pgmcrater(1)            UNIX System V(15 October 1991)             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 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.




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pgmcrater(1)            UNIX System V(15 October 1991)             pgmcrater(1)


      -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:


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pgmcrater(1)            UNIX System V(15 October 1991)             pgmcrater(1)


      (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.
















































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