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 -
curses color manipulation routines
#include <ncurses/curses.h>
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);
/* extensions */
void reset_color_pairs(void);
int COLOR_PAIR(int n);
PAIR_NUMBER(attrs);
curses supports color attributes on terminals with that capability. To
use these routines start_color must be called, usually right after
initscr. Colors are always used in pairs (referred to as color-pairs).
A color-pair consists of a foreground color (for characters) and a background
color (for the blank field on which the characters are displayed). A
programmer initializes a color-pair with the routine init_pair. After
it has been initialized, COLOR_PAIR(n) can be used to convert
the pair to a video attribute.
If a terminal is capable of redefining colors, the programmer can
use the routine init_color to change the definition of a color. The
routines has_colors and can_change_color return TRUE or
FALSE, depending on whether the terminal has color capabilities and
whether the programmer can change the colors. The routine
color_content allows a programmer to extract the amounts of red,
green, and blue components in an initialized color. The routine
pair_content allows a programmer to find out how a given color-pair
is currently defined.
The curses library combines these inputs to produce the actual foreground
and background colors shown on the screen:
- per-character video attributes (e.g., via waddch),
- the window attribute (e.g., by wattrset), and
- the background character (e.g., wbkgdset).
Per-character and window attributes are usually set by a parameter
containing video attributes including a color pair value. Some functions
such as wattr_set use a separate parameter which is the color pair
number.
The background character is a special case: it includes a
character value, 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:
- •
- If the parameter passed to waddch is blank, and it uses the
special color pair 0,
- curses next checks the window attribute.
- If the window attribute does not use color pair 0, curses uses the
color pair from the window attribute.
- Otherwise, curses uses the background character.
- •
- 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.
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.
is initialized by start_color to the maximum number of colors the
terminal can support.
is initialized by start_color to the maximum number of color pairs the
terminal can support.
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:
- It initializes two global variables, COLORS and COLOR_PAIRS
(respectively defining the maximum number of colors and color-pairs the
terminal can support).
- It initializes the special color pair 0 to the default foreground
and background colors. No other color pairs are initialized.
- It restores the colors on the terminal to the values they had when the
terminal was just turned on.
- 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 legal, and may result in a runtime error:
- COLORS corresponds to the terminal database's max_colors
capability, (see terminfo(5)).
- color values are expected to be in the range 0 to COLORS-1,
inclusive (including 0 and COLORS-1).
- a special color value -1 is used in certain extended functions to
denote the default color (see use_default_colors(3X)).
- COLOR_PAIRS corresponds to the terminal database's max_pairs
capability, (see terminfo(5)).
- legal color pair values are in the range 1 to COLOR_PAIRS-1,
inclusive.
- 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.
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.
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.
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:
- The first argument must be a legal 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.
- The second and third arguments must be legal 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.
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.
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).
- The first argument must be a legal color value; default colors are not
allowed here. (See the section Colors for the default color
index.)
- 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.
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.
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.
- The first argument must be a legal color value, i.e., 0 through
COLORS-1, inclusive.
- 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.
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.
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.
- The first argument must be a legal color value, i.e., in the range
1 through COLOR_PAIRS-1, inclusive.
- The values that are stored at the addresses pointed to by the second and
third arguments are in the range 0 through COLORS,
inclusive.
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.
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.
PAIR_NUMBER(attrs) extracts the color value from its attrs
parameter and returns it as a color pair number.
Its inverse 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.
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:
- •
- 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.
- •
- 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.
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:
- COLOR_YELLOW is actually brown. To get yellow, use COLOR_YELLOW combined
with the A_BOLD attribute.
- 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).
- Color RGB values are not settable.
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:
- PCCurses (1987-1990) provided for only eight (8) colors.
- 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.
- 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.
- 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.
- •
- 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.
This implementation satisfies XSI 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.