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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 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

       -width width
	      Sets  the  width	of the generated image to width pixels.  The default width is 256

       -xsize width
	      Sets the width of the generated image to width pixels.  The default  width  is  256

       -ysize height
	      Sets the height of the generated image to height pixels.	The default height is 256

       -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.

	      This is the seed for the random number generator that generates the pixels.

	      Use this to ensure you get the same image on separate invocations.

	      By default, pgmnoise uses a seed derived from the time of day and process ID, which
	      gives you fairly uncorrelated results in multiple invocations.

	      This option was new in Netpbm 10.61 (December 2012).

       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.

       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)
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