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Maxwell Render 2.5 User Manual

Chapter 10. Maxwell Materials | 64

The following images demonstrate this:

F.03 Nd=1 F.04 Nd=40

From the images above you can see that raising the Nd increases the effect of the

Reectance 90° color, while the Reectance 0° color (in this case set to nearly black) has

less and less effect when increasing the Nd.

Of course the value of the Reectance 90º color will also affect the maximum strength

of reectance. For example, you can set a high Nd but set the Reectance 90° to a dark

color. The result will be a surface which is reecting equally, no matter the viewing angle,

but the reections will appear weaker.

For opaque materials, an Nd of at least 3 is recommended if you want them to be reective.

Metals have an even higher Nd. The Nd setting can go as high as 1000. For a mirror-like

surface it is usually enough to set the Nd to 100.

The Nd has no inuence on the look of the material when working with Lambertian

materials. Lambertian is a special mode which makes a surface reect all light in a diffuse

way. A Lambertian material has a roughness 100. It is the highest roughness a surface can

have. It is also important to remember that the Nd has less inuence on the reectance

of an object the higher you set the Roughness (See the Surface Properties chapter for

more details on the Roughness parameter). The smoother a surface is (low roughness),

the bigger the effect of Nd on the surface will be. You will see the biggest changes to the

reectiveness of the object as you change the Nd, when the surface roughness is low.

With transparent materials the Nd also controls the amount of refraction. Seawater for

example has an Nd of about 1.333. You can nd common Nd values for transparent

materials on the internet.

• Force Fresnel: The amount of light that is reected back from the object is

determined by the re 0° and 90° colors. The Nd controls the Fresnel curve, or the

inuence of these two colors, depending on viewing angle. In reality however, the Nd

itself species how much light is reected back from the surface, with a higher Nd

specifying a more reective material. These three settings (re0°, re90°, Nd) can

come into conict when you put in, for example, a black re 0° color but a very high

Nd. Force Fresnel ensures that the reectance ramp (curve) between Ref0 and Ref90

will be maintained by the given Nd. It discards the luminance of a given reectance

color and enforces physically correct Fresnel while only deriving the hues of the re.

0° and re. 90° color chips. When Force Fresnel is unchecked the reectance value

of a surface is derived both from the Nd and the luminance of the re0° color. When

Force Fresnel is checked, the reectance between re0 and 90° (the Fresnel curve)

is maintained according to the Nd, and only the hue of a given reectance color is

taken in consideration.

This option should be turned off when you texture the re. 0° or re.90° channels,

otherwise your textures may not appear as you expect when using a lower roughness.

As you can see in the images below, because the luminance information of a given

color is discarded, a bright red in your texture may appear darker than expected

if the Nd is low and the Force Fresnel option is checked. Here, Maxwell Render is

determining the nal reectance of the object, just looking at the Nd.

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