Materials - Blender Master Class: A Hands-on Guide to Modeling, Sculpting, Materials, and Rendering - page 185

define transparent—or, in this case, nonexistent—
areas of the surface. These areas can then be ren-
dered completely transparent (invisible), which will
make the surface appear more complex than its
geometry alone would allow (see Figure 12-4).

Transmission and Refraction
In addition to reflecting light, surfaces like water or
glass also transmit light (allow light to pass through
them). In the real world, a part of the incident light
(light hitting the surface) is usually reflected, and
the rest is transmitted and bent slightly ( refracted ).
The light is bent because as it enters a different
medium, its speed changes. This bending of light
results in a distorted image when you look through
a transparent material. The amount of distortion
depends on the object's index of refraction (IOR) ,
which determines how much the direction of a
ray of light is altered at the boundary of the object
(see Figure 12-8 on page 189), and the shape and
thickness of the object. For some objects, this effect
is significant—for example, when you look through
a lens or a glass of water, you'll notice considerable
distortion. Other times, the effect is minimal—for
example, there is little distortion when you look
through the thin glass of a window.

Z Transparency
We can use Blender to simulate transmission and
refraction of light using ray-traced transparency.
But we can also cheat by ignoring refraction and
simply rendering whatever is behind an object. This
is called Z Transparency . This technique works well
with objects like windows, where the refraction is
minimal. Figure 12-3 compares ray tracing with
Z Transparency.
Z Transparency is also useful for rendering thin
surfaces with holes or complex outlines, where the
mesh can be left simple and a texture can be used to

Figure 12-4: Using Z Transparency to give this wire
fence material a more complex appearance without
requiring extra geometry (using the Blender Internal
renderer). This material has been applied to a single
plane. (The Transparent Shadows option has been
turned on for the floor material to create accurate
shadows.)

Light can interact with a surface in still more
complex ways, such as scattering beneath the sur-
face or exhibiting translucency or anisotropic reflec-
tion, which is the type of reflection produced by
brushed metal. We'll examine these other types of
interaction as we move through this chapter.

Blender Internal Materials
Blender Internal is Blender's default render
engine, and you can create materials for Blender
Internal using the Materials and Textures tabs of
the Properties editor. You can find the properties
of materials in the Material tab (and the textures
and aspects of a material they can be used to affect)
in the Textures tab.
Blender Internal's materials are pretty modu-
lar, allowing you to add and combine different
material properties—such as specular reflections,
mirror reflectivity, transparency, and subsurface
scattering—simply by turning on the relevant
options from the different panels in the Material

Figure 12-3: Ray tracing vs. Z Transparency (using the Blender
Internal renderer). Left: Z Transparency with an alpha of 0.25
and specular highlights. The specular and slight diffuse visibil-
ity make this material visible. Right: Ray-traced transparency
with an IOR of 1.5. This image is more realistic but will take
much longer to render.