User manual

© Next Limit Technologies 2010
Maxwell Render 2.5 User Manual
Chapter 10. Maxwell Materials | 67
F.01 r2 values 15 0 F.02 r2 values 55 0
IOR Custom
This option allows you to set the value of the index of refraction (Nd) manually, as
explained above.
IOR Measured Data
As an alternative to the Reectance, Nd and Abbe values, you can use an .ior le which
provides Maxwell Render with the exact index or refraction for each wavelength of a
material. Complex IOR (also called Full IOR) data is collected from precise measurements
in laboratories and describe the optical properties of a material to the highest degree of
accuracy. These materials have the advantage of being extremely realistic, with some
drawbacks:
The use of complex IOR data means Maxwell Render has a lot more math to do. Complex
IOR computations require evaluation of more mathematical functions and they are
dependent on viewing angle and on spectral wavelength. In addition, they involve the
computation of full dispersion (which in itself introduces a performance hit; taking longer
to resolve image noise). This method does not allow for many optimizations without
sacricing the intended physical accuracy.
Surface properties such as roughness, anisotropy and bump can still be changed when
using IOR les. Naturally, a BSDF with an IOR le loaded can still be mixed with other
“regular” BSDFs to create material variations.
• When to use regular BSDF or IOR les
Suppose you work in jewelry and would like to render gold (and only gold in its pure
form) and you do not mind waiting longer for a high resolution image, as long as the
result is physically accurate in the most precise way possible; capturing the subtle
effects of light and the unexpected shifts in color as it would if a real gold object was
present. In this situation the use of a complex IOR material is recommended.
On the other hand, suppose you are working on a two-minute animation of a gold
robot for a TV production. In this case you need speed and exibility. For instance, you
might want the gold to look a bit redder and you might want it to reect some blue
light in some areas. In this case the extreme accuracy of an .IOR le is not needed.
Instead, you can opt for a regular BSDF material and adjust the parameters until you
get something that resembles gold in many respects. Your custom-made gold follows
the physical laws of light for accuracy, while still being entirely customizable and
production-friendly.
As a general rule, .ior les that describe metals will not render much slower, but .ior
les describing transparent materials involve the calculation of dispersion, increasing
the render time. When using .ior les, dispersion cannot be turned off; it is “built-in”
in the .ior le data.
10.04.02 Surface Properties
Roughness
Roughness allows you to add tiny imperfections and miniscule details on a surface to
make it reect light in a more diffuse way. The roughness of a surface can range from 0
(perfectly smooth surface) to 100 (pure diffuse). A roughness value of 100 corresponds
to a “lambertian” or perfectly diffuse model. It is important to remember that if you want
to have very smooth, mirror-like surfaces or clear glass, the roughness should be set to a
low value (i.e. 0 to 10).
You can also set a black and white texture to control the roughness. Brighter values in
the texture create a higher roughness (a more diffuse surface). When using a texture, the
roughness number just applies to the maximum roughness needed. For example, if you