man/man3/Tk_GetColorByValue.3

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.TH Tk_AllocColorFromObj 3 8.1 Tk "Tk Library Procedures"
.BS
.SH NAME
Tk_AllocColorFromObj, Tk_GetColor, Tk_GetColorFromObj, Tk_GetColorByValue, Tk_NameOfColor, Tk_FreeColorFromObj, Tk_FreeColor \- maintain database of colors
.SH SYNOPSIS
.nf
\fB#include <tk.h>\fR
.VS 8.1
.sp
XColor *
\fBTk_AllocColorFromObj(\fIinterp, tkwin, objPtr\fB)\fR
.sp
XColor *
\fBTk_GetColor(\fIinterp, tkwin, name\fB)\fR
.sp
XColor *
\fBTk_GetColorFromObj(\fItkwin, objPtr\fB)\fR
.VE
.sp
XColor *
\fBTk_GetColorByValue(\fItkwin, prefPtr\fB)\fR
.sp
CONST char *
\fBTk_NameOfColor(\fIcolorPtr\fB)\fR
.sp
GC
\fBTk_GCForColor(\fIcolorPtr, drawable\fB)\fR
.sp
.VS 8.1
\fBTk_FreeColorFromObj(\fItkwin, objPtr\fB)\fR
.VE
.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.
.VS 8.1 br
.AP Tcl_Obj *objPtr in/out
String value describes desired color; internal rep will be
modified to cache pointer to corresponding (XColor *).
.AP char *name in
Same as \fIobjPtr\fR except description of color is passed as a string and
resulting (XColor *) isn't cached.
.VE
.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_AllocColorFromObj\fR, \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
.VS 8.1
.PP
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.
.PP
Given a textual description of a color, \fBTk_AllocColorFromObj\fR
locates a pixel value that may be used to render the color
in a particular window.  The desired color is specified with an
object whose string value must have one of the following forms:
.VE
.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
.VS 8.1
\fBTk_AllocColorFromObj\fR 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 \fItkwin\fR.
If an error occurs in \fBTk_AllocColorFromObj\fR (such as an unknown
color name) then NULL is returned and an error message is stored in
\fIinterp\fR's result if \fIinterp\fR isn't NULL.
If the colormap for \fItkwin\fR is full, \fBTk_AllocColorFromObj\fR
will use the closest existing color in the colormap.
\fBTk_AllocColorFromObj\fR caches information about
the return value in \fIobjPtr\fR, which speeds up future calls to procedures
such as \fBTk_AllocColorFromObj\fR and \fBTk_GetColorFromObj\fR.
.PP
\fBTk_GetColor\fR is identical to \fBTk_AllocColorFromObj\fR except
that the description of the color is specified with a string instead
of an object.  This prevents \fBTk_GetColor\fR from caching the
return value, so \fBTk_GetColor\fR is less efficient than
\fBTk_AllocColorFromObj\fR.
.PP
\fBTk_GetColorFromObj\fR returns the token for an existing color, given
the window and description used to create the color.
\fBTk_GetColorFromObj\fR doesn't actually create the color; the color
must already have been created with a previous call to
\fBTk_AllocColorFromObj\fR or \fBTk_GetColor\fR.  The return
value is cached in \fIobjPtr\fR, which speeds up
future calls to \fBTk_GetColorFromObj\fR with the same \fIobjPtr\fR
and \fItkwin\fR.
.VE
.PP
\fBTk_GetColorByValue\fR is similar to \fBTk_GetColor\fR except that
the desired color is indicated with the \fIred\fR, \fIgreen\fR, and
\fIblue\fR fields of the structure pointed to by \fIcolorPtr\fR.
.PP
This package maintains a database
of all the colors currently in use.
If the same color is requested multiple times from
\fBTk_GetColor\fR or \fBTk_AllocColorFromObj\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 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
\fBTk_AllocColorFromObj\fR, \fBTk_GetColor\fR, or \fBTk_GetColorByValue\fR
instead of lower level procedures like \fBXAllocColor\fR.
.PP
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
\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_AllocColorFromObj\fR or \fBTk_GetColor\fR then the return value
is the string that was used 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
.VS 8.1
When a color is no longer needed \fBTk_FreeColorFromObj\fR or
\fBTk_FreeColor\fR should be called to release it.
For \fBTk_FreeColorFromObj\fR the color to release is specified
with the same information used to create it; for
\fBTk_FreeColor\fR the color to release is specified
with a pointer to its XColor structure.
There should be exactly one call to \fBTk_FreeColorFromObj\fR
or \fBTk_FreeColor\fR for each call to \fBTk_AllocColorFromObj\fR,
\fBTk_GetColor\fR, or \fBTk_GetColorByValue\fR.
.VE
.SH KEYWORDS
color, intensity, object, pixel value