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 for-
       mula 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  specifica-
                 tion 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  ren-
       dering  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  suf-
       ficiently  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, with-
       out 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: Sci-
       ence 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  compre-
       hensive fractal forgery generator which creates three-dimensional land-
       scapes 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)