After adding some dust beams of light become visible.
The type keyword selects the type of atmospheric scattering we want to use. In this case we use the isotropic scattering that equally scatters light in all directions (see "Atmosphere" for more details about the different scattering types).
The samples keyword determines the number of samples used in accumulating the atmospheric effect. For every ray samples are taken along the ray to determine wether a sample is lit by a light source or not. If the sample is lit the amount of light scattered into the direction of the viewer is determined and added to the total intensity.
You can always start with an arbitrary number of samples. If the results do not fit your ideas you can increase the sampling rate to get better results. The problem of choosing a good sampling rate is the trade-off between a satisfying image and a fast rendering. A high sampling rate will almost always work but the rendering will also take a very long time. That's something to experiment with.
The distance keyword specifies the density of the atmosphere. It works in the same way as the distance parameter of the fog feature.
Last but not least will the scattering value determine the amount of light that is scattered by the particles (the remaining light is absorbed). As you'll later see this parameter is very useful in adjusting the overall brightness of the atmosphere.
Looking at the image created from the above scene you'll notice some very ugly anti-aliasing artefacts known as mach-bands. They are the result of a low sampling rate.
How this effect can be avoid is described in the following section.
The brute force approach is to increase the sampling rate until the artefacts vanish and you get a satisfying image. Though this will always work it is a bad idea because it is very time consuming. A better approach is to use jittering and anti-aliasing first. If both features don't help you'll have to increase the sampling rate.
Jittering moves each sample point by a small, random amount along the sampling direction. This helps to reduce regular features resulting from aliasing. There is (hardly) nothing more annyoing to the human visual system than the regular features resulting from a low sampling rate. It's much better to add some extra noise to the image by jittering the sample positions. The human eye is much more forgiving to that.
Use the jitter keyword followed by the amount of jittering you want to use. Good jittering values are up to 0.5, higher values result in too much noise.
You should be aware that jittering can not fix the artefacts introduced by a too low sampling rate. It can only make them less visible.
An additional and better way of reducing aliasing artefacts is to use (adaptive) super-sampling. This method casts additional samples where it is likely that they are needed. If the intensity between two adjactent samples differs too much additional samples are taken inbetween. This step is done recursively until a specified recursion level is reached or the sample get close to each other.
The aa_level and aa_threshold keywords are used to control the super-sampling. The aa_level keyword determines the maximum recursion level while the aa_threshold keyword specifies the maximum allowed difference between two sample before the super-sampling is done.
After all this theory we get back to our sample scene and add the appropriate keywords to use both jittering and supersamling ( atmos3.pov ).
A very low threshold value was choosen to super-sample even between adjactent points with a very similar intensity. The maximum recursion level of 4 will lead to a maximum of fifteen super-samples.
If you are looking at the results that you get after adding jittering and super-sampling you won't be satisfied. The only way of reducing the still visible artefacts is to increase the sampling rate by choosing a higher number of samples.
Doing this you'll get a good result showing (almost) no artefacts. Btw. the amount of dust floating around in this room may be a little bit exaggerated but it's just an example. And examples tend to be exaggerated.
If you want to create a reddish atmosphere for example, you can add the following line to the atmosphere statement used in the above example.
Just using rgb <1,0,0> does not work because the color's filter value will be zero and thus no light will be filtered by the color, i. e. no light will be multiplied with the color's RGB components.
The filter value of 0.25 means that 25% of the light passing through the atmosphere will be filtered by the red color and 75% will pass unfiltered.
The transmittance channel of the atmosphere's color is used to specify a minimum translucency. By default the transmittance channel is zero and thus there is no such minimum translucency. Using a positive value lets you determine the amount of background light that will always pass through the atmosphere, regardless of its thickness set by the distance keyword.
If you use e.g. a color of rgbt <0,0,0,0.3> with our room example you can make the blue background become visible. Until now it was hidden by the atmosphere.
The best approach is to choose the distance value first. This value determines the visibility of the objects in the scene regardless of atmospheric light scattering. It works in the same way as the distance value of the fog feature.
Since fog is very similar to the unlit atmosphere you can use a fog instead of an atmosphere to quickly choose a working distance value. If you do this with room scene we used earlier you would use the following fog statement instead of the atmosphere ( atmos4.pov ).
The black color is used to simulate the attenuation you'll get in those parts of the atmosphere scene lying in shadow.
If you want to use a colored atmosphere you'll have to use the same color for the fog as you want to use for the atmosphere, including the filter and transmittance channel values (see "Using a Coloured Atmosphere" and "Atmosphere" for an explanation of the atmosphere's color).
If you (roughly) want to simulate the appearance of those parts lit by a light source you can use the color of the atmosphere inside the fog statement instead.
After you are satisfied with the distance value you'll have to choose a scattering value. This value lets you fit the atmosphere's intensity to your needs. Starting with a value of one you have to increase the value if the atmosphere effects are hardly visible. If you don't see anything in the lit parts of the atmosphere you'll have to decrease the value.
You should be aware that you may have to use very small or very large values to get the desired results.