cs781 - Colour for Computer Graphics - Winter 2012
Course Notes
Lecture 13 - Light in Motion
And Now from our Sponsor
- Projects
- Next Wednesday's class
Surface Reflectances
According to Jim Kajiya, writing in the mid 1980s,, the Bidirectional
Reflectance Distribution Function (BRDF) should tell the whole story. What is
it, and where does it come from?
- R( incoming angle (2 dof), outgoing angle (2 dof), wavelength (1 dof) )
- incoming and outgoing angles: 0 < \theta < \pi/2, 0 < \phi
< 2\pi
- wavelength 400 nm < \lambda < 700nm
- we won't even talk about sampling density (3.14 million values at
10 degree, 10 nm sampling)
- potentially also a function of distance along the surface (2 dof) and
difference in surface normal (1 dof )
- if the surface varies, then also parameters that control variation
Maybe it could come from measurement
- even if you could measure it how would you encode the result
Maybe you could simulate the system
- just how complex is the system
Maybe you could find a simple model
- capturing only the important aspects of the system
Smooth Surfaces
Surface reflectance
- For smooth surfaces the surface reflectance is mirror-like
Body reflectance
- just like selective absorption
- doesn't come out at the same place: Why doesn't it matter?
- edges of a surface
Lambertian reflectance
Lambert 1728-1777
Consider a small circular hole in the surface and ask how much light comes
through it in different directions
- Assume light direction is completely randomized
- The directional factor is the cosine zenith angle
The luminance of light in any direction is constant.
When the volume immediately below the surface has spatial structure, the
direction of light re-emitted through the surface is not necessarily
isotropic.
The Moon
- What would the full moon look like if its surface were Lambertian?
- What does the full moon actually look like?
Rough Surfaces
Surface Reflectance
Surface Gloss
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