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Left: Specular reflection
Right: Diffuse reflection
The way in which the photons are reflected from a surface depends primarily
on the smoothness of the surface. Rough surfaces tend to reflect photons in
all directions. These are known as diffuse surfaces, and this type of reflection
is known as diffuse reflection (shown above). A wall painted with flat paint
is a good example of a diffuse surface.
The final illumination of the kitchen is determined by the interaction between
the surfaces and the billions of photons emitted from the light source. At any
given point on a surface, it is possible that photons have arrived directly from
the light source (direct illumination) or else indirectly through one or more
bounces off other surfaces (indirect illumination). If you were standing in the
kitchen, a very small number of the photons in the room would enter your
eye and stimulate the rods and cones of your retina. This stimulation would,
in effect, form an image that is perceived by your brain.
In computer graphics we replace the rods and cones of a retina with the pixels
of the computer screen. One goal of a global illumination algorithm is to
re-create, as accurately as possible, what you would see if you were standing
in a real environment. A second goal is to accomplish this task as quickly as
5980 | Chapter 18 Rendering