pgmcrater
create cratered terrain by fractal forgery
see also :
pnmgamma - pnmsmooth
Synopsis
pgmcrater
[-number n] [-height|-ysize
s] [-width|-xsize s]
[-gamma g]
All options can
be abbreviated to their shortest unique prefix.
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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,
pnmgamma , pnmsmooth
[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[:at:]Autodesk[:dot:]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.