maintain database of colors

## NAME

Tk_GetColor, Tk_GetColorByValue, Tk_NameOfColor, Tk_FreeColor - maintain database of colors

## SYNOPSIS

#include <tk.h>

XColor * Tk_GetColor(interp, tkwin, nameId)

XColor * Tk_GetColorByValue(tkwin, prefPtr)

char * Tk_NameOfColor(colorPtr)

GC Tk_GCForColor(colorPtr, drawable)

Tk_FreeColor(colorPtr)

## ARGUMENTS

`Tcl_Interp *interp` (in)
  Interpreter to use for error reporting.
`Tk_Window tkwin` (in)
  Token for window in which color will be used.
`Tk_Uid nameId` (in)
  Textual description of desired color.
`XColor *prefPtr` (in)
  Indicates red, green, and blue intensities of desired color.
`XColor *colorPtr` (in)
  Pointer to X color information. Must have been allocated by previous call to Tk_GetColor or Tk_GetColorByValue, except when passed to Tk_NameOfColor.
`Drawable drawable` (in)
  Drawable in which the result graphics context will be used. Must have same screen and depth as the window for which the color was allocated.

## DESCRIPTION

The Tk_GetColor and Tk_GetColorByValue procedures locate pixel values that may be used to render particular colors in the window given by tkwin. In Tk_GetColor the desired color is specified with a Tk_Uid (nameId), which may have any of the following forms:

colorname
  Any of the valid textual names for a color defined in the server's color database file, such as red or PeachPuff.
#RGB
#RRGGBB
#RRRGGGBBB
#RRRRGGGGBBBB
  A numeric specification of the red, green, and blue intensities to use to display the color. Each R, G, or B represents a single hexadecimal digit. The four forms permit colors to be specified with 4-bit, 8-bit, 12-bit or 16-bit values. When fewer than 16 bits are provided for each color, they represent the most significant bits of the color. For example, #3a7 is the same as #3000a0007000.

In Tk_GetColorByValue, the desired color is indicated with the red, green, and blue fields of the structure pointed to by colorPtr.

If Tk_GetColor or Tk_GetColorByValue is successful in allocating the desired color, then it returns a pointer to an XColor structure; the structure indicates the exact intensities of the allocated color (which may differ slightly from those requested, depending on the limitations of the screen) and a pixel value that may be used to draw in the color. If the colormap for tkwin is full, Tk_GetColor and Tk_GetColorByValue will use the closest existing color in the colormap. If Tk_GetColor encounters an error while allocating the color (such as an unknown color name) then NULL is returned and an error message is stored in interp->result; Tk_GetColorByValue never returns an error.

Tk_GetColor and Tk_GetColorByValue maintain a database of all the colors currently in use. If the same nameId is requested multiple times from Tk_GetColor (e.g. by different windows), or if the same intensities are requested multiple times from Tk_GetColorByValue, then existing pixel values will be re-used. Re-using an existing pixel avoids any interaction with the X server, which makes the allocation much more efficient. For this reason, you should generally use Tk_GetColor or Tk_GetColorByValue instead of Xlib procedures like XAllocColor, XAllocNamedColor, or XParseColor.

Since different calls to Tk_GetColor or Tk_GetColorByValue may return the same shared pixel value, callers should never change the color of a pixel returned by the procedures. If you need to change a color value dynamically, you should use XAllocColorCells to allocate the pixel value for the color.

The procedure Tk_NameOfColor is roughly the inverse of Tk_GetColor. If its colorPtr argument was created by Tk_GetColor, then the return value is the nameId string that was passed to Tk_GetColor to create the color. If colorPtr was created by a call to Tk_GetColorByValue, or by any other mechanism, then the return value is a string that could be passed to Tk_GetColor to return the same color. Note: the string returned by Tk_NameOfColor is only guaranteed to persist until the next call to Tk_NameOfColor.

Tk_GCForColor returns a graphics context whose Foreground field is the pixel allocated for colorPtr and whose other fields all have default values. This provides an easy way to do basic drawing with a color. The graphics context is cached with the color and will exist only as long as colorPtr exists; it is freed when the last reference to colorPtr is freed by calling Tk_FreeColor.

When a pixel value returned by Tk_GetColor or Tk_GetColorByValue is no longer needed, Tk_FreeColor should be called to release the color. There should be exactly one call to Tk_FreeColor for each call to Tk_GetColor or Tk_GetColorByValue. When a pixel value is no longer in use anywhere (i.e. it has been freed as many times as it has been gotten) Tk_FreeColor will release it to the X server and delete it from the database.

## KEYWORDS

color, intensity, pixel value