Rendering Effects Created by Ray Tracing
In order to examine a number of effects that can be
created using ray tracing, it will be helpful to disccus the topic
in terms of a specific example. Suppose we were to create a scene
to raytrace consisting of a single room. The room is empty except
for a single table positioned in the center of the room. As our light
source we introduce one bare 100-watt light bulb which is fixed on the ceiling
directly above the center of the table.
It is easy to see that the ceiling, the walls, and the
surface top of the table would be directly illuminated by the light emitted
by the bulb. The majority of the floor in our room would also be
"lit" by the bulb, but what about the area directly underneath the table.
There is no ray of light that has a direct path from the bulb to the area
below the table, yet we know that if this model were real, the area under
the table would still be visible. Every illuminated surface in the
room (in our scene, the ceiling, walls, table top, and the rest of the
floor) reflects a portion of the light it receives. As we trace the
paths of these rays about the room, we see that many of the rays eventually
illuminate the floor space that is under the table. The floor space
is, as a result, shadowed, but not completely black. The ray tracing
model takes this real-world effect into account and is capable of
rendering images that incorporate realistic shadows.
Now suppose we add a highly
reflective surface into our scene. We can lay a mirror on top of
the table. Now, a much larger portion of the light that directly
and indirectly illuminates the table top will be reflected back into the
room. In viewing the room we would notice the reflected images of
the walls, ceiling, and bulb appearing in the mirror. The illumaination
effects in the rest of the room, including the area under the table, would
also be affected by our addition.
Transparent objects produce both reflected and transmitted
light. If we were to place a transparent drinking glass on top of
a cleared table in our scene, and we then set a small plastic toy
on two sides of the glass, we would notice several things. Viewing
from one side of the glass we would be able to see light transmitted from
the object behind the glass. We would also see a reflection in the glass
of the object that is on our side of the glass. We might also notice
light transmission and relection of the walls, ceiling, and table top.
A transparent "bubble" enclosed by four colored
if we placed a prism onto our table. (The glass and toys have been
removed.) Much of the light from the bulb would be transmitted through
the prism. The light would be bent at different angles according
to its frequency, and the result would be a rainbow effect. This
is an example of refraction.
Refraction must be considered
in order to model realistic transparency effects. When a ray of light
intersects the surface of an object that has some degree of transparency,
part of the light is reflected and part is refracted. This means
that instead of "bouncing" off the surface, some of the light continues
through the material. The speed of light differs in different materials,
so the path of refracted light is a function of each material and the direction
of incident light. The light is bent, and the image is a distorted
view of what we would otherwise expect.
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