The clipped_by statement is technically an object modifier but it provides a type of CSG similar to CSG intersection. The syntax is:

CLIPPED_BY:
    clipped_by { UNTEXTURED_SOLID_OBJECT... } |
    clipped_by { bounded_by }

Where UNTEXTURED_SOLID_OBJECT is one or more solid objects which have had no texture applied. For example:

  object {
    My_Thing
    clipped_by{plane{y,0}}
  }

Every part of the object My_Thing that is inside the plane is retained while the remaining part is clipped off and discarded. In an intersection object the hole is closed off. With clipped_by it leaves an opening. For example the following figure shows object A being clipped by object B.

You may use clipped_by to slice off portions of any shape. In many cases it will also result in faster rendering times than other methods of altering a shape. Occasionally you will want to use the clipped_by and bounded_by options with the same object. The following shortcut saves typing and uses less memory.

  object {
    My_Thing
    bounded_by { box { <0,0,0>, <1,1,1> } }
    clipped_by { bounded_by }
  }

This tells POV-Ray to use the same box as a clip that was used as a bound.

The calculations necessary to test if a ray hits an object can be quite time consuming. Each ray has to be tested against every object in the scene. POV-Ray attempts to speed up the process by building a set of invisible boxes, called bounding boxes, which cluster the objects together. This way a ray that travels in one part of the scene does not have to be tested against objects in another, far away part of the scene. When a large number of objects are present the boxes are nested inside each other. POV-Ray can use bounding boxes on any finite object and even some clipped or bounded quadrics. However infinite objects (such as a planes, quartic, cubic and poly) cannot be automatically bound. CSG objects are automatically bound if they contain finite (and in some cases even infinite) objects. This works by applying the CSG set operations to the bounding boxes of all objects used inside the CSG object. For difference and intersection operations this will hardly ever lead to an optimal bounding box. It is sometimes better (depending on the complexity of the CSG object) to have you place a bounding shape yourself using a bounded_by statement.

Normally bounding shapes are not necessary but there are cases where they can be used to speed up the rendering of complex objects. Bounding shapes tell the ray-tracer that the object is totally enclosed by a simple shape. When tracing rays, the ray is first tested against the simple bounding shape. If it strikes the bounding shape the ray is further tested against the more complicated object inside. Otherwise the entire complex shape is skipped, which greatly speeds rendering. The syntax is:

BOUNDED_BY:
    bounded_by { UNTEXTURED_SOLID_OBJECT... } |
    bounded_by { clipped_by }

Where UNTEXTURED_SOLID_OBJECT is one or more solid objects which have had no texture applied. For example:

  intersection {
    sphere { <0,0,0>, 2 }
    plane  { <0,1,0>, 0 }
    plane  { <1,0,0>, 0 }
    bounded_by { sphere { <0,0,0>, 2 } }
  }

The best bounding shape is a sphere or a box since these shapes are highly optimized, although, any shape may be used. If the bounding shape is itself a finite shape which responds to bounding slabs then the object which it encloses will also be used in the slab system.

While it may a good idea to manually add a bounded_by to intersection, difference and merge, it is best to never bound a union. If a union has no bounded_by POV-Ray can internally split apart the components of a union and apply automatic bounding slabs to any of its finite parts. Note that some utilities such as raw2pov may be able to generate bounds more efficiently than POV-Ray's current system. However most unions you create yourself can be easily bounded by the automatic system. For technical reasons POV-Ray cannot split a merge object. It is maybe best to hand bound a merge, especially if it is very complex.

Note: if bounding shape is too small or positioned incorrectly it may clip the object in undefined ways or the object may not appear at all. To do true clipping, use clipped_by as explained in the previous section. Occasionally you will want to use the clipped_by and bounded_by options with the same object. The following shortcut saves typing and uses less memory.

  object {
    My_Thing
    clipped_by{ box { <0,0,0>,<1,1,1 > }}
    bounded_by{ clipped_by }
  }

This tells POV-Ray to use the same box as a bound that was used as a clip.

You may specify the no_shadow keyword in an object to make that object cast no shadow. This is useful for special effects and for creating the illusion that a light source actually is visible. This keyword was necessary in earlier versions of POV-Ray which did not have the looks_like statement. Now it is useful for creating things like laser beams or other unreal effects. During test rendering it speeds things up if no_shadow is applied.

Simply attach the keyword as follows:

  object {
    My_Thing
    no_shadow
  }

Syntax:

  OBJECT {
    [OBJECT_ITEMS...]
    no_image
    no_reflection
  }

These two keywords are very similar in usage and function to the no_shadow keyword, and control an object's visibility.
You can use any combination of the three with your object.

When no_image is used, the object will not be seen by the camera, either directly or through transparent/refractive objects. However, it will still cast shadows, and show up in reflections (unless no_reflection and/or no_shadow is used also).

When no_reflection is used, the object will not show up in reflections. It will be seen by the camera (and through transparent/refractive objects) and cast shadows, unless no_image and/or no_shadow is used.

Using these three keywords you can produce interesting effects like a sphere casting a rectangular shadow, a cube that shows up as a cone in mirrors, etc.

Syntax:

  OBJECT {
    [OBJECT_ITEMS...]
    double_illuminate
  }

A surface has two sides; usually, only the side facing the light source is illuminated, the other side remains in shadow. When double_illuminate is used, the other side is also illuminated.
This is useful for simulating effects like translucency (as in a lamp shade, sheet of paper, etc).

Note: double_illuminate only illuminates both sides of the same surface, so on a sphere, for example, you will not see the effect unless the sphere is either partially transparent, or if the camera is inside and the light source outside of the sphere (or vise versa).

Some of POV-Ray's objects allow you to choose between a fast but sometimes inaccurate root solver and a slower but more accurate one. This is the case for all objects that involve the solution of a cubic or quartic polynomial. There are analytic mathematical solutions for those polynomials that can be used.

Lower order polynomials are trivial to solve while higher order polynomials require iterative algorithms to solve them. One of those algorithms is the Sturmian root solver. For example:

  blob {
    threshold .65
    sphere { <.5,0,0>, .8, 1 }
    sphere { <-.5,0,0>,.8, 1 }
    sturm
  }

The keyword may optionally be followed by a float expression which is interpreted as a boolean value. For example sturm off may be used to force it off. When the keyword is specified alone, it is the same as sturm on. By default sturm is off when not specified.

The following list shows all objects for which the Sturmian root solver can be used.

• blob
• cubic
• lathe (only with quadratic splines)
• poly
• prism (only with cubic splines)
• quartic
• sor