1Pgmcrater User Manual(0) Pgmcrater User Manual(0)
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6 pgmcrater - create cratered terrain by fractal forgery
7
10 pgmcrater
11 [-number n]
13 [-height|-ysize s]
15 [-width|-xsize s]
17 [-gamma g]
19
23 This program is part of Netpbm(1).
24 pgmcrater creates a PGM image which mimics cratered terrain. The PGM
26 image is created by simulating the impact of a given number of craters
27 with random position and size, then rendering the resulting terrain
28 elevations based on a light source shining from one side of the screen.
29 The size distribution of the craters is based on a power law which
30 results in many more small craters than large ones. The number of
31 craters of a given size varies as the reciprocal of the area as
32 described on pages 31 and 32 of Peitgen and Saupe[1]; cratered bodies
33 in the Solar System are observed to obey this relationship. The for‐
34 mula used to obtain crater radii governed by this law from a uniformly
35 distributed pseudorandom sequence was developed by Rudy Rucker.
36 High resolution images with large numbers of craters often benefit from
38 being piped through pnmsmooth. The averaging performed by this process
39 eliminates some of the jagged pixels and lends a mellow ``telescopic
40 image'' feel to the overall picture.
41 pgmcrater simulates only small craters, which are hemispherical in
43 shape (regardless of the incidence angle of the impacting body, as long
44 as the velocity is sufficiently high). Large craters, such as Coperni‐
45 cus and Tycho on the Moon, have a ``walled plain'' shape with a cross-
46 section more like:
47 /\ /\
49 _____/ \____________/\____________/ \_____
50 Larger craters should really use this profile, including the central
53 peak, and totally obliterate the pre-existing terrain.
54 The randomness in the image is limited before Netpbm 10.37 (December
56 2006) -- if you run the program twice in the same second, you may get
57 identical output.
58
61 All options can be abbreviated to their shortest unique prefix.
62 -number n
66 Causes n craters to be generated. If no -number specification
67 is given, 50000 craters will be generated. Don't expect to see
68 them all! For every large crater there are many, many more tiny
69 ones which tend simply to erode the landscape. In general, the
70 more craters you specify the more realistic the result; ideally
71 you want the entire terrain to have been extensively turned over
72 again and again by cratering. High resolution images containing
73 five to ten million craters are stunning but take quite a while
74 to create.
75 -height height
78 Sets the height of the generated image to height pixels. The
79 default height is 256 pixels.
80 -width width
83 Sets the width of the generated image to width pixels. The
84 default width is 256 pixels.
85 -xsize width
88 Sets the width of the generated image to width pixels. The
89 default width is 256 pixels.
90 -ysize height
93 Sets the height of the generated image to height pixels. The
94 default height is 256 pixels.
95 -gamma factor
98 The specified factor is used to gamma adjust the image in the
99 same manner as performed by pnmgamma. The default value is 1.0,
100 which results in a medium contrast image. Values larger than 1
101 lighten the image and reduce contrast, while values less than 1
102 darken the image, increasing contrast.
103 Note that this is separate from the gamma correction that is
105 part of the definition of the PGM format. The image pnmgamma
106 generates is a genuine, gamma-corrected PGM image in any case.
107 This option simply changes the contrast and may compensate for a
108 display device that does not correctly render PGM images.
109
114 The-gamma option isn't really necessary since you can achieve the same
115 effect by piping the output from pgmcrater through pnmgamma. However,
116 pgmcrater performs an internal gamma map anyway in the process of ren‐
117 dering the elevation array into the PGM format, so there's no addi‐
118 tional overhead in allowing an additional gamma adjustment.
119 Real craters have two distinct morphologies.
121
124 pnmgamma(1), pnmsmooth(1) pgm(1),
125 [1] Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal
129 Images, New York: Springer Verlag, 1988.
130
135 John Walker
136 Autodesk SA
137 Avenue des Champs-Montants 14b
138 CH-2074 MARIN
139 Suisse/Schweiz/Svizzera/Svizra/Switzerland
140 Usenet:kelvin@Autodesk.com
141 Fax:038/33 88 15
142 Voice:038/33 76 33
143 Permission to use, copy, modify, and distribute this software and its
145 documentation for any purpose and without fee is hereby granted, with‐
146 out any conditions or restrictions. This software is provided 'as is'
147 without express or implied warranty.
148
151 The original 1991 version of this manual contains the following:
152 PLUGWARE!
155 If you like this kind of stuff, you may also enjoy 'James Gleick's
156 Chaos--The Software' for MS-DOS, available for $59.95 from your local
157 software store or directly from Autodesk, Inc., Attn: Science Series,
158 2320 Marinship Way, Sausalito, CA 94965, USA. Telephone: (800)
159 688-2344 toll-free or, outside the U.S. (415) 332-2344 Ext 4886. Fax:
160 (415) 289-4718. 'Chaos--The Software' includes a more comprehensive
161 fractal forgery generator which creates three-dimensional landscapes as
162 well as clouds and planets, plus five more modules which explore other
163 aspects of Chaos. The user guide of more than 200 pages includes an
164 introduction by James Gleick and detailed explanations by Rudy Rucker
165 of the mathematics and algorithms used by each program.
166netpbm documentation 20 November 2008 Pgmcrater User Manual(0)