Materials - Blender Master Class: A Hands-on Guide to Modeling, Sculpting, Materials, and Rendering - page 194

Adding a Normal Map
Though I had finished adding textures to the mate-
rial, you may remember that in Chapter 11, I also
baked a normal map for the Bat Creature as a poten-
tial alternative to using displacement. At this stage, I
could add the normal map in another image texture
slot and set it to affect the Normals of the material
in the Influence options. I also had to tell Blender
that this is a tangent space normal map (not a bump
map or an object space normal map), so in the Image
Sampling panel of the Textures tab, I enabled the
Normal Map option and made sure the Normal
Map Space option just below was set to Tangent.
The resulting material will use the textures to dis-
tort its normals when rendered, making it look more
detailed even with subdivision and displacement
mapping turned off.
As I was using displacement mapping, I didn't
need to use my normal map at all, but it's a fair
alternative if you want to keep render times low or
if your computer is struggling with the poly count
of the scene.

To replicate this effect in Blender Internal, we
can enable subsurface scattering for a material. This
effect approximates the scattering of light within an
object by precomputing the lighting over its surface
and then blurring the shadows. It also causes light
to scatter through thin parts of the model. This
produces a nice effect when combined with light-
ing the subject from the back (as I will light the Bat
Creature in Chapter 13).
Enabling subsurface scattering in the Materials
tab of the Properties editor gives you a lot of options
to play with. The settings for subsurface scattering
are as follows:

IOR This is the index of refraction of the material.
High IORs result in slightly softer boundaries
between light and dark.

Scale This is the most important setting, because
it determines the scale of the scattering effect as
a ratio between Blender's units in the 3D View-
port and real-world scales. If you've built your
scene to a scale of 1 Blender unit = 1m, then
a value of 0.001 is “correct,” though you can
set this to whatever value achieves the desired
results (see Figure 12-16).

Subsurface Scattering
Subsurface scattering (SSS) is the effect of light
bouncing around below the surface of a material,
and it's common in materials like skin, wax, and
marble. It results in the edges of shadows look-
ing somewhat blurred, as light diffuses out from
fully lit areas to areas in shadow. As the light scat-
ters, some of it is absorbed, with the remaining
light usually taking on the color of the underlying
substance. In the case of skin, the flesh and blood
below the skin cause the scattered light to appear
an orange-red color.

Scattering Color This determines the color of light
scattering. Its effect depends largely upon how
you set the color blend (as discussed below).
When the color blend is set high, it colors all of
the scattered light with the color you choose,
generally turning your whole mesh that color.
When the color blend is set to 0, it has the oppo-
site effect, causing the scattered light to take on
the inverse color—that is, the color on the oppo-
site side of the circle in the color picker (see
Figure 12-17).

Figure 12-16: The effect of different scale values on subsurface scattering. Left to right: Scale values of 0.01, 0.1, and 0.5.