166 lines
4.7 KiB
Text
166 lines
4.7 KiB
Text
$NetBSD: patch-ao,v 1.1 1999/01/10 23:52:16 tron Exp $
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--- pnm/pnmhisteq.1.orig Sun Jan 10 23:48:41 1999
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+++ pnm/pnmhisteq.1 Sun Jan 10 23:48:41 1999
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@@ -0,0 +1,161 @@
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+.TH pnmhisteq 1 "19 March 1995"
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+.IX "pnmhisteq command"
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+.IX histogram
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+.SH NAME
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+pnmhisteq \- histogram equalise a portable anymap
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+.SH SYNOPSIS
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+.na
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+.B pnmhisteq
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+'ti 15
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+.RB [ \-gray ]
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+.RB [ \-rmap
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+.IR pgmfile ]
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+.RB [ \-wmap
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+.IR pgmfile ]
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+.RB [ \-verbose ]
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+.RI [ pnmfile ]
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+.ad
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+.SH DESCRIPTION
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+.B pnmhisteq
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+increases the contrast of a portable graymap or pixmap through the
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+technique of
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+.IR "histogram equalisation" [1].
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+A histogram of the luminance of pixels in the map is computed, from
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+which a transfer function is calculated which spreads out intensity levels
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+around histogram peaks and compresses them at troughs. This has the
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+effect of using the available levels of intensity more efficiently and
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+thereby increases the detail visible in the image.
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+.PP
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+Mathematically, if
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+.I N[i]
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+is the number of pixels of luminosity
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+.I i
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+in the image and
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+.I T
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+is the total number of pixels, luminosity
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+.I j
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+is replaced by:
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+.PP
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+.NF
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+.vs 9p
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+ j
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+ ---
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+ \\
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+ \> N[i] / T
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+ \/
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+ ---
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+ i=0
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+.vs
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+.FI
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+.PP
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+If you're processing a related set of images, for example frames of an
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+animation, it's generally best to apply the same intensity map to
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+every frame, since otherwise you'll get distracting frame-to-frame
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+changes in the brightness of objects.
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+.BR pnmhisteq 's
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+.B \-wmap
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+option
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+allows you to save, as a portable graymap, the luminosity map computed
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+from an image (usually a composite of the images you intend
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+to process created with
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+.BR pnmcat ).
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+Then, you can subsequently process each of the individual images using
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+the luminosity map saved in the file, supplied with the
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+.B \-rmap
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+option.
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+.SH OPTIONS
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+.TP 10
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+.B \-gray
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+When processing a pixmap, only gray pixels (those with identical red,
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+green, and blue values) are included in the histogram and modified
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+in the output image. This is a special purpose option intended
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+for images where the actual data are gray scale, with
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+colour annotations you don't want modified. Weather
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+satellite images that show continent outlines in colour are best
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+processed using this option. The option has no effect when the input
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+is a graymap.
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+.TP
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+.BI \-rmap " mapfile"
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+Process the image using the luminosity map specified by the portable
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+graymap
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+.I mapfile.
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+The graymap, usually created by an earlier run of
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+.B pnmhisteq
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+with the
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+.B \-wmap
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+option, contains a single row with number of columns equal to
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+the
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+.I maxval
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+(greatest intensity) of the image. Each pixel in the image is
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+transformed by looking up its luminosity in the corresponding
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+column in the map file and changing it to the value
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+given by that column.
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+.TP
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+.BI \-wmap " mapfile"
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+Creates a portable graymap,
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+.I mapfile,
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+containing the luminosity map computed from the histogram of the
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+input image. This map file can be read on subsequent runs of
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+.B pnmhisteq
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+with the
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+.B \-rmap
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+option, allowing a group of images to be processed with
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+an identical map.
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+.TP
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+.B \-verbose
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+Prints the histogram and luminosity map on standard error.
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+.PP
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+All flags can be abbreviated to their shortest unique prefix.
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+.SH BUGS
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+Histogram equalisation is effective for increasing the visible detail
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+in scientific imagery and in some continuous-tone pictures. It is
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+often too drastic, however, for scanned halftone images, where it
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+does an excellent job of making halftone artifacts apparent. You
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+might want to experiment with
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+.BR pgnnorm ,
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+.BR ppmnorm ,
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+and
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+.B pnmgamma
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+for more subtle contrast enhancement.
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+.PP
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+The luminosity map file supplied by the
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+.B \-rmap
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+option must have the same
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+.I maxval
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+as the input image. This is always the case when the
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+map file was created by the
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+.B \-wmap
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+option of
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+.BR pnmhisteq .
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+If this restriction causes a problem, simply adjust the
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+.I maxval
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+of the map with
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+.B pnmdepth
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+to agree with the input image.
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+.PP
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+If the input is a PBM file (on which histogram equalisation is an
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+identity operation), the only effect of passing the file through
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+.B pnmhisteq
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+will be the passage of time.
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+.SH "SEE ALSO"
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+.PD
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+.BR pgmnorm (1),
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+.BR pnm (5),
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+.BR pnmcat (1),
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+.BR pnmdepth (1),
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+.BR pnmgamma (1),
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+.BR pnmnorm (1)
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+.TP 5
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+[1]
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+Russ, John C. The Image Processing Handbook.
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+Boca Raton: CRC Press, 1992. Pages 105-110.
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+.ne 10
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+.SH AUTHOR
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+.ce 2
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+Copyright (C) 1995 by John Walker (kelvin@fourmilab.ch).
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+WWW home page: http://www.fourmilab.ch/
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+.PP
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+Permission to use, copy, modify, and distribute this software and its
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+documentation for any purpose and without fee is hereby granted,
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+without any conditions or restrictions. This software is provided ``as
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+is'' without express or implied warranty.
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