While ray tracing certainly adds dynamic capabilities
to the field of computer graphics, it also has it's limitations.
Other methods of rendering images that model light are also widely used.
Two such methods are ray casting and radiosity. These methods
incorporate a process that is similar to the one used in used in ray tracing,
however, they vary in technique and capability.
Ray casting is a three-dimensional rendering technique
that has served as a foundation in computer graphics, especially in computer
games. Like ray tracing, ray casting traces imaginary rays of light
in order to render realistic images of model worlds. The appeal of
choosing ray casting, though, is its capability to render images at extremely
high frame rates.
In ray casting, 3-D worlds are described by 2-D
maps which are overhead views of the worlds. The designer assigns
the location and orientation of the point of view (POV). In order
to simulate a realistic human viewpoint, the POV generally encompasses
a 60-degree field of vision. Light sources are then positioned within
the model of the world. Individual rays are traced, and intersections
between the rays and the 3-D world are determined. This, in
turn, determines what, in the 3-D world, should be visible from the POV.
In its basic structure, the technique of ray casting
closely resembles that of ray tracing. But when a model is ray traced,
rays of light are simulated in order to accurately model real physical
environments. Rays "interact" with the objects they encounter, and
light is reflected, refracted, and absorbed within a given space, before
the color of an individual pixel is determined. In ray casting, rays
are sent from a viewpoint through pixels of the screen into the model scene
and the intersections between the ray and the objects it hits are calculated.
The ray is not "recast" from that point, therefore, mirroring, refraction,
and shadowing effects cannot be determined.
When models are rendered using ray casting as opposed
to ray tracing, the accuracy of the method is severly reduced. Beacuse
of the constraints it places on the 3-D world, ray casting sacrifices realism.
The usefulness of ray casting is that it can be performed in real time.
Ray casting is a useful tool in applications such as the preliminary steps
of ray tracing, where speed is a primary concern. [This images is re-produced, with permission, from the
Stanford Computer Graphics Lab Web Site @ http://graphics.stanford.edu. Reproduction is prohibited.]
Ray tracing certainly adds dynamic effects
to the field of computer graphics. It also has its limitations.
Radiocity is a technique that offers a short cut to ray tracing.
When an image is ray traced, individual rays move throughout a model world
traveling from one surface to another. However, each intersection
between the ray and a surface is only a dimensionless point.
In order to complete a rendering we must still approximate the amount of
light that has been reflected, refracted, and absorbed by any given surface
between the calculated points. Radiosity is a way of tracing the
general flow of light between two surfaces. The technique is combined
with Gouraud shading to smooth across the calculated patches of light.
The result is a technique that has significantly different capablities
than conventional ray tracing.
Ray tracing is particularly good at rendering point
light, specular reflection, and refraction. Radiocity, on the other
hand, effectively renders models containing area light sources, and can
produce diffuse reflections, realistic shadows and color bleeding effects.
It is said that radiocity is view independent, this is perhaps the most
important difference between radiocity and ray tracing. View independence
means that once radiocity calculartions have been performed for a given
environment, the distribution of light throughout the entire environment
is known. This makes it possible to run walkthrough animations in
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