1Ppmforge User Manual(0)                                Ppmforge User Manual(0)
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NAME

6       ppmforge - fractal forgeries of clouds, planets, and starry skies
7

SYNOPSIS

9       ppmforge
10
11       [-clouds]  [-night] [-dimension dimen] [-hour hour] [-inclination|-tilt
12       angle] [-mesh size] [-power  factor]  [-glaciers  level]  [-ice  level]
13       [-saturation  sat]  [-seed  seed]  [-stars  fraction]  [{-xsize|-width}
14       width] [{-ysize|-height} height]
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16

DESCRIPTION

18       This program is part of Netpbm(1).
19
20       ppmforge generates three kinds of  ``random  fractal  forgeries,''  the
21       term  coined  by  Richard  F. Voss of the IBM Thomas J. Watson Research
22       Center for seemingly realistic pictures of natural objects generated by
23       simple  algorithms  embodying  randomness  and fractal self-similarity.
24       The techniques used by ppmforge are essentially those given by Voss[1],
25       particularly  the technique of spectral synthesis explained in more de‐
26       tail by Dietmar Saupe[2].
27
28       The program generates two varieties of pictures:  planets  and  clouds,
29       which  are  just different renderings of data generated in an identical
30       manner, illustrating the unity of the fractal structure of  these  very
31       different  objects.   A third type of picture, a starry sky, is synthe‐
32       sised directly from pseudorandom numbers.
33
34       The generation of planets or clouds begins with the preparation  of  an
35       array  of random data in the frequency domain.  The size of this array,
36       the ``mesh size,'' can be set with the -mesh  option;  the  larger  the
37       mesh  the more realistic the pictures but the calculation time and mem‐
38       ory requirement increases as the square of the mesh size.  The  fractal
39       dimension, which you can specify with the -dimension option, determines
40       the roughness of the terrain on the planet or the scale  of  detail  in
41       the clouds.  As the fractal dimension is increased, more high frequency
42       components are added into the random mesh.
43
44       Once the mesh is generated, an inverse two dimensional  Fourier  trans‐
45       form is performed upon it.  This converts the original random frequency
46       domain data into spatial amplitudes.  We scale the real components that
47       result  from  the Fourier transform into numbers from 0 to 1 associated
48       with each point on the mesh.  You can further modify this number by ap‐
49       plying a ``power law scale'' to it with the -power option.  Unity scale
50       leaves the numbers unmodified; a power scale of 0.5  takes  the  square
51       root  of the numbers in the mesh, while a power scale of 3 replaces the
52       numbers in the mesh with their cubes.  Power law scaling is best  envi‐
53       sioned  by  thinking  of the data as representing the elevation of ter‐
54       rain; powers less than 1 yield landscapes  with  vertical  scarps  that
55       look like glacially-carved valleys; powers greater than one make fairy-
56       castle spires (which require large mesh sizes and high  resolution  for
57       best results).
58
59       After  these  calculations,  we have a array of the specified size con‐
60       taining numbers that range from 0 to 1.  ppmforge generates as follows:
61
62       The randomness in the image is limited before  Netpbm  10.37  (December
63       2006)  --  if you run the program twice in the same second, you may get
64       identical output.
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66
67
68       Clouds A color map is created that ranges from pure blue  to  white  by
69              increasing admixture (desaturation) of blue with white.  Numbers
70              less than 0.5 are colored blue, numbers between 0.5 and 1.0  are
71              colored  with corresponding levels of white, with 1.0 being pure
72              white.
73
74
75       Planet The mesh is projected onto a sphere.  Values less than  0.5  are
76              treated  as  water  and values between 0.5 and 1.0 as land.  The
77              water areas are colored based upon the  water  depth,  and  land
78              based  on its elevation.  The random depth data are used to cre‐
79              ate clouds over the oceans.  An  atmosphere  approximately  like
80              the  Earth's is simulated; its light absorption is calculated to
81              create a blue cast around the limb of the  planet.   A  function
82              that rises from 0 to 1 based on latitude is modulated by the lo‐
83              cal elevation to generate polar ice caps--high altitude  terrain
84              carries  glaciers  farther from the pole.  Based on the position
85              of the star with respect to the observer, the apparent color  of
86              each  pixel  of the planet is calculated by ray-tracing from the
87              star to the planet to the observer and applying a lighting model
88              that sums ambient light and diffuse reflection (for most planets
89              ambient light is zero, as their primary star is the only  source
90              of  illumination).   Additional random data are used to generate
91              stars around the planet.
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93
94       Night  A sequence of pseudorandom numbers is  used  to  generate  stars
95              with a user specified density.
96
97
98       Cloud  pictures always contain 256 or fewer colors and may be displayed
99       on most color mapped devices without further processing.   Planet  pic‐
100       tures  often  contain  tens  of  thousands of colors which must be com‐
101       pressed with pnmquant or ppmdither before encoding in  a  color  mapped
102       format.   If the display resolution is high enough, ppmdither generally
103       produces better looking planets.  pnmquant  tends  to  create  discrete
104       color bands, particularly in the oceans, which are unrealistic and dis‐
105       tracting.  The number of colors in starry sky pictures  generated  with
106       the  -night  option  depends  on  the  value specified for -saturation.
107       Small values limit the color temperature distribution of the stars  and
108       reduce the number of colors in the image.  If the -saturation is set to
109       0, none of the stars will be colored and the resulting image will never
110       contain  more  than 256 colors.  Night sky pictures with many different
111       star colors often look best when color compressed  by  pamdepth  rather
112       than  pnmquant  or  ppmdither.  Try newmaxval settings of 63, 31, or 15
113       with pamdepth to reduce the number of colors in the picture to  256  or
114       fewer.
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117

OPTIONS

119       In  addition  to  the options common to all programs based on libnetpbm
120       (most notably -quiet, see
121        Common Options ⟨index.html#commonoptions⟩ ), ppmforge  recognizes  the
122       following command line options:
123
124       You can abbreviate any options to its shortest unique prefix.
125
126
127
128       -clouds
129              Generate  clouds.  An image of fractal clouds is generated.  Se‐
130              lecting clouds sets the default for fractal  dimension  to  2.15
131              and power scale factor to 0.75.
132
133
134       -dimension dimen
135               Sets the fractal dimension to the specified dimen, which may be
136              any real number between 0 and 5 inclusive.  Higher  fractal  di‐
137              mensions  create  more  ``chaotic'' images, which require higher
138              resolution output and a larger FFT mesh size to look  good.   If
139              no  dimension is specified, the program uses 2.4 when generating
140              planets and 2.15 for clouds.
141
142
143       -glaciers level
144              The floating point level setting controls the  extent  to  which
145              terrain  elevation causes ice to appear at lower latitudes.  The
146              default value of 0.75 makes the polar  caps  extend  toward  the
147              equator  across  high  terrain and forms glaciers in the highest
148              mountains, as on Earth.  Higher  values  make  ice  sheets  that
149              cover  more  and more of the land surface, simulating planets in
150              the midst of an ice age.  Lower values tend to  be  boring,  re‐
151              sulting in unrealistic geometrically-precise ice cap boundaries.
152
153
154       -hour hour
155              When  generating a planet, ppmforge uses hour as the "hour angle
156              at the central meridian."  If you specify -hour 12, for example,
157              the planet will be fully illuminated, corresponding to high noon
158              at the longitude at the center of the screen.  You  can  specify
159              any  floating  point value between 0 and 24 for hour, but values
160              which place most of the planet in darkness (0 to 4 and 20 to 24)
161              result in crescents which, while pretty, don't give you many il‐
162              luminated pixels for the amount of  computing  that's  required.
163              If  no -hour option is specified, a random hour angle is chosen,
164              biased so that only 25% of the images generated  will  be  cres‐
165              cents.
166
167
168       -ice level
169              Sets  the  extent  of  the  polar ice caps to the given floating
170              point level.  The default level of 0.4 produces ice caps similar
171              to those of the Earth.  Smaller values reduce the amount of ice,
172              while larger -ice settings create more prominent ice caps.  Suf‐
173              ficiently large values, such as 100 or more, in conjunction with
174              small settings for -glaciers (try 0.1) create "ice  balls"  like
175              Europa.
176
177
178       -inclination|-tilt angle
179              The  inclination  angle of the planet with regard to its primary
180              star is set to angle, which can be any floating point value from
181              -90  to 90.  The inclination angle can be thought of as specify‐
182              ing, in degrees, the ``season'' the planet is currently  experi‐
183              encing  or, more precisely, the latitude at which the star tran‐
184              sits the zenith at local noon.  If 0, the planet is at  equinox;
185              the  star  is directly overhead at the equator.  Positive values
186              represent summer in the  northern  hemisphere,  negative  values
187              summer  in the southern hemisphere.  The Earth's inclination an‐
188              gle, for example, is about 23.5 at the June solstice, 0  at  the
189              equinoxes in March and September, and -23.5 at the December sol‐
190              stice.  If no inclination angle is specified, a random value be‐
191              tween -21.6 and 21.6 degrees is chosen.
192
193
194       -mesh size
195              A  mesh of size by size will be used for the fast Fourier trans‐
196              form (FFT).  Note that memory requirements and computation speed
197              increase as the square of size; if you double the mesh size, the
198              program will use four times the memory and  run  four  times  as
199              long.   The  default  mesh is 256x256, which produces reasonably
200              good looking pictures  while  using  half  a  megabyte  for  the
201              256x256  array  of  single precision complex numbers required by
202              the FFT.  On machines with limited memory capacity, you may have
203              to reduce the mesh size to avoid running out of RAM.  Increasing
204              the mesh size produces better looking pictures;  the  difference
205              becomes  particularly noticeable when generating high resolution
206              images with relatively high fractal dimensions (between 2.2  and
207              3).
208
209
210       -night A  starry  sky  is generated.  The stars are created by the same
211              algorithm used for the stars that surround planet pictures,  but
212              the output consists exclusively of stars.
213
214
215       -power factor
216              Sets  the  "power  factor"  used to scale elevations synthesised
217              from the FFT to factor, which can be any floating  point  number
218              greater  than  zero.  If no factor is specified a default of 1.2
219              is used if a planet is being generated, or 0.75  if  clouds  are
220              selected  by  the  -clouds  option.  The result of the FFT image
221              synthesis is an array of elevation values between 0  and  1.   A
222              non-unity power factor exponentiates each of these elevations to
223              the specified power.  For example, a power factor of  2  squares
224              each  value,  while a power factor of 0.5 replaces each with its
225              square root.  (Note that exponentiating values between 0  and  1
226              yields  values  that  remain  within that range.)  Power factors
227              less than 1 emphasise large-scale elevation changes at  the  ex‐
228              pense  of  small  variations.   Power factors greater than 1 in‐
229              crease the roughness of the terrain and, like high  fractal  di‐
230              mensions,  may  require  a  larger  FFT  mesh size and/or higher
231              screen resolution to look good.
232
233
234       -saturation sat
235              Controls the degree of color saturation of the stars  that  sur‐
236              round  planet  pictures  and  fill starry skies created with the
237              -night option.  The default value of 125 creates stars which re‐
238              semble  the  sky  as seen by the human eye from Earth's surface.
239              Stars are dim; only the brightest activate the cones in the  hu‐
240              man retina, causing color to be perceived.  Higher values of sat
241              approximate the appearance of stars from Earth orbit, where bet‐
242              ter  dark  adaptation, absence of skyglow, and the concentration
243              of light from a given star onto a smaller  area  of  the  retina
244              thanks  to  the lack of atmospheric turbulence enhances the per‐
245              ception of color.  Values greater than 250 create ``science fic‐
246              tion'' skies that, while pretty, don't occur in this universe.
247
248              Thanks  to the inverse square law combined with Nature's love of
249              mediocrity, there are many, many dim stars for every bright one.
250              This  population  relationship  is  accurately  reflected in the
251              skies created by ppmforge.  Dim, low mass stars live much longer
252              than  bright  massive stars, consequently there are many reddish
253              stars for every blue giant.  This relationship is  preserved  by
254              ppmforge.  You can reverse the proportion, simulating the sky as
255              seen in a starburst galaxy, by specifying a negative sat value.
256
257
258       -seed num
259              Sets the seed for the random number  generator  to  the  integer
260              num.  The seed used to create each picture is displayed on stan‐
261              dard output (unless suppressed with the  -quiet  option).   Pic‐
262              tures  generated  with  the  same seed will be identical.  If no
263              -seed is specified, a random seed derived from the date and time
264              will  be  chosen.  Specifying an explicit seed allows you to re-
265              render a picture you particularly like at a higher resolution or
266              with different viewing parameters.
267
268
269       -stars fraction
270              Specifies  the  percentage  of  pixels,  in tenths of a percent,
271              which will appear as stars, either surrounding a planet or fill‐
272              ing  the entire frame if -night is specified.  The default frac‐
273              tion is 100.
274
275
276       -xsize|-width width
277              Sets the width of the generated image to width pixels.  The  de‐
278              fault  width  is 256 pixels.  Images must be at least as wide as
279              they are high; if a width less than the height is specified,  it
280              will  be increased to equal the height.  If you must have a long
281              skinny image, make a square one with ppmforge, then  use  pamcut
282              to extract a portion of the shape and size you require.
283
284
285       -ysize|-height height
286              Sets  the  height  of the generated image to height pixels.  The
287              default height is 256 pixels.  If the height  specified  exceeds
288              the width, the width will be increased to equal the height.
289
290
291
292

LIMITATIONS

294       The algorithms require the output image to be at least as wide as it is
295       high, and the width to be an even number of pixels.  These  constraints
296       are  enforced  by  increasing the size of the requested image if neces‐
297       sary.
298
299       You may have to reduce the FFT mesh size on machines with 16 bit  inte‐
300       gers and segmented pointer architectures.
301
302

SEE ALSO

304       pamcut(1), pamdepth(1), ppmdither(1), pnmquant(1), ppm(1)
305
306
307
308       [1]    Voss,  Richard F., ``Random Fractal Forgeries,'' in Earnshaw et.
309              al.,  Fundamental  Algorithms  for  Computer  Graphics,  Berlin:
310              Springer-Verlag, 1985.
311
312
313       [2]    Peitgen,  H.-O.,  and Saupe, D. eds., The Science Of Fractal Im‐
314              ages, New York: Springer Verlag, 1988.
315
316
317
318

AUTHOR

320       John Walker
321       Autodesk SA
322       Avenue des Champs-Montants 14b
323       CH-2074 MARIN
324       Suisse/Schweiz/Svizzera/Svizra/Switzerland
325           Usenet:kelvin@Autodesk.com
326           Fax:038/33 88 15
327           Voice:038/33 76 33
328
329
330       Permission to use, copy, modify, and distribute this software  and  its
331       documentation  for any purpose and without fee is hereby granted, with‐
332       out any conditions or restrictions.  This  software  is  provided  ``as
333       is'' without express or implied warranty.
334
335
336   PLUGWARE!
337       If  you  like  this  kind of stuff, you may also enjoy ``James Gleick's
338       Chaos--The Software'' for MS-DOS, available for $59.95 from your  local
339       software  store  or directly from Autodesk, Inc., Attn: Science Series,
340       2320  Marinship  Way,  Sausalito,  CA  94965,  USA.   Telephone:  (800)
341       688-2344  toll-free or, outside the U.S. (415) 332-2344 Ext 4886.  Fax:
342       (415) 289-4718.  ``Chaos--The Software'' includes a more  comprehensive
343       fractal forgery generator which creates three-dimensional landscapes as
344       well as clouds and planets, plus five more modules which explore  other
345       aspects  of  Chaos.   The user guide of more than 200 pages includes an
346       introduction by James Gleick and detailed explanations by  Rudy  Rucker
347       of the mathematics and algorithms used by each program.
348

DOCUMENT SOURCE

350       This  manual  page was generated by the Netpbm tool 'makeman' from HTML
351       source.  The master documentation is at
352
353              http://netpbm.sourceforge.net/doc/ppmforge.html
354
355netpbm documentation             27 March 2021         Ppmforge User Manual(0)
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