Pgmcrater User Manual(0) Pgmcrater User Manual(0)NAMEpgmcrater - create cratered terrain by fractal forgery
SYNOPSISpgmcrater
[-number n]
[-height|-ysize s]
[-width|-xsize s]
[-gamma g]
All options can be abbreviated to their shortest unique prefix.
DESCRIPTION
This program is part of Netpbm(1).
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.
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 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.
SEE ALSOpnmgamma(1), pnmsmooth(1)pgm(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, with‐
out any conditions or restrictions. This software is provided 'as is'
without express or implied warranty.
HISTORY
The original 1991 version of this manual contains the following:
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.
netpbm documentation 15 October 1991 Pgmcrater User Manual(0)