1Pnmshear User Manual(0) Pnmshear User Manual(0)
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6 pnmshear - shear a PNM image by a specified angle
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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
24 angle and produce a PNM image as output. If the input file is in
25 color, the output will be too, otherwise it will be grayscale. The
26 angle is 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:
35 +-------+ |-an+-------+
36 | | |gl/ /
37 | OLD | |e/ NEW /
38 | | |/ /
39 +-------+ +-------+
40 The angle should not get too close to 90 or -90, or the resulting image
42 will be unreasonably wide.
43 pnmshear does the shearing by looping over the source pixels and dis‐
45 tributing fractions to each of the destination pixels. This has an
46 'anti-aliasing' effect - it avoids jagged edges and similar artifacts.
47 However, it also means that the original colors in the image are modi‐
48 fied and there are typically more of them than you started with. If
49 you need to keep precisely the same set of colors, see the -noantialias
50 option. If the expanded palette is a problem, you can run the result
51 through pnmquant.
52
55 -background=color
56 This determines the color of the background on which the sheared
57 image sits.
58 Specify the color (color) as described for the argument of the
60 ppm_parsecolor() library routine ⟨libppm.html#colorname⟩ .
61 By default, if you don't specify this option, pnmshear selects
63 what appears to it to be the background color of the original
64 image. It determines this color rather simplisticly, by taking
65 an average of the colors of the two top corners of the image.
66 This option was new in Netpbm 10.37 (December 2006). Before
68 that, pnmshear always behaved as is the default now.
69 -noantialias
72 This option forces pnmrotate to simply move pixels around
73 instead of synthesizing output pixels from multiple input pix‐
74 els. The latter could cause the output to contain colors that
75 are not in the input, which may not be desirable. It also prob‐
76 ably makes the output contain a large number of colors. If you
77 need a small number of colors, but it doesn't matter if they are
78 the exact ones from the input, consider using pnmquant on the
79 output instead of using -noantialias.
80 Note that to ensure the output does not contain colors that are
82 not in the input, you also must consider the background color.
83 See the -background option.
84
89 pnmrotate(1), pamflip(1), pnmquant(1), pnm(1)
90
94 Copyright (C) 1989, 1991 by Jef Poskanzer.
95netpbm documentation 27 November 2006 Pnmshear User Manual(0)