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 ).

A black fog can be used to get a working distance value for the atmosphere.

The black color is used to simulate the attenuation you'll get in those parts of the atmosphere scene lying in shadow.

Even though this is not obvious this holds for infinite and patch objects like quadrics, quartics, triangles, polygons, etc. Whenever you add one of those objects you should add the hollow keyword as long as you are not absolutely sure you don't need it. You also have to make sure that all objects the camera is inside are set to be hollow.

Whenever you get unexpected results you should check for solid objects and set them to be hollow.

The size and shape of the rainbow are determined by the angle and width keywords. The direction keyword is used to set the direction of the incoming light, thus setting the rainbow's position. The rainbow is visible when the angle between the direction vector and the incident light direction is larger than angle-width/2 and smaller than angle+width/2.

The incoming light is the virtual light source that is responsible for the rainbow. There needn't be a real light source to create the rainbow effect.

The rainbow is a fog-like effect, i.e. the rainbow's color is mixed with the background color based on the distance to the intersection point. If you choose small distance values the rainbow will be visible on objects, not just in the background. You can avoid this by using a very large distance value.

The color map is the crucial part of the rainbow since it contains all the colors that normally can be seen in a rainbow. The color of the innermost color band is taken from the color map entry 0 while the outermost band is take from entry 1. You should note that due to the limited color range any monitor can display it is impossible to create a real rainbow. There are just some colors that you cannot display.

The filter channel of the rainbow's color map is used in the same way as with fogs. It determines how much of the light passing through the rainbow is filtered by the color.

The following example shows a simple scene with a ground plane, three spheres and a somewhat exaggerated rainbow ( rainbow1.pov ).

Some irregularity is added to the color bands using the jitter keyword.

A colorful rainbow.

The rainbow in our sample is much too bright. You'll never see a rainbow like this in reality. You can decrease the rainbow's colors by decreasing the RGB values in the color map.

We can use the transmittance channel of the colors in the color map to specify a minimum translucency, just like we did with the fog. To get realistic results we have to use very large transmittance values as you can see in the following example ( rainbow2.pov ).

The transmittance values increase at the outer bands of the rainbow to make it softly blend into the background.

A much more realistc rainbow.

The resulting image looks much more realistic than our first rainbow.

If you use arc_angle 120 for example you'll get a rainbow arc that abruptly vanishes at the arc's ends. This does not look good. To avoid this the falloff_angle keyword can be used to specify a region where the arc smoothly blends into the background.

povray is the name of the program and it is followed by several switches. Each switch begins with a plus or minus sign. The +I switch with the filename tells POV-Ray what scene file it should use as input and +V tells the program to output its status to the text screen as it's working. The +W and +H switches set the width and height of the image in pixels. This image will be 80 pixels wide by 60 pixels high.

In switches which toggle a feature, the plus turns it on and minus turns it off. For example +P turns on the pause for keypress when finished option while -P turns it off. Other switches are used to specify values and do not toggle a feature. Either plus or minus may be used in that instance. For example +W 320 sets the width to 320 pixels. You could also use -W 320 and get the same results.

Switches may be specified in upper or lower case. They are read left to right but in general may be specified in any order. If you specify a switch more than once, the previous value is generally overwritten with the last specification. The only exception is the +L switch for setting library paths. Up to ten unique paths may be specified.

Almost all + / - switches have an equivalent option which can be used in an INI file which is described in the next section. A detailed description of each switch is given in the option reference section.