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       treadcolmap.3 (1531B)
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            1 .TH READCOLMAP 3
            2 .SH NAME
            3 RGB, readcolmap, writecolmap \- access display color map
            4 .SH SYNOPSIS
            5 .B #include <u.h>
            6 .br
            7 .B #include <libc.h>
            8 .br
            9 .B #include <draw.h>
           10 .PP
           11 .PP
           12 .ta \w'\fLvoid 'u
           13 .PP
           14 .B
           15 int        readcolmap(Display *d, RGB *map)
           16 .PP
           17 .B
           18 int        writecolmap(Display *d, RGB *map)
           19 .fi
           20 .SH DESCRIPTION
           21 Colors are described by their red, green, and blue
           22 light intensities, in an
           23 .B RGB
           24 datum:
           25 .IP
           26 .EX
           27 .ta 6n
           28 typedef
           29 struct RGB {
           30         ulong red;
           31         ulong green;
           32         ulong blue;
           33 } RGB;
           34 .EE
           35 .PP
           36 Black is represented by zero in all three positions and
           37 white has the maximum
           38 .B unsigned
           39 .B long
           40 value in all three positions.
           41 .PP
           42 A color map is an array of
           43 .BR RGB s,
           44 of length
           45 .if t \x'-.8n'2\u\s-1\fIdepth\fP\s+1\d,
           46 .if n 2^\fIdepth\fP,
           47 giving the colors for pixels 0, 1, 2, etc.
           48 On displays with color mapped pixels
           49 (typically 8-bit displays),
           50 one retrieves RGB color information
           51 by treating the pixel data as an offset
           52 into the color map.
           53 .PP
           54 .I Readcolmap
           55 reads the color map for the given display into the provided
           56 .IR map ,
           57 which must have enough space to hold it.
           58 .I Writecolmap
           59 associates the given color map with the given display, if possible.
           60 (The hardware might not allow this.)
           61 Both return 0 on success, or \-1 on error, setting
           62 .IR errstr .
           63 .PP
           64 Changing the hardware color map does not change
           65 the color map used by the
           66 .MR draw (3)
           67 operator to convert between
           68 mapped and true color or greyscale images,
           69 which is described in 
           70 .MR color (7) .
           71 .SH SOURCE
           72 .B \*9/src/libdraw
           73 .SH "SEE ALSO"
           74 .MR graphics (3) ,
           75 .MR draw (3) ,
           76 .MR color (7)