pgmcrater(1)						      General Commands Manual						      pgmcrater(1)

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 gener-
		 ates is a genuine, gamma-corrected PGM image in any case.  This option simply changes the contrast and may compensate for a  dis-
		 play 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.  How-
       ever, 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  com-
       prehensive 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.

								  15 October 1991						      pgmcrater(1)