Graphics Reference
In-Depth Information
(1) The interaction of light with
boundaries
between materials, and
(2) the scattering and absorption of light as it passes
through
material.
Therefore, the two basic ingredients to all models are properties of surfaces and prop-
erties of light. An important property of a surface is its
reflectance
. Different
reflectances for different wavelengths cause color. Another property is
transparency
.
Also, in order to model color accurately, one needs to maintain the wavelength or
spectrally dependent information throughout the visible range. We shall indicate the
dependence of a variable on wavelength by giving it a wavelength parameter l. In
practice we are only interested in the three wavelengths that correspond to the red,
green, and blue colors of an RGB monitor, so that each of the intensity equations we
shall see in this chapter really represent three equations, one for each of those three
colors.
To summarize, several shading models are used in graphics ranging from the
simple to the complex. The Phong reflectance model ([BuiT75]), which has been
around since the mid-1970s, still seems to be the most popular of the local reflectance
models, even though there have been many improvements (see [Glas95]). Phong
shading together with texture maps and shadows does a pretty good job for local
shading models. Furthermore, one can mix in ray tracing for good result. For really
eye-catching images, full-blown ray-tracing methods, radiosity methods, or a combi-
nation of the two are the current candidates. Of course, the latter also take corre-
spondingly more computer power.
Finally, although photorealism was the holy grail of computer graphics for many
years, now that it has been pretty much achieved, it is no longer the driving force it
once was and one is more concerned with finding innovative uses for this new
medium.
Section 9.2 starts this chapter's topics with a discussion of local reflectance
models. We then move on to shading models in Section 9.3, global reflectance models
in Section 9.4, and tie things all together with the rendering equation in Section 9.5.
Sections 9.6-9.8 describe ways of enhancing images without increasing geometric
complexity. An overview of the whole rendering process is presented in Section 9.9.
The chapter ends with a discussion of ways to deal with rendering colors when there
are limitations on the size of the color palette.
9.2
Local Reflectance Models
Geometrical optics
(or
ray theory
) treats reflected light as having three components, an
ambient one and a diffuse and specular one.
Ambient
(or background) light is that
light that is uniformly incident from the
environment
and which is reflected equally
in all directions by surfaces.
Diffuse
and
specular
light is reflected light from
specific
sources. Diffuse light is light that is scattered equally in all directions and has two
sources. It comes either from internal scattering at those places where light penetrates
or from multiple surface reflections if the surface is rough. For example, dull or matte
material generates diffuse light. Specular light is the part of light that is concentrated
in the mirror direction. It gives objects their highlights.
