User manual
© Next Limit Technologies 2010
Maxwell Render 2.5 User Manual
Chapter 10. Maxwell Materials | 66
Abbe
F.01 Without Dispersion F.02 With Dispersion active
Different wavelengths of light can be refracted at slightly different angles as they pass
through a material, and this is what causes dispersion, the effect seen when a beam of
light passes through a prism and is split up into the different wavelengths of light. The
name Abbe comes from the German physicist Ernst Abbe, who dened the Abbe number.
Abbe controls the amount of dispersion. The higher the Abbe number, the less dispersion
will be visible in the render. An Abbe number higher than 60-70 will render as if dispersion
was not activated.
Dispersion must be enabled in the Material Properties panel; otherwise the Abbe parameter
in the BSDF will be grayed-out. Because dispersion generally takes longer to render, it is
disabled by default.
R2
The “R2” name derives from the rst time it was suggested, for the Mike Verta’s R2D2
project. Generally, the falloff between the 0° color and the 90° is controlled by the Nd
and roughness parameters. A high Nd number means more of the 90° color will be visible
when using a low roughness value, but as you increase the roughness, the 90° color will
start to disappear until at roughness 99 (or lambert), the 90° color will not be visible at all,
no matter the Nd setting of the material.
This is normal behavior, but there may be special cases where you would like to control
the inuence of roughness and still keep the 90° color visible at high roughness. Examples
would be car paints that quickly change their color to another color based on viewing
angle, or velvet which has a high roughness but a bright “sheen” of a different color, which
would not be possible to create without the .r2 parameters.
The rst parameter can be set between 0-90 and controls the falloff angle between the 0°
and 90° colors. The second parameter can be set between 0-100 and controls how much
inuence the roughness should have. For instance, if you write 45 0, the 90° color will
start appearing when the viewing angle increases more than 45°, and roughness will have
no inuence on the effect. If you set the second parameter to 100, you are specifying that
roughness should have maximum inuence on the 0° and 90° colors; it would be as if you
had disabled the .r2 functionality.
• Other R2 examples
If we set the r2 values to 15 0, the 90° color will quickly become visible, right after
the viewing angle is 15° or more. Roughness will not have any inuence. Setting the
r2 values to 70 50 means the 90° color will become visible only at a large viewing
angle, making it appear just at the edges of the object. The increased roughness
would make this effect less visible, but it would still show up. Using 85 99 means
the 90° color will be visible only at the very edges of the object, but with the second
parameter set to 99, which controls the inuence of the roughness, the effect would
be virtually invisible.