Ubuntu Manpage: start_color, has_colors, can_change_color, init_pair, init_color, color

Provided by: ncurses-doc_6.1-1ubuntu1.18.04.1_all

NAME

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

SYNOPSIS

       #include <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);

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) 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.

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.

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.

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:

• 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, (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, (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.

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:

• 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(3X)
routine, or to specify the use of default colors (color number -1) if you first invoke the
use_default_colors(3X) routine.

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.

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).

• 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.

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.

• 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.

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.

• 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.

PAIR_NUMBER
PAIR_NUMBER(attrs) extracts the color value from its attrs parameter and returns it as a
color pair number.

COLOR_PAIR
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.

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.

       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  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.

PORTABILITY

       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.