pgmcrater
Section: User Commands (1)
Updated: 15 October 1991
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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.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- DESIGN NOTES
-
- SEE ALSO
-
- AUTHOR
-
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