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CentOS 7.0 - man page for ppmforge (centos section 0)

Ppmforge User Manual(0) 						  Ppmforge User Manual(0)

NAME
       ppmforge - fractal forgeries of clouds, planets, and starry skies

SYNOPSIS
       ppmforge

       [-clouds] [-night] [-dimension dimen] [-hour hour] [-inclination|-tilt angle] [-mesh size]
       [-power factor] [-glaciers level] [-ice level]  [-saturation  sat]  [-seed  seed]  [-stars
       fraction] [{-xsize|-width} width] [{-ysize|-height} height]

DESCRIPTION
       This program is part of Netpbm(1)

       ppmforge generates three kinds of ``random fractal forgeries,'' the term coined by Richard
       F. Voss of the IBM Thomas J. Watson Research Center for seemingly  realistic  pictures  of
       natural objects generated by simple algorithms embodying randomness and fractal self-simi-
       larity.	The techniques used by ppmforge are essentially those given by Voss[1],  particu-
       larly the technique of spectral synthesis explained in more detail by Dietmar Saupe[2].

       The  program  generates two varieties of pictures: planets and clouds, which are just dif-
       ferent renderings of data generated in an identical manner, illustrating the unity of  the
       fractal structure of these very different objects.  A third type of picture, a starry sky,
       is synthesised directly from pseudorandom numbers.

       The generation of planets or clouds begins with the preparation of an array of random data
       in  the	frequency domain.  The size of this array, the ``mesh size,'' can be set with the
       -mesh option; the larger the mesh the more realistic the pictures but the calculation time
       and  memory  requirement increases as the square of the mesh size.  The fractal dimension,
       which you can specify with the -dimension option, determines the roughness of the  terrain
       on  the	planet	or  the  scale	of  detail  in	the  clouds.  As the fractal dimension is
       increased, more high frequency components are added into the random mesh.

       Once the mesh is generated, an inverse two dimensional Fourier transform is performed upon
       it.   This converts the original random frequency domain data into spatial amplitudes.  We
       scale the real components that result from the Fourier transform into numbers from 0 to	1
       associated  with each point on the mesh.  You can further modify this number by applying a
       ``power law scale'' to it with the -power option.  Unity scale leaves the numbers  unmodi-
       fied; a power scale of 0.5 takes the square root of the numbers in the mesh, while a power
       scale of 3 replaces the numbers in the mesh with their cubes.  Power law scaling  is  best
       envisioned  by  thinking of the data as representing the elevation of terrain; powers less
       than 1 yield landscapes with vertical scarps that look like glacially-carved valleys; pow-
       ers  greater  than  one	make fairy-castle spires (which require large mesh sizes and high
       resolution for best results).

       After these calculations, we have a array of the specified size	containing  numbers  that
       range from 0 to 1.  ppmforge generates as follows:

       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.

       Clouds A color map is created that ranges from pure blue to white by increasing	admixture
	      (desaturation) of blue with white.  Numbers less than 0.5 are colored blue, numbers
	      between 0.5 and 1.0 are colored with corresponding levels of white, with 1.0  being
	      pure white.

       Planet The mesh is projected onto a sphere.  Values less than 0.5 are treated as water and
	      values between 0.5 and 1.0 as land.  The water areas are	colored  based	upon  the
	      water  depth,  and  land based on its elevation.	The random depth data are used to
	      create clouds over the oceans.  An atmosphere approximately  like  the  Earth's  is
	      simulated; its light absorption is calculated to create a blue cast around the limb
	      of the planet.  A function that rises from 0 to 1 based on latitude is modulated by
	      the  local  elevation  to  generate  polar  ice caps--high altitude terrain carries
	      glaciers farther from the pole.  Based on the position of the star with respect  to
	      the  observer, the apparent color of each pixel of the planet is calculated by ray-
	      tracing from the star to the planet to the observer and applying a  lighting  model
	      that  sums  ambient light and diffuse reflection (for most planets ambient light is
	      zero, as their primary star is the only source of illumination).	Additional random
	      data are used to generate stars around the planet.

       Night  A  sequence of pseudorandom numbers is used to generate stars with a user specified
	      density.

       Cloud pictures always contain 256 or fewer colors and  may  be  displayed  on  most  color
       mapped  devices	without  further processing.  Planet pictures often contain tens of thou-
       sands of colors which must be compressed with pnmquant or ppmdither before encoding  in	a
       color  mapped  format.  If the display resolution is high enough, ppmdither generally pro-
       duces better looking planets.  pnmquant tends to create discrete color bands, particularly
       in  the oceans, which are unrealistic and distracting.  The number of colors in starry sky
       pictures generated with the -night option depends on the value specified for  -saturation.
       Small  values  limit the color temperature distribution of the stars and reduce the number
       of colors in the image.	If the -saturation is set to 0, none of the stars will be colored
       and  the resulting image will never contain more than 256 colors.  Night sky pictures with
       many different star colors often look best when color compressed by pamdepth  rather  than
       pnmquant  or  ppmdither.   Try newmaxval settings of 63, 31, or 15 with pamdepth to reduce
       the number of colors in the picture to 256 or fewer.

OPTIONS
       You can abbreviate any options to its shortest unique prefix.

       -clouds
	      Generate clouds.	An image of fractal clouds is generated.  Selecting  clouds  sets
	      the default for fractal dimension to 2.15 and power scale factor to 0.75.

       -dimension dimen
	       Sets the fractal dimension to the specified dimen, which may be any floating point
	      value between 0 and 3.  Higher fractal dimensions create more  ``chaotic''  images,
	      which require higher resolution output and a larger FFT mesh size to look good.  If
	      no dimension is specified, 2.4 is used when generating planets and 2.15 for clouds.

       -glaciers level
	      The floating point level setting controls the extent  to	which  terrain	elevation
	      causes ice to appear at lower latitudes.	The default value of 0.75 makes the polar
	      caps extend toward the equator across high terrain and forms glaciers in the  high-
	      est mountains, as on Earth.  Higher values make ice sheets that cover more and more
	      of the land surface, simulating planets in the midst of an ice age.   Lower  values
	      tend  to	be  boring, resulting in unrealistic geometrically-precise ice cap bound-
	      aries.

       -hour hour
	      When generating a planet, ppmforge uses hour as the  'hour  angle  at  the  central
	      meridian.'   If you specify -hour 12, for example, the planet will be fully illumi-
	      nated, corresponding to high noon at the longitude at the  center  of  the  screen.
	      You  can	specify  any  floating	point value between 0 and 24 for hour, but values
	      which place most of the planet in darkness (0 to 4 and 20 to 24)	result	in  cres-
	      cents which, while pretty, don't give you many illuminated pixels for the amount of
	      computing that's required.  If no -hour option is specified, a random hour angle is
	      chosen, biased so that only 25% of the images generated will be crescents.

       -ice level
	      Sets  the  extent  of  the  polar  ice caps to the given floating point level.  The
	      default level of 0.4 produces ice caps similar to those of the Earth.  Smaller val-
	      ues  reduce the amount of ice, while larger -ice settings create more prominent ice
	      caps.  Sufficiently large values, such as 100 or more, in  conjunction  with  small
	      settings for -glaciers (try 0.1) create 'ice balls' like Europa.

       -inclination|-tilt angle
	      The  inclination	angle  of  the	planet	with regard to its primary star is set to
	      angle, which can be any floating point value from -90 to 90.  The inclination angle
	      can be thought of as specifying, in degrees, the ``season'' the planet is presently
	      experiencing or, more precisely, the latitude at which the star transits the zenith
	      at  local  noon.	 If 0, the planet is at equinox; the star is directly overhead at
	      the equator.  Positive values represent summer in the northern hemisphere, negative
	      values summer in the southern hemisphere.  The Earth's inclination angle, for exam-
	      ple, is about 23.5 at the June solstice, 0 at the equinoxes in March and September,
	      and -23.5 at the December solstice.  If no inclination angle is specified, a random
	      value between -21.6 and 21.6 degrees is chosen.

       -mesh size
	      A mesh of size by size will be used for the fast	Fourier  transform  (FFT).   Note
	      that  memory  requirements and computation speed increase as the square of size; if
	      you double the mesh size, the program will use four times the memory and	run  four
	      times as long.  The default mesh is 256x256, which produces reasonably good looking
	      pictures while using half a megabyte for the 256x256 array of single precision com-
	      plex  numbers  required  by the FFT.  On machines with limited memory capacity, you
	      may have to reduce the mesh size to avoid running out of RAM.  Increasing the  mesh
	      size  produces better looking pictures; the difference becomes particularly notice-
	      able when generating high resolution images with relatively high fractal dimensions
	      (between 2.2 and 3).

       -night A  starry  sky  is generated.  The stars are created by the same algorithm used for
	      the stars that surround planet pictures, but the	output	consists  exclusively  of
	      stars.

       -power factor
	      Sets  the  'power factor' used to scale elevations synthesised from the FFT to fac-
	      tor, which can be any floating point number greater than zero.   If  no  factor  is
	      specified  a  default  of  1.2  is  used if a planet is being generated, or 0.75 if
	      clouds are selected by the -clouds option.  The result of the FFT  image	synthesis
	      is an array of elevation values between 0 and 1.	A non-unity power factor exponen-
	      tiates each of these elevations to the specified power.  For example, a power  fac-
	      tor  of  2  squares  each value, while a power factor of 0.5 replaces each with its
	      square root.  (Note that exponentiating values between 0 and 1 yields  values  that
	      remain  within that range.)  Power factors less than 1 emphasise large-scale eleva-
	      tion changes at the expense of small variations.	 Power	factors  greater  than	1
	      increase	the  roughness	of  the  terrain  and,	like high fractal dimensions, may
	      require a larger FFT mesh size and/or higher screen resolution to look good.

       -saturation sat
	      Controls the degree of color saturation of the stars that surround planet  pictures
	      and  fill  starry  skies	created with the -night option.  The default value of 125
	      creates stars which resemble the sky as seen by the human eye from Earth's surface.
	      Stars  are  dim; only the brightest activate the cones in the human retina, causing
	      color to be perceived.  Higher values of sat approximate the  appearance	of  stars
	      from Earth orbit, where better dark adaptation, absence of skyglow, and the concen-
	      tration of light from a given star onto a smaller area of the retina thanks to  the
	      lack  of	atmospheric  turbulence enhances the perception of color.  Values greater
	      than 250 create ``science fiction'' skies that, while pretty, don't occur  in  this
	      universe.

	      Thanks  to  the inverse square law combined with Nature's love of mediocrity, there
	      are many, many dim stars for every bright one.   This  population  relationship  is
	      accurately  reflected  in  the skies created by ppmforge.  Dim, low mass stars live
	      much longer than bright massive stars, consequently there are  many  reddish  stars
	      for every blue giant.  This relationship is preserved by ppmforge.  You can reverse
	      the proportion, simulating the sky as seen in a starburst galaxy, by  specifying	a
	      negative sat value.

       -seed num
	      Sets the seed for the random number generator to the integer num.  The seed used to
	      create each picture is displayed on standard output  (unless  suppressed	with  the
	      -quiet  option).	 Pictures  generated with the same seed will be identical.  If no
	      -seed is specified, a random seed derived from the date and time	will  be  chosen.
	      Specifying an explicit seed allows you to re-render a picture you particularly like
	      at a higher resolution or with different viewing parameters.

       -stars fraction
	      Specifies the percentage of pixels, in tenths of a percent, which  will  appear  as
	      stars,  either surrounding a planet or filling the entire frame if -night is speci-
	      fied.  The default fraction is 100.

       -xsize|-width width
	      Sets the width of the generated image to width pixels.  The default  width  is  256
	      pixels.  Images must be at least as wide as they are high; if a width less than the
	      height is specified, it will be increased to equal the height.  If you must have	a
	      long  skinny  image,  make a square one with ppmforge, then use pamcut to extract a
	      portion of the shape and size you require.

       -ysize|-height height
	      Sets the height of the generated image to height pixels.	The default height is 256
	      pixels.	If the height specified exceeds the width, the width will be increased to
	      equal the height.

LIMITATIONS
       The algorithms require the output image to be at least as wide as  it  is  high,  and  the
       width  to  be  an even number of pixels.  These constraints are enforced by increasing the
       size of the requested image if necessary.

       You may have to reduce the FFT mesh size on machines with 16 bit  integers  and	segmented
       pointer architectures.

SEE ALSO
       pamcut(1) , pamdepth(1) , ppmdither(1) , pnmquant(1) , ppm(1)

       [1]    Voss,  Richard  F.,  ``Random Fractal Forgeries,'' in Earnshaw et. al., Fundamental
	      Algorithms for Computer Graphics, Berlin: Springer-Verlag, 1985.

       [2]    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.

netpbm documentation			 25 October 1991		  Ppmforge User Manual(0)


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