PGMCRATER
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 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
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.