Tk_GetColor(3)

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
Interpreter to use for error reporting. Token for window in which color will be used. Textual description of desired color. Indicates red, green, and blue intensities of desired color. 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 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