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<H1><A ID="SECTION00500000000000000000">
Specifying a View</A>
</H1>
<P>
When designing a <#218#><#1793#><EM>rayshade</EM><#1793#><#218#> input file, there are two main
issues that must be considered. The first and more complex
is the selection of the objects to be rendered and the appearances
they should be assigned. The second and usually easier issue is
the choice of viewing parameters. This chapter deals with the
latter problem; the majority of the following chapters
discuss aspects of objects and their appearances.
<P>
<#219#><#1795#><EM>Rayshade</EM><#1795#><#219#> uses a camera model to describe
the geometric relationship between the objects to be rendered
and the image that is produced. This relationship describes
a perspective projection from world space onto the image plane.
<P>
The geometry of the perspective projection
may be thought of as an
infinite pyramid, known as the viewing <#220#><EM>frustum</EM><#220#>.
The apex of the frustum is defined by the camera's position,
and the main axis of the frustum by a ``look'' vector.
The four sides of
the pyramid are differentiated by their relationship to a
reference ``up'' vector from the camera's position.
<P>
The image
ultimately produced by <#221#><#1797#><EM>rayshade</EM><#1797#><#221#> may then be thought of as
the projection of the objects closest to the eye onto
a rectangular screen formed by the intersection of the pyramid with
a plane orthogonal to the pyramid's axis. The overall shape
of the frustum (the lengths of the top and bottom sides compared
to left and right)
is described by the horizontal and vertical fields
of view.
<P>