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

20 Chapter 3: A Brief Survey of Ray Tracing Acceleration Methods
puter Graphics (like e.g. Glassner [Glassner89], Shirley [Shirley02, Shirley03],
Möller-Haines [Akenine-Möller02] or introductory graphics [Foley97]), or to
the original publications 4 .
Outline
All the different techniques that have been proposed over the last two decades
can be structured and ordered in many different ways. Most commonly, this
ordering is performed on the kind of “coherence” these different techniques
employ 5 . However, though exploiting coherence undoubtedly most often is
the key to fast and efficient ray tracing, the term coherence is used inconsistently
by many different researchers, and many acceleration techniques
actually use a mix of different kinds of coherence. As a consequence, the following
summary of techniques will be ordered based on the goals that they
try to achieve.
Based on that, the different techniques that have been proposed over the
last two decades can be roughly grouped into two categories: One category
consists of techniques that aim at reducing the number of rays to be traced,
which can be performed either by (re-)constructing an image with less samples
on the image plane (e.g. through adaptive sampling), or by reducing
the number of rays to be traced for each such sample (e.g. by shooting less
shadow rays, or through pruning of the shading tree 6 ).
Please note that many of these techniques are applicable to only a small
subset of all the variants of ray tracing. For example, shadow caching (see
below) can not be used to accelerate ray casting, and first hit optimizations
will hardly benefit global illumination algorithms.
4 Very extensive surveys of ray tracing articles (though usually not including the newer
ones) can also be found in [Wilson93]
5 Coherence refers the degree of similarity between two problems. Obviously, two similar
problems can be solved faster if this similarity can be exploited in one or another way.
Ray tracing contains many different forms of coherence that can be exploited, like e.g. ray
coherence, temporal coherence, image space coherence, object coherence, ray coherence,
memory access coherence, frame-to-frame coherence, etc.
6 Note that the term “shading tree” (or shade tree) also has another, totally different
meaning: The term “shade tree” is (as in this context) commonly used to refer to the tree
formed by all secondary rays that have recursively been invoked by a given ray. This may
not be confused with Cook’s “shade trees” (see [Cook84b]), which essentially form a way
of elegantly expressing the way that a certain (single) ray is shaded, similar to a shading
language. Though the term “ray tree” might be a better name for the former concept, the
ambiguous term “shade tree” is already widely used in practive.