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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 AT 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.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/ppmforge.html
netpbm documentation 25 October 1991 Ppmforge User Manual(0)
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