pgmcrater(1)
NAME
pgmcrater - create cratered terrain by fractal forgery
SYNOPSIS
pgmcrater [-number n] [-height|-ysize s] [-width|-xsize s] [-gamma g] All options can be abbreviated to their shortest unique prefix.
DESCRIPTION
pgmcrater creates a PGM image which mimics cratered terrain. The PGM
image 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.
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.
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 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 adjust the image 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.
Note that this is separate from the gamma correction that is
part of the definition of the PGM format. The image pnmgamma
generates is a genuine, gamma-corrected PGM image in any
case. This option simply changes the contrast and may
compensate for a display device that does not correctly
render PGM images.
DESIGN NOTES
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 the PGM format, so there's no additional overhead in allowing an additional gamma adjustment. Real craters have two distinct morphologies.
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
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.
15 October 1991 pgmcrater(1)
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