1Ppmforge User Manual(0) Ppmforge User Manual(0)
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6 ppmforge - fractal forgeries of clouds, planets, and starry skies
7
9 ppmforge
10 [-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]
15
18 This program is part of Netpbm(1).
19 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
26 detail by Dietmar Saupe[2].
27 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 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 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
49 applying a ``power law scale'' to it with the -power option. Unity
50 scale leaves the numbers unmodified; a power scale of 0.5 takes the
51 square root of the numbers in the mesh, while a power scale of 3
52 replaces the numbers in the mesh with their cubes. Power law scaling
53 is best envisioned by thinking of the data as representing the eleva‐
54 tion of terrain; powers less than 1 yield landscapes with vertical
55 scarps that look like glacially-carved valleys; powers greater than one
56 make fairy-castle spires (which require large mesh sizes and high reso‐
57 lution for best results).
58 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 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.
65 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 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
83 local elevation to generate polar ice caps--high altitude ter‐
84 rain carries glaciers farther from the pole. Based on the posi‐
85 tion of the star with respect to the observer, the apparent
86 color of each pixel of the planet is calculated by ray-tracing
87 from the star to the planet to the observer and applying a
88 lighting model that sums ambient light and diffuse reflection
89 (for most planets ambient light is zero, as their primary star
90 is the only source of illumination). Additional random data are
91 used to generate stars around the planet.
92 Night A sequence of pseudorandom numbers is used to generate stars
95 with a user specified density.
96 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.
115
119 You can abbreviate any options to its shortest unique prefix.
120 -clouds
124 Generate clouds. An image of fractal clouds is generated.
125 Selecting clouds sets the default for fractal dimension to 2.15
126 and power scale factor to 0.75.
127 -dimension dimen
130 Sets the fractal dimension to the specified dimen, which may be
131 any floating point value between 0 and 3. Higher fractal dimen‐
132 sions create more ``chaotic'' images, which require higher reso‐
133 lution output and a larger FFT mesh size to look good. If no
134 dimension is specified, 2.4 is used when generating planets and
135 2.15 for clouds.
136 -glaciers level
139 The floating point level setting controls the extent to which
140 terrain elevation causes ice to appear at lower latitudes. The
141 default value of 0.75 makes the polar caps extend toward the
142 equator across high terrain and forms glaciers in the highest
143 mountains, as on Earth. Higher values make ice sheets that
144 cover more and more of the land surface, simulating planets in
145 the midst of an ice age. Lower values tend to be boring,
146 resulting in unrealistic geometrically-precise ice cap bound‐
147 aries.
148 -hour hour
151 When generating a planet, ppmforge uses hour as the "hour angle
152 at the central meridian." If you specify -hour 12, for example,
153 the planet will be fully illuminated, corresponding to high noon
154 at the longitude at the center of the screen. You can specify
155 any floating point value between 0 and 24 for hour, but values
156 which place most of the planet in darkness (0 to 4 and 20 to 24)
157 result in crescents which, while pretty, don't give you many
158 illuminated pixels for the amount of computing that's required.
159 If no -hour option is specified, a random hour angle is chosen,
160 biased so that only 25% of the images generated will be cres‐
161 cents.
162 -ice level
165 Sets the extent of the polar ice caps to the given floating
166 point level. The default level of 0.4 produces ice caps similar
167 to those of the Earth. Smaller values reduce the amount of ice,
168 while larger -ice settings create more prominent ice caps. Suf‐
169 ficiently large values, such as 100 or more, in conjunction with
170 small settings for -glaciers (try 0.1) create "ice balls" like
171 Europa.
172 -inclination|-tilt angle
175 The inclination angle of the planet with regard to its primary
176 star is set to angle, which can be any floating point value from
177 -90 to 90. The inclination angle can be thought of as specify‐
178 ing, in degrees, the ``season'' the planet is presently experi‐
179 encing or, more precisely, the latitude at which the star tran‐
180 sits the zenith at local noon. If 0, the planet is at equinox;
181 the star is directly overhead at the equator. Positive values
182 represent summer in the northern hemisphere, negative values
183 summer in the southern hemisphere. The Earth's inclination
184 angle, for example, is about 23.5 at the June solstice, 0 at the
185 equinoxes in March and September, and -23.5 at the December sol‐
186 stice. If no inclination angle is specified, a random value
187 between -21.6 and 21.6 degrees is chosen.
188 -mesh size
191 A mesh of size by size will be used for the fast Fourier trans‐
192 form (FFT). Note that memory requirements and computation speed
193 increase as the square of size; if you double the mesh size, the
194 program will use four times the memory and run four times as
195 long. The default mesh is 256x256, which produces reasonably
196 good looking pictures while using half a megabyte for the
197 256x256 array of single precision complex numbers required by
198 the FFT. On machines with limited memory capacity, you may have
199 to reduce the mesh size to avoid running out of RAM. Increasing
200 the mesh size produces better looking pictures; the difference
201 becomes particularly noticeable when generating high resolution
202 images with relatively high fractal dimensions (between 2.2 and
203 3).
204 -night A starry sky is generated. The stars are created by the same
207 algorithm used for the stars that surround planet pictures, but
208 the output consists exclusively of stars.
209 -power factor
212 Sets the "power factor" used to scale elevations synthesised
213 from the FFT to factor, which can be any floating point number
214 greater than zero. If no factor is specified a default of 1.2
215 is used if a planet is being generated, or 0.75 if clouds are
216 selected by the -clouds option. The result of the FFT image
217 synthesis is an array of elevation values between 0 and 1. A
218 non-unity power factor exponentiates each of these elevations to
219 the specified power. For example, a power factor of 2 squares
220 each value, while a power factor of 0.5 replaces each with its
221 square root. (Note that exponentiating values between 0 and 1
222 yields values that remain within that range.) Power factors
223 less than 1 emphasise large-scale elevation changes at the
224 expense of small variations. Power factors greater than 1
225 increase the roughness of the terrain and, like high fractal
226 dimensions, may require a larger FFT mesh size and/or higher
227 screen resolution to look good.
228 -saturation sat
231 Controls the degree of color saturation of the stars that sur‐
232 round planet pictures and fill starry skies created with the
233 -night option. The default value of 125 creates stars which
234 resemble the sky as seen by the human eye from Earth's surface.
235 Stars are dim; only the brightest activate the cones in the
236 human retina, causing color to be perceived. Higher values of
237 sat approximate the appearance of stars from Earth orbit, where
238 better dark adaptation, absence of skyglow, and the concentra‐
239 tion of light from a given star onto a smaller area of the
240 retina thanks to the lack of atmospheric turbulence enhances the
241 perception of color. Values greater than 250 create ``science
242 fiction'' skies that, while pretty, don't occur in this uni‐
243 verse.
244 Thanks to the inverse square law combined with Nature's love of
246 mediocrity, there are many, many dim stars for every bright one.
247 This population relationship is accurately reflected in the
248 skies created by ppmforge. Dim, low mass stars live much longer
249 than bright massive stars, consequently there are many reddish
250 stars for every blue giant. This relationship is preserved by
251 ppmforge. You can reverse the proportion, simulating the sky as
252 seen in a starburst galaxy, by specifying a negative sat value.
253 -seed num
256 Sets the seed for the random number generator to the integer
257 num. The seed used to create each picture is displayed on stan‐
258 dard output (unless suppressed with the -quiet option). Pic‐
259 tures generated with the same seed will be identical. If no
260 -seed is specified, a random seed derived from the date and time
261 will be chosen. Specifying an explicit seed allows you to re-
262 render a picture you particularly like at a higher resolution or
263 with different viewing parameters.
264 -stars fraction
267 Specifies the percentage of pixels, in tenths of a percent,
268 which will appear as stars, either surrounding a planet or fill‐
269 ing the entire frame if -night is specified. The default frac‐
270 tion is 100.
271 -xsize|-width width
274 Sets the width of the generated image to width pixels. The
275 default width is 256 pixels. Images must be at least as wide as
276 they are high; if a width less than the height is specified, it
277 will be increased to equal the height. If you must have a long
278 skinny image, make a square one with ppmforge, then use pamcut
279 to extract a portion of the shape and size you require.
280 -ysize|-height height
283 Sets the height of the generated image to height pixels. The
284 default height is 256 pixels. If the height specified exceeds
285 the width, the width will be increased to equal the height.
286
291 The algorithms require the output image to be at least as wide as it is
292 high, and the width to be an even number of pixels. These constraints
293 are enforced by increasing the size of the requested image if neces‐
294 sary.
295 You may have to reduce the FFT mesh size on machines with 16 bit inte‐
297 gers and segmented pointer architectures.
298
301 pamcut(1), pamdepth(1), ppmdither(1), pnmquant(1), ppm(1)
302 [1] Voss, Richard F., ``Random Fractal Forgeries,'' in Earnshaw et.
306 al., Fundamental Algorithms for Computer Graphics, Berlin:
307 Springer-Verlag, 1985.
308 [2] Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal
311 Images, New York: Springer Verlag, 1988.
312
317 John Walker
318 Autodesk SA
319 Avenue des Champs-Montants 14b
320 CH-2074 MARIN
321 Suisse/Schweiz/Svizzera/Svizra/Switzerland
322 Usenet:kelvin@Autodesk.com
323 Fax:038/33 88 15
324 Voice:038/33 76 33
325 Permission to use, copy, modify, and distribute this software and its
327 documentation for any purpose and without fee is hereby granted, with‐
328 out any conditions or restrictions. This software is provided ``as
329 is'' without express or implied warranty.
330 PLUGWARE!
333 If you like this kind of stuff, you may also enjoy ``James Gleick's
334 Chaos--The Software'' for MS-DOS, available for $59.95 from your local
335 software store or directly from Autodesk, Inc., Attn: Science Series,
336 2320 Marinship Way, Sausalito, CA 94965, USA. Telephone: (800)
337 688-2344 toll-free or, outside the U.S. (415) 332-2344 Ext 4886. Fax:
338 (415) 289-4718. ``Chaos--The Software'' includes a more comprehensive
339 fractal forgery generator which creates three-dimensional landscapes as
340 well as clouds and planets, plus five more modules which explore other
341 aspects of Chaos. The user guide of more than 200 pages includes an
342 introduction by James Gleick and detailed explanations by Rudy Rucker
343 of the mathematics and algorithms used by each program.
344
346 This manual page was generated by the Netpbm tool 'makeman' from HTML
347 source. The master documentation is at
348 http://netpbm.sourceforge.net/doc/ppmforge.html
350netpbm documentation 25 October 1991 Ppmforge User Manual(0)