Manpages - curs_color.3x

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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 - 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), 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.

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*(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.

init_extended_pair

Because init_pair uses signed short*s for its parameters, that limits color-pairs and color-values to 32767 on modern hardware. The extension *init_extended_pair uses *int*s 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).

  • 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.

init_extended_color

Because init_color uses signed short*s 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 *int*s 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 *short*s 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.

extended_color_content

Because color_content uses signed short*s 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 *int*s 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 *short*s 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.

extended_pair_content

Because pair_content uses signed short*s for its parameters, that limits color-pair and color-values to 32767 on modern hardware. The extension *extended_pair_content uses *int*s 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.

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. 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.

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:

  • 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.

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:

  • 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.

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.

SEE ALSO

*curses*(3X), *curs_initscr*(3X), *curs_attr*(3X), *curs_variables*(3X), *default_colors*(3X)

Author: dt

Created: 2022-02-20 Sun 14:44