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

10 Chapter 2: An Introduction to Ray Tracing
2.2 The Ray Tracing Rendering Algorithm
Whereas the concept of finding the intersection between a ray and a scene can
be used for many different applications (e.g. [Durgin97, Plasi97, Agelet97]), it
is most commonly used in the context of the traditional recursive ray tracing
algorithm.
2.2.1 Appel: First Approaches to Ray Tracing
The idea of using ray shooting for computing images is as old as 1968, and
was first introduced by Arthur Appel [Appel68] as an alternative method for
solving the “hidden surface” problem for rendering solid objects. In order
to compute a two-dimensional image of a three-dimensional scene, rays are
generated from the virtual camera through each pixel, traced into the scene,
and the closest object is determined (see Figure 2.1). The color of this ray
is then determined based on the properties of this object.
primary rays
scene primitives
eye
eye
virtual image plane
image
plane
scene primitives
Figure 2.1: The principle of ray casting: In order to compute an image of a
scene as seen by a virtual camera, a “primary ray” is shot through each pixel
of the virtual image plane, and cast into the scene (left). For each such ray,
the closest object hit by this ray is determined by intersecting the ray with the
geometric primitives that make up the scene (right).
Additionally, the same concept of shooting rays for determining visibility
can be used to shade the hit-point. For example, shadows can be computed by
shooting “shadow rays” 5 into the direction of the light sources, to determine
whether the respective light source is occluded from the hit surface point, or
whether it is occluded. This concept of using shadow rays to influence the
techniques usually are spatial techniques.
5 Other commonly used names for shadow rays are “illumination rays”, “shadow feelers”,
or “light feelers”.