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Arch & Design Material (mental ray) – Overview 1565
mental ray material) to be able to work properly,
it is imperative that the energy of these photons to
matchthedirectlightcastbythatsamelight.And
since photons model light in a physical manner,
deca y is built in.
Hence, when using GI:
• Light sources must emit photons at the correct
energy.
• Thedirectlightmustdecayinaphysically
correct wa y to match the decay of the photons.
Therefore it is important to make sure the light
shader and the photon emission shader of the ligh ts
work well together.
In 3ds Max this is most easily solved by using the
photometric lights (page 2–1301).Alloftheselights
are guaranteed to have their photon energy in sync
w ith their direct light. It is built in and automatic
and one does not need to worry about it.
Featu re s
The Shading M odel
From a usage perspective, the shading mo del
consists of three components:
• Diff use - diffuse channel /including Oren
Nayar “roughness”).
• Reflections - glossy anisotropic reflections
(and hig h li g hts).
• Refraction - glossy anisotropic transparency
(and tr anslucency).
The Arch & Design material shading model
Direct and indirect light f rom the scene cause
diffuse reflections as well as translucency effects.
Direct light sources also create specular highlights.
Ray tracing is used to create reflective and
refractive effects, and advanced importance-driven
multi-sampling is used to create glossy reflect ions
and refraction.
The rendering speed of the g lossy
reflect ions/refraction can further b e enhanced by
int erpolation as well as “emulated” reflections with
the help of Final Gathering.
Conservation of Energy
Oneofthemostimportantfeaturesofthematerial
is that it is automatically energy conserving. This
meansthatitmakessurethatdiffuse+reflection
+ refract ion <= 1. In other words, no energy is
magically creat ed and the incoming light energy is
properly distributed to the diff use, reflection and
refraction components in a way that maintains the
first law of thermodynamics.
Inpractice,thismeans,forexample,thatwhen
adding reflectivit y, the energy must be taken
from somewhere, and hence the d iffuse level and
thetransparencywillbeautomaticallyreduced
accordingly. Similarly, adding transparency
happens at the cost of the diffuse level.
Therulesareasfollows: