pgmcrater(1)pgmcrater(1)NAMEpgmcrater - create cratered terrain by fractal forgery
SYNOPSISpgmcrater [-number n] [-height|-ysize s] [-width|-xsize s]
[-gamma g]
All options can be abbreviated to their shortest unique
prefix.
DESCRIPTIONpgmcrater creates a PGM image which mimics cratered ter
rain. 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 for
mula used to obtain crater radii governed by this law from
a uniformly distributed pseudorandom sequence was devel
oped 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 hemi
spherical in shape (regardless of the incidence angle of
the impacting body, as long as the velocity is suffi
ciently 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 land
scape. 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 con
trast, while values less than 1 darken the
image, increasing contrast.
Note that this is separate from the gamma cor
rection 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 pgm
crater 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 addi
tional overhead in allowing an additional gamma adjust
ment.
Real craters have two distinct morphologies.
SEE ALSOpgm(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 soft
ware 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)
