curs_color(3) Library calls curs_color(3)
NAME
start_color, has_colors, can_change_color, init_pair, init_color,
init_extended_pair, init_extended_color, color_content, pair_content,
extended_color_content, extended_pair_content, reset_color_pairs,
COLOR_PAIR, PAIR_NUMBER, COLORS, COLOR_PAIRS, COLOR_BLACK, COLOR_RED,
COLOR_GREEN, COLOR_YELLOW, COLOR_BLUE, COLOR_MAGENTA, COLOR_CYAN,
COLOR_WHITE - manipulate terminal colors with curses
SYNOPSIS
#include <curses.h>
/* variables */
int COLOR_PAIRS;
int COLORS;
int start_color(void);
bool has_colors(void);
bool can_change_color(void);
int init_pair(short pair, short f, short b);
int init_color(short color, short r, short g, short b);
/* extensions */
int init_extended_pair(int pair, int f, int b);
int init_extended_color(int color, int r, int g, int b);
int color_content(short color, short *r, short *g, short *b);
int pair_content(short pair, short *f, short *b);
/* extensions */
int extended_color_content(int color, int *r, int *g, int *b);
int extended_pair_content(int pair, int *f, int *b);
/* extension */
void reset_color_pairs(void);
int COLOR_PAIR(int n);
PAIR_NUMBER(int attr);
DESCRIPTION
Overview
curses supports color attributes on terminals with that capability.
Call start_color (typically right after initscr(3X)) to enable this
feature. Colors are always used in pairs. A color pair couples a
foreground color for characters with a background color for the blank
field on which characters are rendered. init_pair initializes a color
pair. The macro COLOR_PAIR(n) can then convert the pair to a video
attribute.
If a terminal has the relevant capability, init_color permits
(re)definition of a color. has_colors and can_change_color return TRUE
or FALSE, depending on whether the terminal has color capability and
whether the programmer can change the colors. color_content permits
extraction of the red, green, and blue components of an initialized
color. pair_content permits discovery of a color pair's current
definition.
Rendering
curses combines the following data to render a character cell. Any of
them can include color information.
o curses character attributes, as from waddch(3X) or wadd_wch(3X)
o window attributes, as from wattrset(3X) or wattr_set(3X)
o window background character attributes, as from wbkgdset(3X) or
wbkgrndset(3X)
Per-character and window attributes are usually set through a function
parameter containing attributes including a color pair value. Some
functions, such as wattr_set, use a separate color pair number
parameter.
The background character is a special case: it includes a character
code, just as if it were passed to waddch.
The curses library does the actual work of combining these color pairs
in an internal function called from waddch:
o If the parameter passed to waddch is blank, and it uses the special
color pair 0,
o curses next checks the window attribute.
o If the window attribute does not use color pair 0, curses uses
the color pair from the window attribute.
o Otherwise, curses uses the background character.
o If the parameter passed to waddch is not blank, or it does not use
the special color pair 0, curses prefers the color pair from the
parameter, if it is nonzero. Otherwise, it tries the window
attribute next, and finally the background character.
Some curses functions such as wprintw call waddch. Those do not
combine its parameter with a color pair. Consequently those calls use
only the window attribute or the background character.
CONSTANTS
In <curses.h> the following macros are defined. These are the standard
colors (ISO-6429). curses also assumes that COLOR_BLACK is the default
background color for all terminals.
COLOR_BLACK
COLOR_RED
COLOR_GREEN
COLOR_YELLOW
COLOR_BLUE
COLOR_MAGENTA
COLOR_CYAN
COLOR_WHITE
Some terminals support more than the eight (8) "ANSI" colors. There
are no standard names for those additional colors.
VARIABLES
COLORS
is initialized by start_color to the maximum number of colors the
terminal can support.
COLOR_PAIRS
is initialized by start_color to the maximum number of color pairs the
terminal can support. Often, its value is the product COLORS x COLORS,
but this is not always true.
o A few terminals use the HLS color space (see start_color below),
ignoring this rule; and
o terminals supporting a large number of colors are limited to the
number of color pairs that a signed short value can represent.
FUNCTIONS
start_color
The start_color routine requires no arguments. It must be called if
the programmer wants to use colors, and before any other color
manipulation routine is called. It is good practice to call this
routine right after initscr. start_color does this:
o It initializes two global variables, COLORS and COLOR_PAIRS
(respectively defining the maximum number of colors and color pairs
the terminal can support).
o It initializes the special color pair 0 to the default foreground
and background colors. No other color pairs are initialized.
o It restores the colors on the terminal to the values they had when
the terminal was just turned on.
o If the terminal supports the initc (initialize_color) capability,
start_color initializes its internal table representing the red,
green, and blue components of the color palette.
The components depend on whether the terminal uses CGA (aka "ANSI")
or HLS (i.e., the hls (hue_lightness_saturation) capability is
set). The table is initialized first for eight basic colors
(black, red, green, yellow, blue, magenta, cyan, and white), using
weights that depend upon the CGA/HLS choice. For "ANSI" colors the
weights are 680 or 0 depending on whether the corresponding red,
green, or blue component is used or not. That permits using 1000
to represent bold/bright colors. After the initial eight colors
(if the terminal supports more than eight colors) the components
are initialized using the same pattern, but with weights of 1000.
SVr4 uses a similar scheme, but uses 1000 for the components of the
initial eight colors.
start_color does not attempt to set the terminal's color palette to
match its built-in table. An application may use init_color to
alter the internal table along with the terminal's color.
These limits apply to color values and color pairs. Values outside
these limits are not valid, and may result in a runtime error:
o COLORS corresponds to the terminal database's max_colors
capability, (see terminfo(5)).
o color values are expected to be in the range 0 to COLORS-1,
inclusive (including 0 and COLORS-1).
o a special color value -1 is used in certain extended functions to
denote the default color (see use_default_colors(3X)).
o COLOR_PAIRS corresponds to the terminal database's max_pairs
capability, (see terminfo(5)).
o valid color pair values are in the range 1 to COLOR_PAIRS-1,
inclusive.
o color pair 0 is special; it denotes "no color".
Color pair 0 is assumed to be white on black, but is actually
whatever the terminal implements before color is initialized. It
cannot be modified by the application.
has_colors
The has_colors routine requires no arguments. It returns TRUE if the
terminal can manipulate colors; otherwise, it returns FALSE. This
routine facilitates writing terminal-independent programs. For
example, a programmer can use it to decide whether to use color or some
other video attribute.
can_change_color
The can_change_color routine requires no arguments. It returns TRUE if
the terminal supports colors and can change their definitions; other,
it returns FALSE. This routine facilitates writing terminal-
independent programs.
init_pair
The init_pair routine changes the definition of a color pair. It takes
three arguments: the number of the color pair to be changed, the
foreground color number, and the background color number. For portable
applications:
o The first argument must be a valid color pair value. If default
colors are used (see use_default_colors(3X)) the upper limit is
adjusted to allow for extra pairs which use a default color in
foreground and/or background.
o The second and third arguments must be valid color values.
If the color pair was previously initialized, the screen is refreshed
and all occurrences of that color pair are changed to the new
definition.
As an extension, ncurses allows you to set color pair 0 via the
assume_default_colors(3X) routine, or to specify the use of default
colors (color number -1) if you first invoke the use_default_colors(3X)
routine.
init_extended_pair
Because init_pair uses signed shorts for its parameters, that limits
color pairs and color-values to 32767 on modern hardware. The
extension init_extended_pair uses ints for the color pair and color-
value, allowing a larger number of colors to be supported.
init_color
The init_color routine changes the definition of a color. It takes
four arguments: the number of the color to be changed followed by three
RGB values (for the amounts of red, green, and blue components).
o The first argument must be a valid color value; default colors are
not allowed here. (See the section Colors for the default color
index.)
o Each of the last three arguments must be a value in the range 0
through 1000.
When init_color is used, all occurrences of that color on the screen
immediately change to the new definition.
init_extended_color
Because init_color uses signed shorts for its parameters, that limits
color-values and their red, green, and blue components to 32767 on
modern hardware. The extension init_extended_color uses ints for the
color value and for setting the red, green, and blue components,
allowing a larger number of colors to be supported.
color_content
The color_content routine gives programmers a way to find the intensity
of the red, green, and blue (RGB) components in a color. It requires
four arguments: the color number, and three addresses of shorts for
storing the information about the amounts of red, green, and blue
components in the given color.
o The first argument must be a valid color value, i.e., 0 through
COLORS-1, inclusive.
o The values that are stored at the addresses pointed to by the last
three arguments are in the range 0 (no component) through 1000
(maximum amount of component), inclusive.
extended_color_content
Because color_content uses signed shorts for its parameters, that
limits color-values and their red, green, and blue components to 32767
on modern hardware. The extension extended_color_content uses ints for
the color value and for returning the red, green, and blue components,
allowing a larger number of colors to be supported.
pair_content
The pair_content routine allows programmers to find out what colors a
given color pair consists of. It requires three arguments: the color
pair number, and two addresses of shorts for storing the foreground and
the background color numbers.
o The first argument must be a valid color value, i.e., in the range
1 through COLOR_PAIRS-1, inclusive.
o The values that are stored at the addresses pointed to by the
second and third arguments are in the range 0 through COLORS,
inclusive.
extended_pair_content
Because pair_content uses signed shorts for its parameters, that limits
color pair and color-values to 32767 on modern hardware. The extension
extended_pair_content uses ints for the color pair and for returning
the foreground and background colors, allowing a larger number of
colors to be supported.
reset_color_pairs
The extension reset_color_pairs tells ncurses to discard all of the
color pair information which was set with init_pair. It also touches
the current- and standard-screens, allowing an application to switch
color palettes rapidly.
COLOR_PAIR
COLOR_PAIR(n) converts a color pair number to an attribute. Attributes
can hold color pairs in the range 0 to 255. If you need a color pair
larger than that, you must use functions such as attr_set (which pass
the color pair as a separate parameter) rather than the legacy
functions such as attrset.
PAIR_NUMBER
PAIR_NUMBER(attr) extracts the color information from its attr
parameter and returns it as a color pair number; it is the inverse
operation of COLOR_PAIR.
RETURN VALUE
The routines can_change_color and has_colors return TRUE or FALSE.
All other routines return the integer ERR upon failure and an OK (SVr4
specifies only "an integer value other than ERR") upon successful
completion.
X/Open defines no error conditions. SVr4 does document some error
conditions which apply in general:
o This implementation will return ERR on attempts to use color values
outside the range 0 to COLORS-1 (except for the default colors
extension), or use color pairs outside the range 0 to
COLOR_PAIRS-1.
Color values used in init_color must be in the range 0 to 1000.
An error is returned from all functions if the terminal has not
been initialized.
An error is returned from secondary functions such as init_pair if
start_color was not called.
o SVr4 does much the same, except that it returns ERR from
pair_content if the pair was not initialized using init_pairs and
it returns ERR from color_content if the terminal does not support
changing colors.
This implementation does not return ERR for either case.
Specific functions make additional checks:
init_color
returns an error if the terminal does not support this feature,
e.g., if the initialize_color capability is absent from the
terminal description.
start_color
returns an error if the color table cannot be allocated.
NOTES
In the ncurses implementation, there is a separate color activation
flag, color palette, color pairs table, and associated COLORS and
COLOR_PAIRS counts for each screen; the start_color function only
affects the current screen. The SVr4/XSI interface is not really
designed with this in mind, and historical implementations may use a
single shared color palette.
Setting an implicit background color via a color pair affects only
character cells that a character write operation explicitly touches.
To change the background color used when parts of a window are blanked
by erasing or scrolling operations, see curs_bkgd(3X).
Several caveats apply on older x86 machines (e.g., i386, i486) with
VGA-compatible graphics:
o COLOR_YELLOW is actually brown. To get yellow, use COLOR_YELLOW
combined with the A_BOLD attribute.
o The A_BLINK attribute should in theory cause the background to go
bright. This often fails to work, and even some cards for which it
mostly works (such as the Paradise and compatibles) do the wrong
thing when you try to set a bright "yellow" background (you get a
blinking yellow foreground instead).
o Color RGB values are not settable.
EXTENSIONS
The functions marked as extensions were designed for ncurses(3X), and
are not found in SVr4 curses, 4.4BSD curses, or any other previous
curses implementation.
PORTABILITY
Applications employing ncurses extensions should condition their use on
the visibility of the NCURSES_VERSION preprocessor macro.
This implementation satisfies X/Open Curses's minimum maximums for
COLORS and COLOR_PAIRS.
The init_pair routine accepts negative values of foreground and
background color to support the use_default_colors(3X) extension, but
only if that routine has been first invoked.
The assumption that COLOR_BLACK is the default background color for all
terminals can be modified using the assume_default_colors(3X)
extension.
This implementation checks the pointers, e.g., for the values returned
by color_content and pair_content, and will treat those as optional
parameters when null.
X/Open Curses does not specify a limit for the number of colors and
color pairs which a terminal can support. However, in its use of short
for the parameters, it carries over SVr4's implementation detail for
the compiled terminfo database, which uses signed 16-bit numbers. This
implementation provides extended versions of those functions which use
short parameters, allowing applications to use larger color- and pair-
numbers.
The reset_color_pairs function is an extension of ncurses.
HISTORY
SVr3.2 introduced color support to curses in 1987.
SVr4 made internal changes, e.g., moving the storage for the color
state from SP (the SCREEN structure) to cur_term (the TERMINAL
structure), but provided the same set of library functions.
SVr4 curses limits the number of color pairs to 64, reserving color
pair zero (0) as the terminal's initial uncolored state. This limit
arises because the color pair information is a bitfield in the chtype
data type (denoted by A_COLOR).
Other implementations of curses had different limits:
o PCCurses (1987-1990) provided for only eight (8) colors.
o PDCurses (1992-present) inherited the 8-color limitation from
PCCurses, but changed this to 256 in version 2.5 (2001), along with
changing chtype from 16-bits to 32-bits.
o X/Open Curses (1992-present) added a new structure cchar_t to store
the character, attributes and color pair values, allowing increased
range of color pairs. Both color pairs and color-values used a
signed short, limiting values to 15 bits.
o ncurses (1992-present) uses eight bits for A_COLOR in chtype
values.
Version 5.3 provided a wide-character interface (2002), but left
color pairs as part of the attributes-field.
Since version 6 (2015), ncurses uses a separate int for color pairs
in the cchar_t values. When those color pair values fit in 8 bits,
ncurses allows color pairs to be manipulated via the functions
using chtype values.
o NetBSD curses used 6 bits from 2000 (when colors were first
supported) until 2004. At that point, NetBSD changed to use 10
bits. As of 2021, that size is unchanged. Like ncurses before
version 6, the NetBSD color pair information is stored in the
attributes field of cchar_t, limiting the number of color pairs by
the size of the bitfield.
SEE ALSO
curses(3X), curs_attr(3X), curs_initscr(3X), curs_variables(3X),
default_colors(3X)
ncurses 6.5 2024-04-20 curs_color(3)
ncurses 6.5 - Generated Wed May 1 12:40:51 CDT 2024