10564b1175
groff 1.23.0 added .MR to its -man macro package. The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020." From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.
Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.
Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized. groff's .MR does not. This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.
77 lines
1.5 KiB
Groff
77 lines
1.5 KiB
Groff
.TH READCOLMAP 3
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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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.MR draw 3
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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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.MR color 7 .
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.SH SOURCE
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.B \*9/src/libdraw
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.SH "SEE ALSO"
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.MR graphics 3 ,
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.MR draw 3 ,
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.MR color 7
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