# curs_color 3x

    curs_color(3x)                                                  curs_color(3x)

## 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  dis-
           played).   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  rou-
           tines  has_colors  and can_change_color return TRUE or FALSE, depending
           on whether the terminal has color capabilities and whether the program-
           mer can change the colors.  The routine color_content allows a program-
           mer 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  fore-
           ground and background colors shown on the screen:

           o   per-character video attributes (e.g., via waddch),

           o   the window attribute (e.g., by wattrset), and

           o   the background character (e.g., wbkgdset).

           Per-character and window attributes are usually set by a parameter con-
           taining video attributes including a color pair value.  Some  functions
           such as wattr_set use a separate parameter which is the color pair num-
           ber.

           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:

           o   If the parameter passed to waddch is blank, and it uses the special
               color pair 0,

               o   curses next checks the window attribute.

               o   If  the window attribute does not use color pair 0, curses uses
                   the color pair from the window attribute.

               o   Otherwise, curses uses the background character.

           o   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 attri-
               bute next, and finally the background character.

           Some  curses  functions such as wprintw call waddch.  Those do not com-
           bine its parameter with a color pair.  Consequently those calls use on-
           ly 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  ter-
           minal 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 manipu-
           lation routine is called.  It is good practice  to  call  this  routine
           right after initscr.  start_color does this:

           o   It  initializes  two  global variables, COLORS and COLOR_PAIRS (re-
               spectively defining the maximum number of  colors  and  color-pairs
               the terminal can support).

           o   It  initializes  the special color pair 0 to the default foreground
               and background colors.  No other color pairs are initialized.

           o   It restores the colors on the terminal to the values they had  when
               the terminal was just turned on.

           o   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 al-
               ter 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:

           o   COLORS  corresponds to the terminal database's max_colors capabili-
               ty, (see terminfo(5)).

           o   color values are expected to be in the range 0 to COLORS-1,  inclu-
               sive (including 0 and COLORS-1).

           o   a  special  color value -1 is used in certain extended functions to
               denote the default color (see use_default_colors(3x)).

           o   COLOR_PAIRS corresponds to the terminal database's max_pairs  capa-
               bility, (see terminfo(5)).

           o   legal color pair values are in the range 1 to COLOR_PAIRS-1, inclu-
               sive.

           o   color pair 0 is special; it denotes "no color".

               Color pair 0 is assumed to be white on black, but is actually what-
               ever  the terminal implements before color is initialized.  It can-
               not 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 rou-
           tine 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-indepen-
           dent 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  fore-
           ground color number, and the background color number.  For portable ap-
           plications:

           o   The first argument must be a legal color pair  value.   If  default
               colors are used (see use_default_colors(3x)) the upper limit is ad-
               justed to allow for extra pairs which use a default color in  fore-
               ground and/or background.

           o   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  defini-
           tion.

           As  an  extension,  ncurses  allows you to set color pair 0 via the as-
           sume_default_colors(3x) routine, or to specify the use of default  col-
           ors  (color  number  -1) if you first invoke the use_default_colors(3x)
           routine.

### init_extended_pair

           Because init_pair uses signed shorts for its  parameters,  that  limits
           color-pairs  and  color-values to 32767 on modern hardware.  The exten-
           sion init_extended_pair uses ints 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).

           o   The  first argument must be a legal color value; default colors are
               not allowed here.  (See the section Colors for  the  default  color
               index.)

           o   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 shorts for its parameters, that limits
           color-values and their red, green, and blue components to 32767 on mod-
           ern hardware.  The extension init_extended_color uses ints for the col-
           or 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 shorts for
           storing the information about the amounts of red, green, and blue  com-
           ponents in the given color.

           o   The  first  argument  must  be a legal color value, i.e., 0 through
               COLORS-1, inclusive.

           o   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 shorts for its parameters, that  lim-
           its  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.

### 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.

           o   The  first argument must be a legal color value, i.e., in the range
               1 through COLOR_PAIRS-1, inclusive.

           o   The values that are stored at the addresses pointed to by the  sec-
               ond  and  third arguments are in the range 0 through COLORS, inclu-
               sive.

### extended_pair_content

           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 col-
           ors 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 com-
           pletion.

           X/Open defines no error conditions.  SVr4 does document some error con-
           ditions which apply in general:

           o   This implementation will return ERR on attempts to use color values
               outside the range 0 to COLORS-1 (except for the default colors  ex-
               tension), 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.

           o   SVr4  does much the same, except that it returns ERR from pair_con-
               tent if the pair was not initialized using init_pairs  and  it  re-
               turns  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 COL-
           OR_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:

           o   COLOR_YELLOW  is  actually  brown.  To get yellow, use COLOR_YELLOW
               combined with the A_BOLD attribute.

           o   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).

           o   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 struc-
           ture), 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:

           o   PCCurses (1987-1990) provided for only eight (8) colors.

           o   PDCurses  (1992-present)  inherited the 8-color limitation from PC-
               Curses, but changed this to 256 in version 2.5 (2001),  along  with
               changing chtype from 16-bits to 32-bits.

           o   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.

           o   ncurses (1992-present) uses eight bits for A_COLOR in  chtype  val-
               ues.

               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  us-
               ing chtype values.

           o   NetBSD  curses  used  6 bits from 2000 (when colors were first sup-
               ported) 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 back-
           ground 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)  exten-
           sion.

           This  implementation checks the pointers, e.g., for the values returned
           by color_content and pair_content, and will treat those as optional pa-
           rameters 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), de-
           fault_colors(3x)

                                                                    curs_color(3x)