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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]

       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 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 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.

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 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,	 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(1)
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