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Arch & Design Mater ial (mental ray) – Tips & Tricks 1575
and the diffuse sh ading, both driven by the same
color. For example, an aluminum material would
need a bit of diffuse blended in, whereas chrome
would not.
Gold, silver, and copper
Brushed Metal
Brushed metal is an interesting special case. In
some cases, creating a brushed meta l requires only
turning dow n the Reflection Glossiness to a level
where you obtain a very blurred reflection. This is
sufficient when the br ushing direction is random
orthebrushesaretoosmalltobevisibleevenas
an ag gregate effect.
For materials that have a clear brushing direction
and/or where the actual brush strokes are visible,
creating a convincing look is slightly more
involved.
The t iny grooves in a brushed metal surface al l
work together to cause anisotropic reflections.
Thiscanbeillustratedbythefollowingschematic,
which simulates the brush grooves by modeling
many tiny adjacent cylinders, shaded with a simple
Phong shader:
Many small adjacent cylinders
As you can see, the specular highlights in the
cylinders work together to c reate an ag gregate
effect which is the anisotropic highlight.
Also note that the hig hlight isn’t cont inuous; it is
actually broken up into small, adjacent segments.
So the primary visual cues that a material is
brushed metal are:
• Anisotropic highlights that stretch out in
a direction perpendicular to the brushing
direction
• A discont inuous highlight with breaks in the
brushing direction
Many attempts to simulate brushed metals hav e
looked only at the first effect: the anisotropy.
Another common mistake is to think that the
highlight stretches in the brushing direction.
Neither is true.
Hence, to portray brushed metals, it is necessary
to simulate these two visual cues. The first is
simple: Use Anisotropy and Anisotropy Rotation
to make anisotropic highlig hts. The second can
be done in several ways:
•withabumpmap
• with a map that varies the Anisotropy or
Reflection Glossiness values
• with a map that varies the Reflection Color
Each has advantages and disadvantages, but the
onewesuggesthereisthelastone.Thereasonfor