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]

       All options can be abbreviated to their shortest unique prefix.


       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 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.

       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 Coperni-
       cus  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.


       -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.


       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 the PGM format,  so  there’s  no  addi-
       tional overhead in allowing an additional gamma adjustment.

       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, with-
       out 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            15 October 1991       Pgmcrater User Manual(0)

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