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

28 Chapter 3: A Brief Survey of Ray Tracing Acceleration Methods
scenes (and a reasonably fast ray tracer) it is often faster to trace the primary
rays than first rasterizing the scene.
Summary
From all the just mentioned techniques, shadow caches are the most commonly
used, and are still widely used in practice. Also adaptive sampling is
used quite often, especially in the “demo community”, which aims more towards
artistic and highly impressive ray tracing demos than for a general ray
tracing engine. However, most of these techniques require special restrictions
(e.g. point lights and spot lights only for efficient shadow caching), or easily
break down in practice. As such, the RTRT/OpenRT system (see Part II)
does not use any of these techniques, and only concentrates on tracing all
rays as quickly as possible 9 .
3.3 Accelerating Ray-Scene Intersection
Apart from trying to reduce the number of rays to be shot for an image,
another obvious method of accelerating ray tracing is to improve the core
ray tracing algorithms such that the rays can be traced faster.
3.3.1 Faster Intersection Tests
Usually a large fraction of the compute time in ray tracing is spent in rayprimitive
intersections. This was already noted in 1980 by Whitted [Whitted80],
who reported 95% of his compute time to be spent on intersection computations.
3.3.1.1 Different Variants of Primitive Intersection Tests
Obviously, the intersection between a ray and any kind of primitive can be
computed in multiple ways. Each of these different algorithms has different
properties such as the number of floating point operations vs. integer
operation vs. conditionals, memory consumption, or numerical accuracy).
9 While RTRT/OpenRT does not in general use these techniques, some of them have still
been used in some special applications: For example, shadow caching has been used in the
original “Instant Global Illumination” system [Wald02b] (see Chapter 13), and probabilistic
pruning of the ray tree has been used for visualizing the “Headlight” model [Benthin02]
(see Chapter 11.2)