PGMCRATER



		

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  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.

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 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 contrast, while
                values less than 1 darken the image, increasing contrast.
      All flags can be abbreviated to their shortest unique prefix.

BUGS

      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 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 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.