# Reference:Bounded By Object Modifier

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.