Ubuntu Manpage: start_color, init_pair, init_color, has_colors, can_change_color, color_content, pair_content, COLOR

Provided by: ncurses-doc_6.0+20160213-1ubuntu1_all

NAME

       start_color, init_pair, init_color, has_colors, can_change_color, color_content, pair_content, COLOR_PAIR
       - curses color manipulation routines

SYNOPSIS

       # include <curses.h>

       int start_color(void);
       int init_pair(short pair, short f, short b);
       int init_color(short color, short r, short g, short b);
       bool has_colors(void);
       bool can_change_color(void);
       int color_content(short color, short *r, short *g, short *b);
       int pair_content(short pair, short *f, short *b);
       int COLOR_PAIR(int n);

DESCRIPTION

Overview
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 rou‐
tine init_pair. After it has been initialized, COLOR_PAIR(n), a macro defined in <curses.h>, can be used
as a new 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.

Color Rendering
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 at‐
tribute.

• 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 at‐
tribute 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.

Routine Descriptions
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 inter‐
nal 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_light‐
ness_saturation) capability is set). The table is initialized first for eight basic colors (black,
red, green, yellow, blue, magenta, cyan, and white), and after that (if the terminal supports more
than eight colors) the components are initialized to 1000.

start_color does not attempt to set the terminal's color palette to match its built-in table. An ap‐
plication 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, which is typically a signed
16-bit integer (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).

• COLOR_PAIRS corresponds to the terminal database's max_pairs capability, which is typically a signed
16-bit integer (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 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_de‐
fault_colors) the upper limit is adjusted to allow for extra pairs which use a default color in fore‐
ground 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 routine, or to
specify the use of default colors (color number -1) if you first invoke the use_default_colors routine.

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.

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 exam‐
ple, 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-inde‐
pendent programs.

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.

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.

Colors
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

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.  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  sec‐
       ondary functions such as init_pair if start_color was not called.

          init_color
               returns an error if the terminal does not support this feature, e.g., if the initialize_color ca‐
               pability 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 ta‐
       ble, 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 im‐
       plementations may use a single shared color palette.

       Note that setting an implicit background color via a color pair affects only character cells that a char‐
       acter write operation explicitly touches.  To change the background color used when parts of a window are
       blanked by erasing or scrolling operations, see bkgd(3NCURSES).

       Several caveats apply on 386 and 486 machines 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  in‐
           stead).

       •   Color RGB values are not settable.

PORTABILITY