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)