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Pgmcrater User Manual(0)                              Pgmcrater User Manual(0)

       pgmcrater - create cratered terrain by fractal forgery


       [-number n]

       [-height|-ysize s]

       [-width|-xsize s]

       [-gamma g]

       This program is part of Netpbm(1).

       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.

       The randomness in the image is limited before Netpbm 10.37 (December 2006) -- if you run the program  twice  in
       the same second, you may get identical output.

       All options can be abbreviated to their shortest unique prefix.

       -number n
              Causes  n  craters  to be generated.  If no -number specification is given, 50000 craters will be gener-
              ated.  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 cra-
              tering.   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.

       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  the  PGM format, so there's no additional overhead in allowing an additional gamma

       Real craters have two distinct morphologies.

       pnmgamma(1), pnmsmooth(1) pgm(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 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.

       The original 1991 version of this manual contains the following:

       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.

netpbm documentation           20 November 2008       Pgmcrater User Manual(0)