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
   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)

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