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       pgmcrater - create cratered terrain by fractal forgery


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

       All options can be abbreviated to their shortest unique prefix.


       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.


       -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

       -xsize width
                 Sets the width of the generated image to width pixels.  The default width is 256

       -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.


       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 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.


       pgm(5), pnmgamma(1), pnmsmooth(1)

       [1]  Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal Images, New York: Springer
            Verlag, 1988.


            John Walker
            Autodesk SA
            Avenue des Champs-Montants 14b
            CH-2074 MARIN
            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 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                             pgmcrater(1)