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pgmcrater(1)					 pgmcrater(1)

NAME
       pgmcrater - create cratered terrain by fractal forgery

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

DESCRIPTION
       pgmcrater creates a portable graymap which mimics cratered
       terrain. The graymap 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  devel-
       oped 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.

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 land-
		 scape. 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.

			 15 October 1991			1

pgmcrater(1)					 pgmcrater(1)

       -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  correct
		 the  graymap  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  con-
		 trast, while	values	less  than  1  darken the
		 image, increasing contrast.

       All flags can be abbreviated to their shortest unique pre-
       fix.

BUGS
       The  -gamma  option  isn't  really necessary since you can
       achieve the same effect by piping  the  output  from  pgm-
       crater  through	pnmgamma.  However, pgmcrater performs an
       internal gamma map anyway in the process of rendering  the
       elevation  array into a graymap, so there's no additional
       overhead in allowing a user-specified gamma.

       Real craters have two  distinct	morphologies.	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.

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

			 15 October 1991			2

pgmcrater(1)					 pgmcrater(1)

       software and its documentation for any purpose and without
       fee  is hereby granted, without any conditions or restric-
       tions.	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.

			 15 October 1991			3

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