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RedHat 9 (Linux i386) - man page for ppmforge (redhat section 1)

ppmforge(1)			     General Commands Manual			      ppmforge(1)

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

       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]

       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 in-
       creased, 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.  The pixmaps are generated as follows:

       Clouds	 A colour map is created that ranges from pure blue to white by increasing admix-
		 ture (desaturation) of blue with white.  Numbers  less  than  0.5  are  coloured
		 blue,	numbers  between  0.5  and  1.0 are coloured 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 coloured 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 lati-
		 tude is modulated by the local elevation to generate polar ice caps--high  alti-
		 tude  terrain	carries glaciers farther from the pole.  Based on the position of
		 the star with respect to the observer, the apparent colour 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 speci-
		 fied density.

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

       -clouds	 Generate clouds.  A pixmap 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 highest 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 boundaries.

       -hour hour
		 When generating a planet, hour is used as the ``hour angle at the central merid-
		 ian.''  If you specify -hour 12, for example, the planet will be  fully  illumi-
		 nated,  corresponding to high noon at the longitude at the centre 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  cres-

       -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
		 values  reduce  the amount of ice, while larger -ice settings create more promi-
		 nent 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  tran-
		 sits  the zenith at local noon.  If 0, the planet is at equinox; the star is di-
		 rectly overhead at the equator.  Positive values represent summer in the  north-
		 ern  hemisphere, negative values summer in the southern hemisphere.  The Earth's
		 inclination angle, for example, 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 in-
		 clination angle is specified, a random value between -21.6 and 21.6  degrees  is

       -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 complex 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.  In-
		 creasing the mesh size produces better looking pictures; the difference  becomes
		 particularly  noticeable  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

       -power factor
		 Sets the ``power factor'' used to scale elevations synthesised from the  FFT  to
		 factor,  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 synthe-
		 sis is an array of elevation values between 0 and 1.  A non-unity  power  factor
		 exponentiates	each  of these elevations to the specified power.  For example, a
		 power factor 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 elevation changes at the expense of small variations.  Power factors
		 greater than 1 increase the roughness of the terrain and, like high fractal  di-
		 mensions,  may require a larger FFT mesh size and/or higher screen resolution to
		 look good.

       -saturation sat
		 Controls the degree of colour saturation of the stars that surround planet  pic-
		 tures	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 colour to be perceived.	Higher values of sat  approximate
		 the  appearance of stars from Earth orbit, where better dark adaptation, absence
		 of skyglow, and the concentration of light from a given star onto a smaller area
		 of  the retina thanks to the lack of atmospheric turbulence enhances the percep-
		 tion of colour.  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	relation-
		 ship  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  gal-
		 axy, 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 par-
		 ticularly 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
		 specified.  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 pixmap, make a square one with ppmforge, then use pnmcut 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 in-
		 creased to equal the height.

       All flags can be abbreviated to their shortest unique prefix.

       The algorithms require the output pixmap 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 pixmap if necessary.

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

       pnmcut(1), pnmdepth(1), ppmdither(1), ppmquant(1), ppm(5)

       [1]  Voss,  Richard F., ``Random Fractal Forgeries,'' in Earnshaw et. al., Fundamental Al-
	    gorithms 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.

	    John Walker
	    Autodesk SA
	    Avenue des Champs-Montants 14b
	    CH-2074 MARIN
	    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.

					 25 October 1991			      ppmforge(1)

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