Ppmforge User Manual(1) General Commands Manual Ppmforge User Manual(1)
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-similarity. The tech-
niques 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 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 memory re-
quirement increases as the square of the mesh size. The fractal dimen-
sion, 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 com-
ponents 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 do-
main 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 replaces the
numbers in the mesh with their cubes. Power law scaling is best envi-
sioned 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 re-
sults).
After these calculations, we have a array of the specified size contain-
ing 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 in-
creasing 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 wa-
ter 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 cre-
ate 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 el-
evation 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 ambi-
ent 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 pic-
tures often contain tens of thousands 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 produces bet-
ter 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 pp-
mdither. Try newmaxval settings of 63, 31, or 15 with pamdepth to re-
duce the number of colors in the picture to 256 or fewer.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see ]8;;index.html#commonoptions\ Common Options]8;;\ ), ppmforge recognizes the
following command line options:
You can abbreviate any options to its shortest unique prefix.
-clouds
Generate clouds. An image of fractal clouds is generated. Se-
lecting 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 real number between 0 and 5 inclusive. Higher fractal dimen-
sions create more ``chaotic'' images, which require higher reso-
lution output and a larger FFT mesh size to look good. If no di-
mension is specified, the program uses 2.4 when generating plan-
ets 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, ppmforge uses hour as the "hour angle
at the central meridian." If you specify -hour 12, for example,
the planet will be fully illuminated, 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 crescents which, while pretty, don't give you many il-
luminated pixels for the amount of computing that's required. If
no -hour option is specified, a random hour angle is chosen, bi-
ased 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 values reduce the amount of ice,
while larger -ice settings create more prominent ice caps. Suf-
ficiently 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 specify-
ing, in degrees, the ``season'' the planet is currently experi-
encing 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 directly overhead at the equator. Positive values
represent summer in the northern hemisphere, negative values sum-
mer 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 sol-
stice. If no inclination angle is specified, a random value be-
tween -21.6 and 21.6 degrees is chosen.
-mesh size
A mesh of size by size will be used for the fast Fourier trans-
form (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. Increasing 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 al-
gorithm 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 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 synthesis 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 empha-
sise large-scale elevation changes at the expense of small varia-
tions. 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 sur-
round planet pictures and fill starry skies created with the
-night option. The default value of 125 creates stars which re-
semble 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 ap-
proximate 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 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 ppm-
forge. 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 gen-
erated 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 dif-
ferent 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 de-
fault 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 de-
fault 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 inte-
gers 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 Im-
ages, 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, 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 introduc-
tion 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 27 March 2021 Ppmforge User Manual(1)
Generated by dwww version 1.16 on Tue Dec 16 06:31:25 CET 2025.