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pgmcrater - create cratered terrain by fractal forgery
pgmcrater
[-number n] [-height|-ysize s] [-width|-xsize s] [-gamma g]
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.
- -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.
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.
pgm(5)
, pnmgamma(1)
,
pnmsmooth(1)
- Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal Images,
New York: Springer Verlag, 1988.
John Walker
Autodesk SA
Avenue des Champs-Montants 14b
CH-2074 MARIN
Suisse/Schweiz/Svizzera/Svizra/Switzerland
- Usenet:kelvin@Autodesk.comFax:038/33 88 15Voice: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.
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