[-number n]

[-height|-ysize s]

[-width|-xsize s]

[-gamma g]

This program is part of Netpbm.

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
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.

The randomness in the image is limited before Netpbm 10.37 (December
2006) -- if you run the program twice in the same second, you may get
identical output.

All options can be abbreviated to their shortest unique prefix.

-number n

-height height

-width width

-xsize width

-ysize height

-gamma factor

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.

pnmgamma, pnmsmooth pgm,

[1] Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal

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.

The original 1991 version of this manual contains the following:

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.

netpbm documentation 20 November 2008 Pgmcrater User Manual(0)