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.