Realtime Ray Tracing and Interactive Global Illumination - Scientific ...

8 Chapter 2: An Introduction to Ray Tracing
Already at this level, the term “ray tracing” actually covers three different
problems that have to be solved: Finding the closest intersection to the origin,
finding any intersection along the ray, and finding all intersections along it.
Finding only the closest intersection is the most fundamental operation in
any ray tracer. It usually requires to find the closest intersecting primitive P
and its distance t hit . Additionally, most ray tracers also determine additional
parameters that are later-on used for shading the ray, such as local surface
properties or the surface normal.
A slightly simpler problem is to determine whether there is any primitive
that intersects the ray. This is actually the same as checking for visibility
between the two points O and O + t max D. As such, this operation is often
termed “visibility test” or “occlusion test”. Obviously, checking for any intersection
is a slightly simpler problem than finding the closest intersection.
Thus, there are algorithms that are more efficient for this special case than
for the general ray shooting case. As checking for occlusion is a very common
operation in a ray tracer, most ray tracers have specially optimized routines
for this task. Most obviously, “normal” rays might have to perform multiple
intersection tests in order to determine which of these intersections in the
closest one, whereas intersection testing for shadow rays can be terminated
as soon as the first successful intersection test has been performed. Except
for this obvious optimization, there are also several optimizations and heuristics
for accelerating shadow rays, such as Shadow Caching, Shadow Maps,
Adaptive Shadow Testing, Local Illumination Environments, and others (see
Section 3.2).
The third sub-problem in ray tracing is finding all intersections with a
given ray. This is required by some advanced lighting algorithms (such as
e.g. [Sbert97]). Except for these special algorithms, however, this special
problem is not very common, and few ray tracers have special optimizations
for this task (though they obviously exist, see e.g. [Havran01]).
2.1.1 Scene Description
The “scene” to be rendered consists of a list of “geometric primitives”, which
are usually simple geometric shapes such as as polygons, spheres, cones, etc.
However, ray tracing primitives may also be as complex objects as parametric
patches (e.g. Bezier- or NURBS patches), subdivision surfaces, ISO-surfaces,
as well as algebraic or implicit surfaces. Ray tracing can even accurately
handle such complex constructions as CSG 2 , fractals, and recursively and
procedurally defined objects. In fact, any kind of object can be used as a ray
2 CSG = Constructive Solid Geometry