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Tk_AllocColorFromObj(3) Tk Library Procedures Tk_AllocColorFromObj(3)
Tk_AllocColorFromObj, Tk_GetColor, Tk_GetColorFromObj,
Tk_GetColorByValue, Tk_NameOfColor, Tk_FreeColorFromObj, Tk_FreeColor -
maintain database of colors
Tk_AllocColorFromObj(interp, tkwin, objPtr)
Tk_GetColor(interp, tkwin, name)
const char *
Tcl_Interp *interp (in) Interpreter to use for error
Tk_Window tkwin (in) Token for window in which color will
Tcl_Obj *objPtr (in/out) String value describes desired
color; internal rep will be modified
to cache pointer to corresponding
char *name (in) Same as objPtr except description of
color is passed as a string and
resulting (XColor *) is not cached.
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_AllocColorFromObj,
Tk_GetColor or Tk_GetColorByValue,
except when passed to
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
These procedures manage the colors being used by a Tk application.
They allow colors to be shared whenever possible, so that colormap
space is preserved, and they pick closest available colors when
colormap space is exhausted.
Given a textual description of a color, Tk_AllocColorFromObj locates a
pixel value that may be used to render the color in a particular
window. The desired color is specified with a value whose string value
must have one 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
#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,
while the lower unfilled bits will be repeatedly
replicated from the available higher bits. For
example, #3a7 is the same as #3333aaaa7777.
Tk_AllocColorFromObj 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 with the
color in tkwin. If an error occurs in Tk_AllocColorFromObj (such as an
unknown color name) then NULL is returned and an error message is
stored in interp's result if interp is not NULL. If the colormap for
tkwin is full, Tk_AllocColorFromObj will use the closest existing color
in the colormap. Tk_AllocColorFromObj caches information about the
return value in objPtr, which speeds up future calls to procedures such
as Tk_AllocColorFromObj and Tk_GetColorFromObj.
Tk_GetColor is identical to Tk_AllocColorFromObj except that the
description of the color is specified with a string instead of a value.
This prevents Tk_GetColor from caching the return value, so Tk_GetColor
is less efficient than Tk_AllocColorFromObj.
Tk_GetColorFromObj returns the token for an existing color, given the
window and description used to create the color. Tk_GetColorFromObj
does not actually create the color; the color must already have been
created with a previous call to Tk_AllocColorFromObj or Tk_GetColor.
The return value is cached in objPtr, which speeds up future calls to
Tk_GetColorFromObj with the same objPtr and tkwin.
Tk_GetColorByValue is similar to Tk_GetColor except that the desired
color is indicated with the red, green, and blue fields of the
structure pointed to by colorPtr.
This package maintains a database of all the colors currently in use.
If the same color is requested multiple times from Tk_GetColor or
Tk_AllocColorFromObj (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 window server, which makes the
allocation much more efficient. These procedures also provide a
portable interface that works across all platforms. For this reason,
you should generally use Tk_AllocColorFromObj, Tk_GetColor, or
Tk_GetColorByValue instead of lower level procedures like XAllocColor.
Since different calls to this package 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_AllocColorFromObj or
Tk_GetColor then the return value is the string that was used 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 color is no longer needed Tk_FreeColorFromObj or Tk_FreeColor
should be called to release it. For Tk_FreeColorFromObj the color to
release is specified with the same information used to create it; for
Tk_FreeColor the color to release is specified with a pointer to its
XColor structure. There should be exactly one call to
Tk_FreeColorFromObj or Tk_FreeColor for each call to
Tk_AllocColorFromObj, Tk_GetColor, or Tk_GetColorByValue.
color, intensity, value, pixel value
Tk 8.1 Tk_AllocColorFromObj(3)