Contents Flat is computationally somewhat faster, but will produce incorrect results in certain circumstances, particularly sunsets and high altitude shots. Round air is slightly slower, but always produces the correct effects at the horizon.The second option in the air is an air quality parameter. The simplest case is perfectly clean air with no haze-like particles present. This produces beautiful blue skies that become blue-white at the horizon. By choosing
dirty rather than clean for air_type, you can add particles to the atmosphere. "Particles" refers to any microscope particle, whether it is bits of dust or water droplets. Thus we use "dirty" air for fog, haze, and smog.The air is a volumetric structure contributing both attentuation and light. RenderWorld provides a second atmospheric structure, a skydome, for importing a sky of your choice while maintaining the volumetric character of the air. A skydome is an image of a sky mapped from a fish-eye lens. As described in detail below, the dome is modeled at being a shell at a particular altitude in the sky, and is translationally invariant.
RenderWorld_data_file air 1 1
number_of_bands 3
band_id 1 2 3
file_type clean
surface_type flat
mol_height_scale 1.0
mol_density_scale 1.0
end
The next two lines choose a clean, flat atmosphere. There are two parameters to specify the size, opacity, and color of the air. mol_height_scale refers to how tall the atmosphere is, while mol_density_scale controls how dense it is. In both cases, a value of 1.0 represents the typical values for Earth at sea level. For producing realistic images, these values shouldn't be changed much if at all.
Choosing a round atmosphere rather than flat introduces one new parameter, the radius of the planet that the air surrounds. The value in the example file is the radius of the Earth in meters. This value can be changed, but note that the earth's radius is an input parameter to other files as well, and so it is important to coordinate the values used. The important thing is that the same radius be entered for the ocean's radius in the ocean file later.
RenderWorld_data_file air 1 1
number_of_bands 3
band_id 1 2 3
file_type dirty
surface_type round
radius 6371230
mol_height_scale 1.0
mol_density_scale 1.0
dirty_height_scale 1.0
dirty_density_scale 0.2
dirty_factor 0.5
sun_threshold 1000
end
The second example uses the dirty air option. Dirty air introduces 3 other new parameters. Two (dirty_height_scale, and dirty_density_scale) are similar to the parameters for the clean air, affecting the height and density of the fog/smog. The guidelines for setting these parameters are that they should both be positive, and a value of much greater than 1 for the dirty_density_scale can result in near total blockage of the sun.
The parameter
dirty_factor relates to the size distribution of the microscopic aerosol particles in the air. A value of 0.5 or less produces a more fog like image, while a value of 0.9 is more smog like. This value should be restricted to be between 0.1 and 1.0. Beyond that, its effects are best learned by experimenting.
For almost any atmosphere, the sun is going to be considerably brighter than any other objects in the scene. Thus, if the camera sees the sun directly, it will generally saturate in all three color bands and produce a white sun in the image. The final line in the air file allows us to solve this problem. By specifying a threshold value, RenderWorld will make sure than the sun's intensity doesn't exceed this value in any one of the bands, and will preserve the color of the sun. The value here is in scientific units, and the exact effect will depend upon the scaling chosen, so some experimentation may be required. Generally though, a value of 1000 works well. Note that this line is optional.
A skydome is an image that is mapped onto an imaginary hemispherical shell in the atmosphere at a user-prescribed altitude. Except for its spherical nature and connection to the atmosphere/camera geometry, the skydome and its parameters behave very similar to the picture object. As with the air object, a skydome is placed in a scene file as follows:
number_of_times 1
time x y z theta phi chi
0 0 0 -1 0 0 0
name: skydomeabc
type: skydome
input: skydome.dat
priority 0
The skydome file skydome.dat contains the following information:
RenderWorld_data_file skydome 2 2
radius 100000.
number_of_bands 3
band_id 0 1 2
file_name jungle.rgb
min_radiance 0. 0. 0.
max_radiance 255. 255. 255.
image_method 0
end
This is similar to the picture file. The radius is the imagined altitude above the surface of the earth (in this example, 100 km). file_name gives the name of the sgi rgb file containing the skydome image. The 8-bit/color image is mapped in radiometric units by using the min_radiance and max_radiance lines. The image_method 0 renders the skydome and its reflections, whereas image_method 1 renders the skydome reflections but not the primary image of the skydome.Note that to use the skydome, the version information at the top of the skydome file must be
2 2.The skydome image itself is a rectangular image which maps a spherical lens (Schwarzschild lens) onto a flat plane.
RenderWorld_data_file skydome 4 4
radius 10000
number_of_bands 3
band_id 1 2 3
file_name jungle.rgb
horizon_angle 0.
animate frames
use_alpha 1
min_radiance 0. 0. 0.
max_radiance 255. 255. 255.
image_method 0
end
The additional lines have the following meaning:The
horizon_angle is the angle below the nominal end of the skydome (at theta=90 degrees) to extend the skydome.
The line beginning with
animate determine whether the skydome images are animated. The options are no and frames. When frames is chosen, RenderWorld tries to read the file jungle.rgb.xxxx for frame xxxx.The alpha channel of the skydome is used when
use_alpha has the value 1 or 2 (0 is the no alpha choice). For the value 1, the background is attenuated by the alpha, and for the choice of 2, the background is attenuated and the skydome texture is conditioned by the alpha.