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The diffusion model used is Lambertian, meaning that the incoming light is diffused uniformly in every outgoing direction (i.e. in the whole hemisphere around the surface hit point). It is controlled giving one color (constant over the surface or by texel using a texture). A light that illuminates a surface produces a more visible lighting if it's aligned with the surface normal.

Reflection

In our realistic material, reflections have greater weight than two others shading components. First, the user has to choose between two different behaviors (see the figure below):

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Fresnel reflections (on the left) compared to uniform ones (on the right); in the Fresnel case, note how the reflection fades off as the angle between the eye vector and the surface normal decreases
Whatever the model you choose, reflections can be made glossy and / or anisotropic. The glossiness is a measure of the dispersion of the reflection rays around the perfectly specular reflection vector. It can be seen as the half-angle of the cone subtended by the reflection vector where reflection samples are taken. The more the angle increases the more the reflections are blurry (and will require samples to avoid too much noise; see figure below).

The sphere on the left is less glossy than the one on the right.
The anisotropy defines the shape of the reflections. Anisotropic materials reflect light according to the surface orientation (see figure below). It means that a local surface coordinate system is needed to handle oriented reflections.

The sphere on the left is isotropic as the one on the right is anisotropic. Notice how reflections are vertically distorted in the anisotropic case

Transmission

Every realistic material has its own index of refraction (IOR) When transparent, the material IOR is used to deviate light rays when they travel through the volume (see figure below).

The same realistic material with two different IOR values (1.02 on the left and 1.1 on the right)
As for reflections, refractions can be glossy to simulate a wide range of materials:

The same realistic material with (on the right) or without (on the left) refraction glossiness

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This group lets you set the glossiness parameter of reflection. The glossiness parameters control how much the reflections/transmissions are blurry. Depending on the chosen glossiness values, the results may vary a lot:

Different reflection glossiness settings
Glossiness is achieved by increasing the "Anisotropy in U" and "Anisotropy in V" parameters. If both parameters are identical, then, the reflection is isotropic. If both parameters have different values, the reflection is anisotropic, like on a CD-ROM surface or on hairs and furs.

Parameters for a strong anisotropy

BRDF - Orientation

Different reflection anisotropic settings

BRDF - Transmission

Like for the BRDF - Reflection group, this one lets you set the glossiness parameter for the transmission. The IOR (Index Of Refraction) is exposed to control the amount of refraction.

A transparent material with different IOR

Bump and Displacement

For bump or displacement, things are a little bit different as those channels can't be color-based. The combo box is then adjusted to propose only the texture mode. The height of the bump/displacement can be set via a parameter.
Like the previous material channels, Offset, Scale and Rotation parameters are used to transform the texture mapping.

Difference between Bump (on the left) and Displacement (on the right)

Advanced options

When it comes to setup reflection/transmission, Fresnel support is essential. It describes the way real surfaces interacts with the light. The strength of the reflection generally depends on the viewing angle (the angle between the observer and the surface normal). For example, see how your office window panes or car paint reflect more when viewed under grazing angles rather than when front facing.

Difference between Fresnel (on the left) and non-Fresnel (on the right) reflections
This is also the place where you can turn on Caustics generation (both reflective and refractive) for your material.

A glass material with caustics turned on (on the left) or off (on the right)
A Double sided option is also available in this category to set the back-face rendering of the shape.

RedSDK Material Properties

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Common Indices of Reflection