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| author | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
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| committer | giraffedata <giraffedata@9d0c8265-081b-0410-96cb-a4ca84ce46f8> | 2006-12-25 03:06:05 +0000 |
| commit | 1017cbebe5d5edd859e0fddad0a8600f509f4821 (patch) | |
| tree | 78bdf336648566f7a7d55f42837357dea3dd674c /ppmforge.html | |
| parent | 16f2ac126651015a376eba864a3a35f738b0b25a (diff) | |
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Place user guide into Subversion repository
git-svn-id: http://svn.code.sf.net/p/netpbm/code/userguide@181 9d0c8265-081b-0410-96cb-a4ca84ce46f8
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diff --git a/ppmforge.html b/ppmforge.html new file mode 100644 index 00000000..85164be5 --- /dev/null +++ b/ppmforge.html @@ -0,0 +1,395 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<HTML><HEAD><TITLE>Ppmforge User Manual</TITLE></HEAD> +<BODY> +<H1>ppmforge</H1> +Updated: 25 October 1991 +<BR> +<A HREF="#index">Table Of Contents</A> + +<A NAME="lbAB"> </A> +<H2>NAME</H2> + +ppmforge - fractal forgeries of clouds, planets, and starry skies +<A NAME="lbAC"> </A> +<H2>SYNOPSIS</H2> + +<B>ppmforge</B> + +[<B>-clouds</B>] +[<B>-night</B>] +[<B>-dimension</B> <I>dimen</I>] +[<B>-hour</B> <I>hour</I>] +[<B>-inclination|-tilt</B> <I>angle</I>] +[<B>-mesh</B> <I>size</I>] +[<B>-power</B> <I>factor</I>] +[<B>-glaciers</B> <I>level</I>] +[<B>-ice</B> <I>level</I>] +[<B>-saturation</B> <I>sat</I>] +[<B>-seed</B> <I>seed</I>] +[<B>-stars</B> <I>fraction</I>] +[{<B>-xsize</b>|<b>-width</B>} <I>width</I>] +[{<B>-ysize</b>|<b>-height</B>} <I>height</I>] + + +<P>You can abbreviate any options to its shortest unique prefix. + + +<A NAME="lbAD"> </A> +<H2>DESCRIPTION</H2> + +<p>This program is part of <a href="index.html">Netpbm</a>. + +<B>ppmforge</B> 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 techniques used by <B>ppmforge</B> are +essentially those given by Voss[1], particularly the technique of +spectral synthesis explained in more detail by Dietmar Saupe[2]. + +<P>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 synthesised directly from pseudorandom numbers. + +<P>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 <B>-mesh</B> 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 +<B>-dimension</B> 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. + +<P>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 +<B>-power</B> 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 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 results). + +<P>After these calculations, we have a array of the specified size +containing numbers that range from 0 to 1. The pixmaps are generated +as follows: + +<DL COMPACT> +<DT><B>Clouds</B> + +<DD> +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. + +<DT><B>Planet</B> + +<DD> +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. + +<DT><B>Night</B> + +<DD> +A sequence of pseudorandom numbers is used to generate stars with a +user specified density. +</DL> + +<P>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 <B>pnmquant</B> or <B>ppmdither</B> before encoding +in a color mapped format. If the display resolution is high enough, +<B>ppmdither</B> generally produces better looking planets. +<B>pnmquant</B> 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 <B>-night</B> option +depends on the value specified for <B>-saturation</B>. Small values +limit the color temperature distribution of the stars and reduce the +number of colors in the image. If the <B>-saturation</B> 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 +<B>pamdepth</B> rather than <B>pnmquant</B> or <B>ppmdither</B>. Try +<I>newmaxval</I> settings of 63, 31, or 15 with <B>pamdepth</B> to +reduce the number of colors in the picture to 256 or fewer. + + +<A NAME="lbAE"> </A> +<H2>OPTIONS</H2> + +<DL COMPACT> +<DT><B>-clouds</B> + +<DD> +Generate clouds. A pixmap of fractal clouds is generated. Selecting clouds +sets the default for fractal dimension to 2.15 and power scale factor +to 0.75. + +<DT><B>-dimension</B> <I>dimen</I> + +<DD> Sets the fractal dimension to the specified <I>dimen</I>, 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. + +<DT><B>-glaciers</B> <I>level</I> + +<DD>The floating point <I>level</I> 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. + +<DT><B>-hour</B> <I>hour</I> + +<DD>When generating a planet, <b>ppmforge</b> uses <I>hour</I> as the +"hour angle at the central meridian." If you specify <B>-hour +12</B>, 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 +<I>hour</I>, 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 <B>-hour</B> option is specified, a random hour angle +is chosen, biased so that only 25% of the images generated will be +crescents. + +<DT><B>-ice</B> <I>level</I> + +<DD>Sets the extent of the polar ice caps to the given floating point +<I>level</I>. The default level of 0.4 produces ice caps similar to +those of the Earth. Smaller values reduce the amount of ice, while +larger <B>-ice</B> settings create more prominent ice caps. +Sufficiently large values, such as 100 or more, in conjunction with +small settings for <B>-glaciers</B> (try 0.1) create "ice +balls" like Europa. + +<DT><B>-inclination|-tilt</B> <I>angle</I> + +<DD>The inclination angle of the planet with regard to its primary +star is set to <I>angle</I>, 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 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. + +<DT><B>-mesh</B> <I>size</I> + +<DD>A mesh of <I>size</I> by <I>size</I> will be used for the fast +Fourier transform (FFT). Note that memory requirements and +computation speed increase as the square of <I>size</I>; 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). + +<DT><B>-night</B> + +<DD>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. + +<DT><B>-power</B> <I>factor</I> + +<DD>Sets the "power factor" used to scale elevations +synthesised from the FFT to <I>factor</I>, 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 <B>-clouds</B> 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 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 dimensions, may require a larger FFT mesh size and/or higher +screen resolution to look good. + +<DT><B>-saturation</B> <I>sat</I> + +<DD>Controls the degree of color saturation of the stars that surround +planet pictures and fill starry skies created with the <B>-night</B> +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 <I>sat</I> 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. + +<p>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 <B>ppmforge</B>. 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 <B>ppmforge</B>. You +can reverse the proportion, simulating the sky as seen in a starburst +galaxy, by specifying a negative <I>sat</I> value. + +<DT><B>-seed</B> <I>num</I> + +<DD>Sets the seed for the random number generator to the integer +<I>num</I>. The seed used to create each picture is displayed on +standard output (unless suppressed with the <B>-quiet</B> option). +Pictures generated with the same seed will be identical. If no +<B>-seed</B> 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. + +<DT><B>-stars</B> <I>fraction</I> + +<DD>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 <B>-night</B> is specified. The default +<I>fraction</I> is 100. + +<DT><B>-xsize|-width</B><I> width</I> + +<DD>Sets the width of the generated image to <I>width</I> 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 pixmap, +make a square one with <B>ppmforge</B>, then use <B>pamcut</B> to +extract a portion of the shape and size you require. + +<DT><B>-ysize|-height</B> <I>height</I> + +<DD>Sets the height of the generated image to <I>height</I> pixels. +The default height is 256 pixels. If the height specified exceeds the +width, the width will be increased to equal the height. + +</DL> + +<A NAME="lbAF"> </A> +<H2>LIMITATIONS</H2> + +<P>The algorithms require the output pixmap 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 +pixmap if necessary. + +<P>You may have to reduce the FFT mesh size on machines with 16 bit +integers and segmented pointer architectures. + +<A NAME="lbAG"> </A> +<H2>SEE ALSO</H2> + +<B><A HREF="pamcut.html">pamcut</A></B>, +<B><A HREF="pamdepth.html">pamdepth</A></B>, +<B><A HREF="ppmdither.html">ppmdither</A></B>, +<B><A HREF="pnmquant.html">pnmquant</A></B>, +<B><A HREF="ppm.html">ppm</A></B> + +<DL COMPACT> +<DT>[1] +<DD> +Voss, Richard F., ``Random Fractal Forgeries,'' in Earnshaw +et. al., Fundamental Algorithms for Computer Graphics, Berlin: +Springer-Verlag, 1985. + +<DT>[2] +<DD> +Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal Images, +New York: Springer Verlag, 1988. + +</DL> + +<A NAME="lbAH"> </A> +<H2>AUTHOR</H2> + +<PRE> +John Walker +Autodesk SA +Avenue des Champs-Montants 14b +CH-2074 MARIN +Suisse/Schweiz/Svizzera/Svizra/Switzerland + <B>Usenet:</B><A HREF="mailto:kelvin@Autodesk.com">kelvin@Autodesk.com</A> + <B>Fax:</B>038/33 88 15 + <B>Voice:</B>038/33 76 33 +</PRE> + +<P> +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. + +<h3>PLUGWARE!</h3> + +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. + +<HR> +<A NAME="index"> </A> +<H2>Table Of Contents</H2> +<UL> +<LI><A HREF="#lbAB">NAME</A> +<LI><A HREF="#lbAC">SYNOPSIS</A> +<LI><A HREF="#lbAD">DESCRIPTION</A> +<LI><A HREF="#lbAE">OPTIONS</A> +<LI><A HREF="#lbAF">LIMITATIONS</A> +<LI><A HREF="#lbAG">SEE ALSO</A> +<LI><A HREF="#lbAH">AUTHOR</A> +</UL> +</BODY> +</HTML> + + + |