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pgmcrater(1) AMIGA (15 October 1991) pgmcrater(1)
NAME
pgmcrater - create cratered terrain by fractal forgery
SYNOPSIS
pgmcrater [-number n] [-height|-ysize s] [-width|-xsize s]
[-gamma g]
DESCRIPTION
pgmcrater creates a portable graymap which mimics cratered
terrain. The graymap is created by simulating the impact of
a given number of craters with random position and size,
then rendering the resulting terrain elevations based on a
light source shining from one side of the screen. The size
distribution of the craters is based on a power law which
results in many more small craters than large ones. The
number of craters of a given size varies as the reciprocal
of the area as described on pages 31 and 32 of Peitgen and
Saupe[1]; cratered bodies in the Solar System are observed
to obey this relationship. The formula used to obtain
crater radii governed by this law from a uniformly
distributed pseudorandom sequence was developed by Rudy
Rucker.
High resolution images with large numbers of craters often
benefit from being piped through pnmsmooth. The averaging
performed by this process eliminates some of the jagged
pixels and lends a mellow ``telescopic image'' feel to the
overall picture.
OPTIONS
-number n Causes n craters to be generated. If no -number
specification is given, 50000 craters will be
generated. Don't expect to see them all! For
every large crater there are many, many more tiny
ones which tend simply to erode the landscape. In
general, the more craters you specify the more
realistic the result; ideally you want the entire
terrain to have been extensively turned over again
and again by cratering. High resolution images
containing five to ten million craters are
stunning but take quite a while to create.
-height height
Sets the height of the generated image to height
pixels. The default height is 256 pixels.
-width width
Sets the width of the generated image to width
pixels. The default width is 256 pixels.
-xsize width
Sets the width of the generated image to width
Page 1 (printed 3/1/94)
pgmcrater(1) AMIGA (15 October 1991) pgmcrater(1)
pixels. The default width is 256 pixels.
-ysize height
Sets the height of the generated image to height
pixels. The default height is 256 pixels.
-gamma factor
The specified factor is used to gamma correct the
graymap in the same manner as performed by
pnmgamma. The default value is 1.0, which results
in a medium contrast image. Values larger than 1
lighten the image and reduce contrast, while
values less than 1 darken the image, increasing
contrast.
All flags can be abbreviated to their shortest unique
prefix.
BUGS
The -gamma option isn't really necessary since you can
achieve the same effect by piping the output from pgmcrater
through pnmgamma. However, pgmcrater performs an internal
gamma map anyway in the process of rendering the elevation
array into a graymap, so there's no additional overhead in
allowing a user-specified gamma.
Real craters have two distinct morphologies. pgmcrater
simulates only small craters, which are hemispherical in
shape (regardless of the incidence angle of the impacting
body, as long as the velocity is sufficiently high). Large
craters, such as Copernicus and Tycho on the Moon, have a
``walled plain'' shape with a cross-section more like:
/\ /\
_____/ \____________/\____________/ \_____
Larger craters should really use this profile, including the
central peak, and totally obliterate the pre-existing
terrain.
SEE ALSO
pgm(5), pnmgamma(1), pnmsmooth(1)
[1] Peitgen, H.-O., and Saupe, D. eds., The Science Of
Fractal Images, New York: Springer Verlag, 1988.
AUTHOR
John Walker
Autodesk SA
Avenue des Champs-Montants 14b
CH-2074 MARIN
Suisse/Schweiz/Svizzera/Svizra/Switzerland
Usenet: kelvin@Autodesk.com
Fax: 038/33 88 15
Page 2 (printed 3/1/94)
pgmcrater(1) AMIGA (15 October 1991) pgmcrater(1)
Voice: 038/33 76 33
Permission to use, copy, modify, and distribute this
software and its documentation for any purpose and without
fee is hereby granted, without any conditions or
restrictions. This software is provided ``as is'' without
express or implied warranty.
PLUGWARE! If you like this kind of stuff, you may also enjoy
``James Gleick's Chaos--The Software'' for MS-DOS, available
for $59.95 from your local software store or directly from
Autodesk, Inc., Attn: Science Series, 2320 Marinship Way,
Sausalito, CA 94965, USA. Telephone: (800) 688-2344 toll-
free or, outside the U.S. (415) 332-2344 Ext 4886. Fax:
(415) 289-4718. ``Chaos--The Software'' includes a more
comprehensive fractal forgery generator which creates
three-dimensional landscapes as well as clouds and planets,
plus five more modules which explore other aspects of Chaos.
The user guide of more than 200 pages includes an
introduction by James Gleick and detailed explanations by
Rudy Rucker of the mathematics and algorithms used by each
program.
Page 3 (printed 3/1/94)