phpman > man > pgmcrater(1)

pgmcrater(1)                           General Commands Manual                          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  ren‐
       dering  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 re‐
       ciprocal 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  de‐
       veloped 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 oblit‐
       erate 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 stun‐
                 ning 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 per‐
                 formed 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 ad‐
       ditional 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 AT 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.  Tele‐
       phone: (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 mod‐
       ules 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)