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1572 Chapter 16: Material Editor, Materials, and Maps
d=Max Distance where attenuation is Color at Max Dis ta n ce
Therayentersthemediumandisattenuated
throughout its travel. The strength of the
attenuation is such that precisely the Max Distance
(d in the figure) the attenuation matches that of
Color At Max Distance. In other words, at this
depth the attenuation is the same as was received
immediately at the surface with the previous scene.
The falloff is exponential, so at double the Max
Distance value t he effect is that of Color At Max
Distance squared, and so on.
There is one minor t radeoff:
To render the shadows of a material correctly using
this method, you must either u s e caustics or make
sure mental ray is rendering shadows in Segments
mode (see Shadows and Displacement Rollout
(mental ray Renderer) (page 3–114)).
Using caustics naturally gives the most
correct-looking shadows (the above image was
rendered without caustics), but requires that the
scene has caustic photons enabled and contains a
physicallightsourcethatshootscausticphotons.
On the other hand, the mental ray Segments
shadows have a slightly lower performance than
the more widely used Simple shadow mode. But
if it is not used, the shadow intensity will not take
the attenuation through the media into account
properly. However, the image might still look
pleasing.
Water and Liquids
Water, like glass, is a dielectric with an IOR of 1.33.
Hence, the same principles as for glass (above)
apply to bodies of water, which truly need to
refract their environment. An example is water
running from a tap. Colored liquids use the same
principles as colored glass.
Water into wine
To create a liquid in a container, as in the preceding
image, it is important to understand how the Arch
& D esign material handles refraction through
multiple surfaces vs. the real-world behavior of
light in such circumstances.
What is important for refraction is the transition
from one medium to another w ith a different IOR.
Such a transition is known as an interface.
For lemonade in a glass, imag ine a ray of l ight
travelling through the air (IOR=1.0). When it
ent ers the glass, it is refracted by the IOR of the
glass(1.5).Theraythenleavestheglassandenters
the liquid; that is, it passes through an interface
from a medium of IOR 1.5 to another medium of
IOR 1.33.
One way to model this in computer graphics is to
maketheglassoneseparateclosedsurface,with
the normals pointing outward from the surface of