User manual

© Next Limit Technologies 2010
Maxwell Render 2.5 User Manual
Chapter 10. Maxwell Materials | 63
material. Choose a Transmittance color by clicking on the color picker, or specify a texture
by clicking on the texture button. The Transmittance color represents the color of the light
when it has reached the Attenuation distance (see below).
u Note: Transmittance must be a color other than black for transparency to appear. Setting
a brighter transmittance color results in clearer transparency, but you should remember that
this parameter is also tied to the Attenuation Distance.
Attenuation
Glass, water, or even air are transparent when thin, but become opaque when a specic
thickness is reached (different for each material). As light travels through a material, it
loses energy. The Attenuation distance parameter allows you to specify how far light can
move through an object before losing half its energy. For example, if you have a 2cm thick
glass window and you set the attenuation distance to 2cm, the light shining through the
glass on the other side will be half as bright.
Attenuation distance and Transmittance work together. You must set the Transmittance
color higher than RGB 0 for the Attenuation distance to become active. If your Attenuation
distance is very small (for example 1nm) the object will remain opaque because the light
can only travel a very small distance into the object, and will not come out on the other
side. On the other hand, if you have a 1cm thick glass window and you set the attenuation
distance very high (for example 900 meters), and you set the Transmittance color to blue,
the glass will not be colored blue at all. It will be completely transparent because there
was not enough distance in the 1cm thick glass to cause any attenuation and reveal the
Transmittance color.
Attenuation is ruled by an exponential curve, therefore: the thicker the object, the more
attenuated the light will be.
To better understand the concept of Attenuation, consider sea water. When the water layer
is very thin (like water in the palm of your hand) you do not see attenuation: the water
looks transparent. When you have more water thickness, you see a typical sea color (grey,
dark-blue or light blue-green, depending on deep or shallow waters). The Transmittance
color represents the color that you want to get approximately at the Attenuation distance.
Beyond this distance, the light is more and more attenuated, it loses more and more
energy, and the view eventually turns opaque if the volume is thick enough.
Nd
Simply put, Nd should be regarded as the IOR (index of refraction) of the material. The
name Nd is used because it is the common way to denote an index of refraction that has
been measured at the wavelength of 583 nm.
Before explaining how Nd inuences the look of a material, it is important to understand
the “Fresnel effect”. This effect states that the strength of reections on a surface is
dependent on the viewing angle. For example, if you look straight on at your monitor
screen, you will see very weak reections, but if you look at the screen at an angle, the
reections will appear stronger.
F.01 Nd=1.6 F.02 Nd=30
The relationship between Nd and the Fresnel effect is that with a higher Nd number the
surface will start to look equally reective, no matter the viewing angle. So in effect, a
higher Nd “weakens” the Fresnel effect.
So changes in the Nd number affect the overall reectivity of an object. For example, you
can have the Reectance 90° color set to full white, but with an Nd of 1 the object will
have very weak reections (F.03).
Setting the Nd to a high value like 40 and keeping the Reectance 90° to that same full
white color will result in a mirror-like surface (F.04).