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.if !"\\$1"" \&\\$1 \\$2 \\$3 \\$4 \\$5 \\$6
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.de L
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.if !"\\$1"" \&\\$1 \\$2 \\$3 \\$4 \\$5 \\$6
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.BR "\\$1" "\\$2" "\\$3" "\\$4" "\\$5" "\\$6"
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.de LR
.BR "\\$1" "\\$2" "\\$3" "\\$4" "\\$5" "\\$6"
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.de CW
.ft B
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.\" This is gross but it avoids relying on internal implementation details
.\" of the -man macros.
.de TF
.IP "" \w'\fB\\$1\ \ \fP'u
.PD0
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.de EX
.CW
.nf
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.de EE
.fi
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.\" delete above this point if your system has F, L, FR, LR, CW and TF macros
.TH RGBPIX 3G
.SH NAME
RGB, rgbpix, rdcolmap, wrcolmap
.SH SYNOPSIS
.nf
.PP
.B
#include <libc.h>
.B
#include <libg.h>
.PP
.ta \w'\fLunsigned long 'u
.B
unsigned long	rgbpix(Bitmap *b, RGB rgb)
.PP
.B
void	rdcolmap(Bitmap *b, RGB *map)
.PP
.B
void	wrcolmap(Bitmap *b, RGB *map)
.fi
.SH DESCRIPTION
Colors are described by the red, green, and blue
light intensities, in an
.B RGB
datum:
.IP
.EX
.ta 6n
typedef
struct RGB {
	unsigned long red;
	unsigned long green;
	unsigned long blue;
} RGB;
.EE
.PP
Zero intensity means there is no component of the given color;
hence, black is represented by zero in all three positions and
white has the maximum unsigned long value in all three positions.
.PP
Some of the graphics functions, such as
.I point
(see
.IR bitblt (3)),
take a
.I pixel value
argument, which is a single unsigned long.
For a given bitmap,
.I rgbpix
returns the pixel value with a color closest to
the color represented by the
.I rgb
argument.
.PP
There is a
.I colormap
associated with each Bitmap.  A colormap is an array of
.BR RGB s,
of length
.if t 2\u\s82\u\s6\fIldepth\fP\d\d\s10,
.if n 2^(2^\fIldepth\fP),
giving the colors for pixels 0, 1, 2, etc.
.PP
.I Rdcolormap
reads the colormap for the given bitmap into the provided
.IR map ,
which must have enough space to hold it.
.I Wrcolormap
associates the given colormap with the given bitmap, if possible.
(The hardware might not allow this.)
.SH CAVEAT
The X implementation of libg uses whatever colourmap is
the default when it starts up.
A call to
.I rgbpix
will result in a call to
.I XAllocColor
and the allocation of a new colour in the application's colourmap
if there is room.
If no new colours can be allocated
.I rgbpix
will return the closest pixel value approximating the requested colour.
If an application calls
.I wrcolmap
the default colourmap is replaced with the new colourmap and from
then on
.I rgbpix
will deal with the new colourmap.
.SH BUGS
These functions only work for the screen bitmap.
This interface will have to be refined for screens with
ldepth > 3.
.SH "SEE ALSO"
.IR graphics (3),
.IR bitblt (3)