Manual: ppmforge
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 embody‐
ing 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 detail by Dietmar Saupe[2].
The program generates two varieties of pictures: planets and clouds, which are just different renderings of data gen‐
erated 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 num‐
ber by applying 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 envisioned by thinking of the data as representing the eleva‐
tion 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 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 simu‐
lated; 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 thousands of colors which must be compressed with pnmquant or pp‐
mdither 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 result‐
ing 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
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options ),
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. Selecting clouds sets the default for fractal di‐
mension 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 dimensions create more ``chaotic'' images, which require higher resolution output and a larger
FFT mesh size to look good. If no dimension is specified, the program uses 2.4 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 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 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 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) 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 currently 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 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 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 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 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 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 gen‐
erated, or 0.75 if clouds are selected by the -clouds option. The result of the FFT image synthesis is an ar‐
ray 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 re‐
places 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 elevation changes at the expense of small
variations. Power factors greater than 1 increase the roughness of the terrain and, like high fractal dimen‐
sions, 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 cre‐
ated with the -night option. The default value of 125 creates stars which resemble the sky as seen by the hu‐
man eye from Earth's surface. Stars are dim; only the brightest activate the cones in the human retina, caus‐
ing 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 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 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 cho‐
sen. Specifying an explicit seed allows you to re-render a picture you particularly like at a higher resolu‐
tion 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 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@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 27 March 2021 Ppmforge User Manual(1)