( const char * name )

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

The color is left invalid if name cannot be parsed.

See also setNamedColor().

QColor::QColor ( const QColor & c )

Constructs a color that is a copy of c.

uint QColor::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 to be 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 enterAllocContext().

int QColor::blue () const

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

void QColor::cleanup () [static]

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

See also initialize().

int QColor::currentAllocContext () [static]

Returns the current color allocation context.

The default context is 0.

See also enterAllocContext() and leaveAllocContext().

QColor QColor::dark ( int factor = 200 ) const

Returns a darker (or lighter) color, but does not change this object.

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. We recommend using lighter() for this purpose. If factor is 0 or negative, the return value is unspecified.

(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 and themes/wood.cpp.

void QColor::destroyAllocContext ( int context ) [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().

Example: showimg/showimg.cpp.

int QColor::enterAllocContext () [static]

Enters a color allocation context and returns a non-zero 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 for colormap displays, 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().

Example: showimg/showimg.cpp.

void QColor::getHsv ( int & h, int & s, int & v ) const

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

int QColor::green () const

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

void QColor::hsv ( int * h, int * s, int * v ) const

Returns the current RGB value as HSV. The contents of the h, s and v pointers are set to the HSV values. If any of the three pointers are null, the function does nothing.

The hue (which h points to) is set to -1 if the color is achromatic.

See also setHsv() and rgb().

Example: themes/metal.cpp.

void QColor::initialize () [static]

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

See also cleanup().

bool QColor::isValid () const

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

Examples: chart/element.cpp, chart/setdataform.cpp and scribble/scribble.cpp.

void QColor::leaveAllocContext () [static]

Leaves a color allocation context.

See enterAllocContext() for a detailed explanation.

See also enterAllocContext() and currentAllocContext().

Example: showimg/showimg.cpp.

QColor QColor::light ( int factor = 150 ) const

Returns a lighter (or darker) color, but does not change this object.

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. We recommend using dark() for this purpose. If factor is 0 or negative, the return value is unspecified.

(This function converts the current RGB color to HSV, multiplies V by factor, and converts the result back to RGB.)

See also dark().

Examples: desktop/desktop.cpp and themes/wood.cpp.

int QColor::maxColors () [static]

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

QString QColor::name () const

Returns the name of the color in the format "#RRGGBB", i.e. a "#" character followed by three two-digit hexadecimal numbers.

See also setNamedColor().

Example: chart/setdataform.cpp.

int QColor::numBitPlanes () [static]

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

The returned value is equal to the default pixmap depth.

See also QPixmap::defaultDepth().

bool QColor::operator!= ( const QColor & c ) const

Returns TRUE if this color has a different RGB value from c; otherwise returns FALSE.

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

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

bool QColor::operator== ( const QColor & c ) const

Returns TRUE if this color has the same RGB value as c; otherwise returns FALSE.

uint QColor::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 alloc().

int QColor::red () const

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

void QColor::rgb ( int * r, int * g, int * b ) const

Sets the contents pointed to by r, g and b to the red, green and blue components of the RGB value respectively. The value range for a component is 0..255.

See also setRgb() and hsv().

QRgb QColor::rgb () const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns the RGB value.

The return type QRgb is equivalent to unsigned int.

For an invalid color, the alpha value of the returned color is unspecified.

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

void QColor::setHsv ( int h, int s, int v )

Sets a HSV color value. h is the hue, s is the saturation and v is the value of the HSV color.

If s or v are not in the range 0-255, or h is < -1, the color is not changed.

See also hsv() and setRgb().

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

void QColor::setNamedColor ( const QString & name )

Sets the RGB value to name, which may be in one of these formats:
  • #RGB (each of R, G and B is a single hex digit)
  • #RRGGBB
  • #RRRGGGBBB
  • #RRRRGGGGBBBB
  • A name from the X color database (rgb.txt) (e.g. "steelblue" or "gainsboro"). These color names also work under Windows.

The color is left invalid if name cannot be parsed.

void QColor::setRgb ( int r, int g, int b )

Sets the RGB value to r, g, b. The arguments, r, g and b must all be in the range 0..255. If any of them are outside the legal range, the color is not changed.

See also rgb() and setHsv().

void QColor::setRgb ( QRgb rgb )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Sets the RGB value to rgb.

The type QRgb is equivalent to unsigned int.

See also rgb() and setHsv().

Related Functions

QDataStream & operator<< ( QDataStream & s, const QColor & c )

Writes a color object, c to the stream, s.

See also Format of the QDataStream operators.

QDataStream & operator>> ( QDataStream & s, QColor & c )

Reads a color object, c, from the stream, s.

See also Format of the QDataStream operators.

int qAlpha ( QRgb rgba )

Returns the alpha component of the RGBA quadruplet rgba.

int qBlue ( QRgb rgb )

Returns the blue component of the RGB triplet rgb.

See also qRgb() and QColor::blue().

int qGray ( int r, int g, int b )

Returns a gray value 0..255 from the (r, g, b) triplet.

The gray value is calculated using the formula (r*11 + g*16 + b*5)/32.

int qGray ( qRgb rgb )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns a gray value 0..255 from the given rgb colour.

int qGreen ( QRgb rgb )

Returns the green component of the RGB triplet rgb.

See also qRgb() and QColor::green().

int qRed ( QRgb rgb )

Returns the red component of the RGB triplet rgb.

See also qRgb() and QColor::red().

QRgb qRgb ( int r, int g, int b )

Returns the RGB triplet (r,g,b).

The return type QRgb is equivalent to unsigned int.

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

QRgb qRgba ( int r, int g, int b, int a )

Returns the RGBA quadruplet (r,g,b,a).

The return type QRgba is equivalent to unsigned int.

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

This file is part of the Qt toolkit. Copyright © 1995-2002 Trolltech. All Rights Reserved.

Copyright © 2002 TrolltechTrademarks
Qt version 3.0.5

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