Tk_GetColor(3)
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NAME
Tk_GetColor, Tk_GetColorByValue, Tk_NameOfColor, Tk_Free-
Color - 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 pre-
vious call to Tk_Get-
Color or Tk_GetColorBy-
Value, except when
passed to Tk_NameOf-
Color.
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.
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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 Peach-
Puff.
#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 limita-
tions 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 exist-
ing pixel avoids any interaction with the X server, which
makes the allocation much more efficient. For this rea-
son, you should generally use Tk_GetColor or Tk_GetColor-
ByValue instead of Xlib procedures like XAllocColor, XAl-
locNamedColor, 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 proce-
dures. 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_Name-
OfColor.
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 col-
orPtr exists; it is freed when the last reference to col-
orPtr is freed by calling Tk_FreeColor.
When a pixel value returned by Tk_GetColor or Tk_GetColor-
ByValue 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_GetColor-
ByValue. When a pixel value is no longer in use anywhere
(i.e. it has been freed as many times as it has been got-
ten) Tk_FreeColor will release it to the X server and
delete it from the database.
KEYWORDS
color, intensity, pixel value