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

16 Chapter 2: An Introduction to Ray Tracing
point, i.e. it returns a vector pointing towards this light source, the distance
to this light source, and the “intensity” that this light sample contributes to
the surface sample 9 .
Environment Shaders: As the scene is often not “closed”, it may easily
happen that rays are cast into directions in which there is no geometry that
they can hit. Such rays get “lost” into the environment. In order to compute
their contribution, they can be passed to an “environment shader” which typically
looks up a value from a texture representing the distant environment.
However, environment shaders can also be used to simulate more complex
effects like skylight models.
Volume Shaders, Pixel Shaders, etc.: Obviously, it is possible to extend
the just mentioned shader concept even further. For example, it is often
common to use “volume shaders” to compute the attenuation that a ray is
subject to when traveling between two surfaces. As the volumetric effects
are beyond the scope of current interactive ray tracing systems, we will not
go into details here. Some ray tracers also support “pixel shaders” or “image
shaders” that perform some post-filtering (e.g. tone mapping) on the image.
This “shader concept” allows for a high degree of flexibility and extendibility.
All the above mentioned shaders – except for volume shaders
– are also supported in the RTRT/OpenRT system (see Part II). For some
examples of what is possible with this concept, see Chapter 11.
2.3 General Ray Tracing based Algorithms
Though recursive ray tracing is undoubtedly the most common form of using
ray shooting for generating images, there is also a wider range of “ray
tracing algorithms” that do not fit the category of recursive ray tracing.
For example, many global illumination algorithms only use ray tracing for
visibility computations (like radiosity), or also start rays/paths at the light
sources (like light path tracing, bidirectional path tracing, photon mapping,
and metropolis light transport). Ray tracing is even being used for several
applications outside the graphics domain. For example, it can be used for
9 Note that “intensity” is actually the wrong term. In a physically correct renderer,
the value returned by a light shader should actually be “radiance”. Still, “color” and
“intensity” of the light sample are more commonly used terms in practice. This may
stem from the fact that in a general ray tracer light sources do not always have physical
meanings (i.e. lights with negative power, or with constant distance falloff), for which the
term “radiance” does not make sense.