Ubuntu Manpage: start_color, init_pair, init_color, has_colors, can_change_color, 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 routine 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 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.

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 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), 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 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, 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_default_colors) 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
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 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 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 secondary 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 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.

       Note that 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
       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 instead).

       •   Color RGB values are not settable.

PORTABILITY