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pgmcrater General Commands Manual pgmcrater


pgmcrater – create cratered terrain by fractal forgery


pgmcrater [-number n] [-height|-ysize s] [-width|-xsize s] [-gamma g]

All options can be abbreviated to their shortest unique prefix.


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 for‐
mula 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 Coperni‐
cus 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.


-number n Causes n craters to be generated. If no -number specifica‐
tion 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 com‐
pensate for a display device that does not correctly render
PGM images.

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 ren‐
dering the elevation array into the PGM format, so there’s no addi‐
tional overhead in allowing an additional gamma adjustment.

Real craters have two distinct morphologies.


pgm(5), pnmgamma, 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
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, with‐
out 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: Sci‐
ence 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 compre‐
hensive fractal forgery generator which creates three-dimensional land‐
scapes 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

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