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Arch & Design Material (mental ray) – Overview 1563
The major features are:
• Eas y to u se, yet f l exible -controlsarearranged
logicallyinamost-used-firstfashion.
• Te m p l a t e s - allow fast access to settings
combinations for common materials.
• Physical ly accurate -thematerialisenergy
conserving, making it impossible to create
shaders that break the laws of physics.
• Glossy performance - advanced per formance
boosts including interpolation, emulated
glossiness, and importance sampling.
• Twea kable B R DF (bidirectional reflectance
distribution function) - the user can define how
reflectivity depends on angle.
• Transparency -“Solid”or“thin”materials:
transparent objects such as glass can be treated
as either solid (refracting, built out of multiple
faces) or thin (nonrefrac ting, can use sing le
faces).
• Round corners - simulate fillets to allow sharp
edges to still catch the light in a realistic fashion.
• Indirect I lluminat ion control -setthefinal
gather accuracy or indirect illumination level
on a per-material basis.
• Oren-Naya r diff u s e -allows“powdery”
surfaces such as clay.
• Built-in Ambient Occlusion -forcontact
shadows and enhancing small details.
• All-in-one shader - photon and shadow shader
built in.
• Waxed floors, frosted g lass and brushed
metals - all fast and easy to set up.
Physics and the display
The Arch & Design material attempts to be
ph ysically accurate, hence its output has a high
dynamic range. How visually pleasing the material
looks depends on how colors inside the renderer
are mapped to colors displayed on the screen.
When working with the Arch & D esign material it
is highly recommended that you operate through
a tone mapper/exposure cont rol such as the
Logarithmic Exposure Control (page 3–297),orat
the very least use gamma correction.
A note on gamma
Describing all the details of gamma correction is
beyond the scope of this topic; this is just a br ief
overview .
Thecolorspaceofanormal,off-the-shelf
computer screen is not linear. The color with RGB
value 200 200 200 is not tw ice as bright as a color
w ith RGB v alue 100 100 100, as one might expect.
This is not a bug because, due to the fact that
our eyes see light in a non-linear way, the former
color is actually perceived to be about twice as
bright as the latter. This makes the color space of
a normal computer screen roughly perceptually
uniform. This is a good thing, and is actually the
main reason 24-bit color (with only 8 bits or 256
discrete levels for each of the red, green and blue
components) looks as good as it does to our eyes.
The problem is that physically correct computer
graphics operates in a true linear color space
where a value represents actual light energy. If
onesimplymapstherangeofcolorsoutputtothe
renderer naively to the 0-255 range of each RGB
color component it is incorrect.
The solution is to introduce a mapping of some
sort. One of these methods is called gamma
correction.
Most computer screens have a gamma of about
2.2 (know n as the sRGB color space), but most
software defaults to a gamma of 1.0, which makes
everything look too dark (especially midtones),
and light does not “add up” correctly.
Usingagammaof2.2isthetheoreticallycorrect
value, making the physically linear light inside