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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
                 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
       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 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)