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

26 Chapter 3: A Brief Survey of Ray Tracing Acceleration Methods
“explosion of the ray tree”: If each surface shader on average shoots more
than one secondary ray (e.g. one for reflection and one for refraction), the
number of secondary rays grows exponentially with the recursion depth.
However, secondary rays are usually weighed with reflection and refraction
coefficients (usually depending on surface properties and incoming ray
direction), which may get rather small. Furthermore, these attenuation factors
multiply with each further surface interaction, resulting in most rays
having a rather small pixel contribution. In order to avoid tracing all these
“unimportant” rays, it is common to “prune” the ray tree by tracking each
rays “pixel contribution”, and to terminate the recursion once this contribution
drops below a certain threshold.
However, simply terminating such rays without compensation results in
a loss of illumination, and in “biased” images 8 Pruning the ray tree can also
be made unbiased by terminating the rays probabilistically with correct reweighting
(so-called russian roulette termination [Arvo90b]). This however
usually leads to noise in the image, which might be even more disturbing to
the viewer.
3.2.6 Reflection Mapping and Shadow Maps
Finally, the number of both secondary and shadow rays can be greatly reduced
by approximating shadows and reflections with reflection maps and
shadow maps. Both reflection maps and shadow maps are well-known concepts
in realtime rendering (see e.g. [Akenine-Möller02]). Though they are
typically used in triangle rasterization to approximate effects that can not be
computed at all, they can also be used in ray tracing to approximate effects
rather than shooting rays for computing them.
However, both methods are infamous for their tendency to generate artifacts.
As such, they somewhat contradict the concepts of ray tracing, and
should be used with extreme care.
3.2.7 Sample Reuse for Animations
In the special case of ray tracing animations, it is also possible to reuse samples
from other frames without re-tracing the respective rays. For example,
Formella et al. [Formella94] has proposed to store the complete “ray tree” of
each pixel, and to only re-trace those parts of a ray tree that are intersected
8 Pruning by pixel contribution becomes especially problematic in scenes with strongly
varying light source intensity: If connected to a bright light source, even a ray with small
attenuation factors may still have a stronger impact than a less attenuated ray illuminated
by a dark source.