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 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.
The following steps are taken to allocate a color. If, at any point,
a suitable color is found then the appropriate pixel value is
returned and the subsequent steps are not taken:
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.
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 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().
QStringList QColor::colorNames () [static]
Returns a QStringList containing the color names Qt knows about.
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
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.
Warning: Colors are stored internally as RGB values, so getHSv()
may return slightly different values to those set by setHsv().
See also setHsv() and rgb().
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.
void QColor::getRgb ( 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 rgb(), setRgb(), and getHsv().
int QColor::green () const
Returns the G (green) component of the RGB value.
void QColor::hsv ( 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.
Use getHsv() instead.
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 if the window system uses a palette.
Otherwise returns -1. Use numBitPlanes() to calculate the available
colors in that case.
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().
uint QColor::pixel ( int screen ) const
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns the pixel value for screen screen.
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.
QRgb QColor::rgb () const
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::rgb ( int * r, int * g, int * b ) const
This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
Use getRgb() instead
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.
Warning: Colors are stored internally as RGB values, so getHSv()
may return slightly different values to those set by setHsv().
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 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
Writes a color object, c to the stream, s.
See also Format of the QDataStream operators.
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().
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