archie/tk4.2/doc/GetColor.3

'\"
'\" Copyright (c) 1990, 1991 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" SCCS: @(#) GetColor.3 1.22 96/08/27 13:21:26
'\" 
.so man.macros
.TH Tk_GetColor 3 4.0 Tk "Tk Library Procedures"
.BS
.SH NAME
Tk_GetColor, Tk_GetColorByValue, Tk_NameOfColor, Tk_FreeColor \- maintain database of colors
.SH SYNOPSIS
.nf
\fB#include <tk.h>\fR
.sp
XColor *
\fBTk_GetColor\fR(\fIinterp, tkwin, nameId\fB)\fR
.sp
XColor *
\fBTk_GetColorByValue\fR(\fItkwin, prefPtr\fB)\fR
.sp
char *
\fBTk_NameOfColor(\fIcolorPtr\fB)\fR
.sp
GC
\fBTk_GCForColor\fR(\fIcolorPtr, drawable\fR)
.sp
\fBTk_FreeColor(\fIcolorPtr\fB)\fR
.SH ARGUMENTS
.AS "Tcl_Interp" *colorPtr
.AP Tcl_Interp *interp in
Interpreter to use for error reporting.
.AP Tk_Window tkwin in
Token for window in which color will be used.
.AP Tk_Uid nameId in
Textual description of desired color.
.AP XColor *prefPtr in
Indicates red, green, and blue intensities of desired
color.
.AP XColor *colorPtr in
Pointer to X color information.  Must have been allocated by previous
call to \fBTk_GetColor\fR or \fBTk_GetColorByValue\fR, except when passed
to \fBTk_NameOfColor\fR.
.AP 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.
.BE

.SH DESCRIPTION
.PP
The \fBTk_GetColor\fR and \fBTk_GetColorByValue\fR procedures
locate pixel values that may be used to render particular
colors in the window given by \fItkwin\fR.  In \fBTk_GetColor\fR
the desired color is specified with a Tk_Uid (\fInameId\fR), which
may have any of the following forms:
.TP 20
\fIcolorname\fR
Any of the valid textual names for a color defined in the
server's color database file, such as \fBred\fR or \fBPeachPuff\fR.
.TP 20
\fB#\fIRGB\fR
.TP 20
\fB#\fIRRGGBB\fR
.TP 20
\fB#\fIRRRGGGBBB\fR
.TP 20
\fB#\fIRRRRGGGGBBBB\fR
A numeric specification of the red, green, and blue intensities
to use to display the color.  Each \fIR\fR, \fIG\fR, or \fIB\fR
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.
.PP
In \fBTk_GetColorByValue\fR, the desired color is indicated with
the \fIred\fR, \fIgreen\fR, and \fIblue\fR fields of the structure
pointed to by \fIcolorPtr\fR.
.PP
If \fBTk_GetColor\fR or \fBTk_GetColorByValue\fR 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 \fItkwin\fR is full, \fBTk_GetColor\fR
and \fBTk_GetColorByValue\fR will use the closest existing color
in the colormap.
If \fBTk_GetColor\fR encounters an error while allocating
the color (such as an unknown color name) then NULL is returned and
an error message is stored in \fIinterp->result\fR;
\fBTk_GetColorByValue\fR never returns an error.
.PP
\fBTk_GetColor\fR and \fBTk_GetColorByValue\fR maintain a database
of all the colors currently in use.
If the same \fInameId\fR is requested multiple times from
\fBTk_GetColor\fR (e.g. by different windows), or if the
same intensities are requested multiple times from
\fBTk_GetColorByValue\fR, 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
\fBTk_GetColor\fR or \fBTk_GetColorByValue\fR
instead of Xlib procedures like \fBXAllocColor\fR,
\fBXAllocNamedColor\fR, or \fBXParseColor\fR.
.PP
Since different calls to \fBTk_GetColor\fR or \fBTk_GetColorByValue\fR
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
\fBXAllocColorCells\fR to allocate the pixel value for the color.
.PP
The procedure \fBTk_NameOfColor\fR is roughly the inverse of
\fBTk_GetColor\fR.  If its \fIcolorPtr\fR argument was created
by \fBTk_GetColor\fR, then the return value is the \fInameId\fR
string that was passed to \fBTk_GetColor\fR to create the
color.  If \fIcolorPtr\fR was created by a call to \fBTk_GetColorByValue\fR,
or by any other mechanism, then the return value is a string
that could be passed to \fBTk_GetColor\fR to return the same
color.  Note:  the string returned by \fBTk_NameOfColor\fR is
only guaranteed to persist until the next call to \fBTk_NameOfColor\fR.
.PP
\fBTk_GCForColor\fR returns a graphics context whose \fBForeground\fR
field is the pixel allocated for \fIcolorPtr\fR 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 \fIcolorPtr\fR exists;  it is freed when the last reference
to \fIcolorPtr\fR is freed by calling \fBTk_FreeColor\fR.
.PP
When a pixel value returned by \fBTk_GetColor\fR or
\fBTk_GetColorByValue\fR is no longer
needed, \fBTk_FreeColor\fR should be called to release the color.
There should be exactly one call to \fBTk_FreeColor\fR for
each call to \fBTk_GetColor\fR or \fBTk_GetColorByValue\fR.
When a pixel value is no longer in
use anywhere (i.e. it has been freed as many times as it has been gotten)
\fBTk_FreeColor\fR will release it to the X server and delete it from
the database.

.SH KEYWORDS
color, intensity, pixel value