1Pnmshear User Manual(0) Pnmshear User Manual(0)
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6 pnmshear - shear a PNM image by a specified angle
7
10 pnmshear
11 [-noantialias] [-background=color] angle [pnmfile]
13 All options can be abbreviated to their shortest unique prefix. You
15 may use two hyphens instead of one to designate an option. You may use
16 either white space or equals signs between an option name and its
17 value.
18
21 This program is part of Netpbm(1).
22 pnmshear reads a PNM image as input and shears it by the specified an‐
24 gle and produce a PNM image as output. If the input file is in color,
25 the output will be too, otherwise it will be grayscale. The angle is
26 in degrees (floating point), and measures this:
27 +-------+ +-------+
29 | | |\ \
30 | OLD | | \ NEW \
31 | | |an\ \
32 +-------+ |gle+-------+
33 If the angle is negative, it shears the other way:
36 +-------+ |-an+-------+
37 | | |gl/ /
38 | OLD | |e/ NEW /
39 | | |/ /
40 +-------+ +-------+
41 The angle should not get too close to 90 or -90, or the resulting image
44 will be unreasonably wide. In fact, if it gets too close, the width
45 will be so large that pnmshear cannot do computations in the word sizes
46 it uses, and the program detects this and fails.
47 pnmshear does the shearing by looping over the source pixels and dis‐
49 tributing fractions to each of the destination pixels. This has an
50 "anti-aliasing" effect - it avoids jagged edges and similar artifacts.
51 However, it also means that the original colors in the image are modi‐
52 fied and there are typically more of them than you started with. If
53 you need to keep precisely the same set of colors, see the -noantialias
54 option. If the expanded palette is a problem, you can run the result
55 through pnmquant.
56
59 In addition to the options common to all programs based on libnetpbm
60 (most notably -quiet, see
61 Common Options ⟨index.html#commonoptions⟩ ), pnmshear recognizes the
62 following command line options:
63 -background=color
67 This determines the color of the background on which the sheared
68 image sits.
69 Specify the color (color) as described for the argument of the
71 pnm_parsecolor() library routine
72 ⟨libnetpbm_image.html#colorname⟩ .
73 By default, if you don't specify this option, pnmshear selects
75 what appears to it to be the background color of the original
76 image. It determines this color rather simplistically, by tak‐
77 ing an average of the colors of the two top corners of the im‐
78 age.
79 This option was new in Netpbm 10.37 (December 2006). Before
81 that, pnmshear always behaved as is the default now.
82 -noantialias
85 This option forces pnmshear to simply move pixels around instead
86 of synthesizing output pixels from multiple input pixels. The
87 latter could cause the output to contain colors that are not in
88 the input, which may not be desirable. It also probably makes
89 the output contain a large number of colors. If you need a
90 small number of colors, but it doesn't matter if they are the
91 exact ones from the input, consider using pnmquant on the output
92 instead of using -noantialias.
93 Note that to ensure the output does not contain colors that are
95 not in the input, you also must consider the background color.
96 See the -background option.
97
102 pnmrotate(1), pamflip(1), pamhomography(1), pnmquant(1), pnm(1)
103
107 Copyright (C) 1989, 1991 by Jef Poskanzer.
108
110 This manual page was generated by the Netpbm tool 'makeman' from HTML
111 source. The master documentation is at
112 http://netpbm.sourceforge.net/doc/pnmshear.html
114netpbm documentation 22 March 2020 Pnmshear User Manual(0)