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| -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 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. |
| -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 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 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 synthesized 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 nonunity 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 emphasize 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 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 that 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 sky glow, 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.