Here are a variety of other topics about POV-Ray features.

POV-Ray includes the ability to render fog. To add fog to a scene, place 
the following declaration outside of any object definitions:

       fog {
         color Gray70      // the fog color
         distance 200.0    // distance for 100% fog color
       }

The fog color is then blended into the current pixel color at a rate 
calculated as:

          1-exp(-depth/distance) =
          1-exp(-200/200) =
          1-exp(-1) =
          1-.37... =
          0.63...

So at depth 0, the color is pure (1.0) with no fog (0.0). At the fog 
distance, you'll get 63% of the color from the object's color and 37% from 
the fog color. 

Subtle use of fog can add considerable realism and depth cuing to a scene 
without adding appreciably to the overall rendering times.  Using a black 
or very dark gray fog can be used to simulate attenuated lighting by 
darkening distant objects.

The "#max_trace_level" directive sets a variable that defines how many 
levels that POV-Ray will trace a ray. This is used when a ray is reflected 
or is passing through a transparent object. When a ray hits a reflective 
surface, it spawns another ray to see what that point reflects, that's 
trace level 1. If it hits another reflective surface, then another ray is 
spawned and it goes to trace level 2. The maximum level by default is 5. 

If max trace level is reached before a non-reflecting surface is found, 
then the color is returned as black. Raise max_trace_level if you see black 
in a reflective surface where there should be a color.

The other symptom you could see is with transparent objects. For instance, 
try making a union of concentric spheres with the Cloud_Sky texture on 
them. Make ten of them in the union with radius's from 1-10 then render the 
Scene. The image will show the first few spheres correctly, then black. 
This is because a new level is used every time you pass through a 
transparent surface.  Raise max_trace_level to fix this problem.  For 
example:

      #max_trace_level 20

Note: Raising max_trace_level will use more memory and time and it could 
cause the program to crash with a stack overflow error. Values for 
max_trace_level are not restricted, so it can be set to any number as long 
as you have the time and memory. 

POV-Ray uses a set of internal stacks to collect ray/object intersection 
points.  The usual maximum number of entries in these "I-Stacks" is 64.  
Complex scenes may cause these stacks to overflow.  POV-Ray doesn't stop 
but it may incorrectly render your scene.  When POV-Ray finishes rendering, 
a number of statistics are displayed.  If you see "I-Stack Overflows" 
reported in the statistics, you should increase the stack size.  Add a 
directive to your scene as follows:

      #max_intersections 200

If the "I-Stack Overflows" remain, increase this value until they stop.

A background color can be specified if desired.  Any ray that doesn't hit 
an object will be colored with this color.  The default background is 
black.  The syntax for background is:

      background { color SkyBlue }

Using a colored background takes up no extra time for the ray tracer, 
making it a very economical, although limited, feature. Only solid colors 
can be specified for a background. Textures cannot be used.  No shadows 
will be cast on it, which makes it very useful, but at the same time, it 
has no "roundness", or shading, and can sometimes cause a scene to look 
"flat".  Use background with restraint.  It's often better, although a bit 
slower, to use a "sky sphere", but there are times when a solid background 
is just what you need.

Although POV-Ray 2.0 has had significant changes to the language over POV-
Ray 1.0, almost all 1.0 scenes will still work if the compatibility mode is 
set to 1.0.  The +MV switch described earlier, sets the initial mode.  The 
default is +MV2.0.  

Inside a scene file you may turn compatibility off or on using the 
"#version" directive.  For example:

      #version 1.0
      // Put some version 1.0 statements here

      #version 2.0
      // Put some version 2.0 statements here

Note you may not change versions inside an object or declaration.

The primary purpose of the switch is to turn off float and expression 
parsing so that commas are not needed.  It also turns off some warning 
messages.

Note some changes in tiles and material_maps cannot be fixed by turning the 
version compatibility on.  It may require hand editing of those statements.  
See the special texture section for details.

Future versions of POV-Ray may not continue to maintain full backward 
compatibility.  We strongly encourage you to phase in 2.0 syntax as much as 
possible.