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<a href="0400-0402.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0406-0406.html">Next</A></CENTER></P>
<A NAME="PAGENUM-403"><P>Page 403</P></A>
<P>Often conversion from ppm to pgm will yield an image with contrast too low for good printer output. The program
maximizes contrast between the gray levels' output.
</P>
<P>A ppm input of n colors is read, and a pgm of n gray levels is written. The gray levels take on the values
0...n-1, while maxval takes on n-1.
</P>
<P>The mapping from color to stepped grayscale can be performed in order of input pixel intensity, or input pixel
frequency (number of repetitions).
</P>
<P><B>
OPTIONS
</B>
</P>
<TABLE>
<TR><TD>
-frequency
</TD><TD>
Sort input colors by the number of times a color appears in the input, before mapping to evenly
distributed gray levels of output.
</TD></TR><TR><TD>
-intensity
</TD><TD>
Sort input colors by their grayscale intensity, before mapping to evenly distributed gray levels of
output. This is the default.
</TD></TR></TABLE>
<P><B>
BUGS
</B>
</P>
<P>Helpful only for images with a very small number of colors. Perhaps should have been an option to
ppmtopgm(1).
</P>
<P><B>
SEE ALSO
</B>
</P>
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<PRE>
ppmtopgm(1), ppmhist(1), ppm(5)
</PRE>
<!-- END CODE SNIP //-->
<P><B>
AUTHOR
</B>
</P>
<P>Copyright" 1993 by Dan Stromberg.
</P>
<P>22 July 1992
</P>
<H3><A NAME="ch01_ 297">
ppmdither
</A></H3>
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<PRE>ppmdither—Ordered dither for color images
</PRE>
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<P><B>
SYNOPSIS
</B>
</P>
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<PRE>
ppmdither [-dim dimension][-red shades][-green shades][-blue shades][ppmfile]
</PRE>
<!-- END CODE SNIP //-->
<P><B>
DESCRIPTION
</B>
</P>
<P>ppmdither reads a portable pixmap as input, and applies dithering to it to reduce the number of colors used down to
the specified number of shades for each primary. The default number of shades is
red=5, green=9, blue=5, for a total of 225 colors. To convert the image to a binary RGB format suitable for color printers, use
-red 2 -green 2 -blue 2. The maximum number of colors that can be used is 256 and can be computed as the product of the number of red, green, and blue shades.
</P>
<P><B>
OPTIONS
</B>
</P>
<TABLE>
<TR><TD>
-dim
</TD><TD>
dimension
</TD><TD>
The size of the dithering matrix. Must be a power of 2.
</TD></TR><TR><TD>
-red
</TD><TD>
shades
</TD><TD>
The number of red shades to be used; minimum of 2.
</TD></TR><TR><TD>
-green
</TD><TD>
shades
</TD><TD>
The number of green shades to be used; minimum of 2.
</TD></TR><TR><TD>
-blue
</TD><TD>
shades
</TD><TD>
The number of blue shades to be used; minimum of 2.
</TD></TR></TABLE>
<P><B>
SEE ALSO
</B>
</P>
<!-- CODE SNIP //-->
<PRE>
pnmdepth(1), ppmquant(1), ppm(5)
</PRE>
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<P><B>
AUTHOR
</B>
</P>
<P>Copyright" 1991 by Christos Zoulas.
</P>
<P>14 July 1991
</P>
<A NAME="PAGENUM-404"><P>Page 404</P></A>
<H3><A NAME="ch01_ 298">
ppmflash
</A></H3>
<P>ppmflash—Brighten a picture up to complete white-out
</P>
<P><B>
SYNOPSIS
</B>
</P>
<!-- CODE SNIP //-->
<PRE>
ppmflash flashfactor [ppmfile]
</PRE>
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<P><B>
DESCRIPTION
</B>
</P>
<P>ppmflash reads a portable pixmap as input and increases its brightness by the specified
flashfactor up to a total white-out image. The
flashfactor may be in the range from 0.0 (original picture's brightness) to 1.0 (full white-out, The
Second After).
</P>
<P>As pnmgamma does not do the brightness correction in the way I wanted it, I wrote this small program.
</P>
<P>This program is similar to ppmbrighten, but not exactly the same.
</P>
<P><B>
SEE ALSO
</B>
</P>
<!-- CODE SNIP //-->
<PRE>
ppm(5), ppmdim(1), pnmgamma(1), ppmbrighten(1)
</PRE>
<!-- END CODE SNIP //-->
<P><B>
AUTHOR
</B>
</P>
<P>Copyright" 1993 by Frank Neumann.
</P>
<P>16 November 1993
</P>
<H3><A NAME="ch01_ 299">
ppmforge
</A></H3>
<P>ppmforge—Fractal forgeries of clouds, planets, and starry skies
</P>
<P><B>
SYNOPSIS
</B>
</P>
<!-- CODE //-->
<PRE>
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]
</PRE>
<!-- END CODE //-->
<P><B>
DESCRIPTION
</B>
</P>
<P>ppmforge generates three kinds of "random fractal forgeries," the term coined by Richard F. Voss of the IBM Thomas<BR>
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
ppmforge are essentially those given by Voss, particularly
the technique of spectral synthesis explained in more detail by Dietmar Saupe. (The "See Also" subsection provides
more detailed information about these men's work.)
</P>
<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 synthesized directly from pseudorandom numbers.
</P>
<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
-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.
</P>
<P>After 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. You 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 unmodified; a power scale of 0.5 takes the square
root
</P>
<A NAME="PAGENUM-405"><P>Page 405</P></A>
<P>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 one yield landscapes
with vertical scarps that look like glacial-carved valleys; powers greater than one make fairy-castle spires (which require large
mesh sizes and high resolution for best results).
</P>
<P>After these calculations, you have an array of the specified size containing numbers that range from 0 to 1. The pixmaps
are generated as follows:
</P>
<TABLE>
<TR><TD>
Clouds
</TD><TD>
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, and numbers between 0.5 and 1.0 are colored with
corresponding levels of white, with 1.0 being pure white.
</TD></TR><TR><TD>
Planet
</TD><TD>
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 on the water depth; 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.
</TD></TR><TR><TD>
Night
</TD><TD>
A sequence of pseudorandom numbers is used to generate stars with a user-specified density.
</TD></TR></TABLE>
<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 that must be compressed with
ppmquant or ppmdither before encoding in a color-mapped format. If the display resolution is high enough,
ppmdither generally produces better-looking planets.
ppmquant 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<BR>
-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
pnmdepth rather than ppmquant or ppmdither.Try newmaxval
settings of 63, 31, or 15 with pnmdepth to reduce the number of colors in the picture
to 256 or fewer.
</P>
<P><B>
OPTIONS
</B>
</P>
<TABLE>
<TR><TD>
-clouds
</TD><TD>
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.
</TD></TR><TR><TD>
-dimension dimen
</TD><TD>
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.
</TD></TR><TR><TD>
-glaciers level
</TD><TD>
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.
</TD></TR><TR><TD>
-hour hour
</TD><TD>
When generating a planet, hour is used 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 percent of the images generated will be crescents.
</TD></TR></TABLE>
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