pgmcrater(1)                                         pgmcrater(1)

       pgmcrater - create cratered terrain by fractal forgery

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

       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  for-
       mula used to obtain crater radii governed by this law from
       a uniformly distributed pseudorandom sequence  was  devel-
       oped 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.

       -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 land-
                 scape.  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

       -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  con-
                 trast,  while  values  less  than  1  darken the
                 image, increasing contrast.

       All flags can be abbreviated to their shortest unique pre-

       The  -gamma  option  isn't  really necessary since you can
       achieve the same effect by piping  the  output  from  pgm-
       crater  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
                       /\                            /\
                 _____/  \____________/\____________/  \_____
       Larger craters should really use this  profile,  including
       the  central peak, and totally obliterate the pre-existing

       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 soft-
       ware 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                        1