ppmforge


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
       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-similarity.
       The techniques used by ppmforge are essentially those given by Voss[1],
       particularly the technique of spectral synthesis explained in more  de-
       tail by Dietmar Saupe[2].

       The  program  generates  two varieties of pictures: planets and clouds,
       which are just different 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  synthe-
       sised 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 memo-
       ry  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 trans-
       form 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 ap-
       plying a ``power law scale'' to it  with  the  -power  option.    Unity
       scale  leaves  the  numbers  unmodified; a power scale of 0.5 takes the
       square root of the numbers in the mesh, while a power scale  of  3  re-
       places  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; powers 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  con-
       taining  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  admixture  (desaturation) of blue with white.
                 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 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 illumi-
                 nation).  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 colours and may be displayed
       on  most colour mapped devices without further processing.  Planet pic-
       tures often contain tens of thousands of colours  which  must  be  com-
       pressed  with  ppmquant or ppmdither before encoding in a colour mapped
       format.  If the display resolution is high enough, ppmdither  generally
       produces  better  looking  planets.   ppmquant tends to create discrete
       colour bands, particularly 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  -saturation.
       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 pictures with many dif-
       ferent  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.

OPTIONS
       -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  gener-
                 ating 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, simulat-
                 ing planets in the midst of an ice age.  Lower values tend to
                 be boring, resulting in unrealistic geometrically-precise ice
                 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 sim-
                 ilar to those of the Earth.  Smaller values 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)  cre-
                 ate ``ice balls'' like Europa.

       -inclination|-tilt angle
                 The inclination angle of the planet with regard to its prima-
                 ry star is set to angle, which can be any floating point val-
                 ue  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 hemi-
                 sphere.  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  incli-
                 nation  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 computa-
                 tion 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  pro-
                 duces  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  run-
                 ning  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  synthe-
                 sised from the FFT to factor, which can be any floating point
                 number greater than zero.  If no factor is  specified  a  de-
                 fault  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 be-

       -saturation sat
                 Controls the degree of colour 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 colour to be perceived.
                 Higher values of sat approximate the appearance of stars from
                 Earth  orbit,  where  better dark adaptation, absence of sky-
                 glow, 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 perception 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 relationship is  accurately  re-
                 flected  in  the  skies  created  by ppmforge.  Dim, low mass
                 stars live much longer than bright massive stars, consequent-
                 ly  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 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  speci-
                 fied,  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 ex-
                 ceeds the width, the width will be  increased  to  equal  the
                 height.

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

       [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, with-
       out 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: Sci-
       ence 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  compre-
       hensive fractal forgery generator which creates three-dimensional land-
       scapes as well as clouds and planets, plus five more modules which  ex-
       plore  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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