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.TH READCOLMAP 2
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.SH NAME
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RGB, readcolmap, writecolmap \- access display color map
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.br
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.B #include <draw.h>
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.PP
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.PP
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.ta \w'\fLvoid 'u
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.PP
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.B
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int	readcolmap(Display *d, RGB *map)
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.PP
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.B
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int	writecolmap(Display *d, RGB *map)
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.fi
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.SH DESCRIPTION
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Colors are described by their red, green, and blue
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light intensities, in an
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.B RGB
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datum:
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.IP
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.EX
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.ta 6n
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typedef
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struct RGB {
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	ulong red;
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	ulong green;
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	ulong blue;
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} RGB;
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.EE
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.PP
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Black is represented by zero in all three positions and
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white has the maximum
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.B unsigned
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.B long
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value in all three positions.
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.PP
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A color map is an array of
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.BR RGB s,
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of length
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.if t \x'-.8n'2\u\s-1\fIdepth\fP\s+1\d,
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.if n 2^\fIdepth\fP,
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giving the colors for pixels 0, 1, 2, etc.
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On displays with color mapped pixels
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(typically 8-bit displays),
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one retrieves RGB color information
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by treating the pixel data as an offset
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into the color map.
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.PP
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.I Readcolmap
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reads the color map for the given display into the provided
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.IR map ,
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which must have enough space to hold it.
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.I Writecolmap
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associates the given color map with the given display, if possible.
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(The hardware might not allow this.)
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Both return 0 on success, or \-1 on error, setting
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.IR errstr .
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.PP
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Changing the hardware color map does not change
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the color map used by the
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.IR draw (2)
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operator to convert between
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mapped and true color or greyscale images,
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which is described in
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.IR color (6).
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.SH SOURCE
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.B /sys/src/libdraw
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.SH "SEE ALSO"
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.IR graphics (2),
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.IR draw (3),
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.IR color (6)