cs781 - Colour for Computer Graphics - Winter 2012
Course Notes
Lecture 16 - Effects of Structure on Colour
And Now from our Sponsor
- Projects
- Last three lectures
- Talk on flicker hallucinations
Dr. Bard Ermentrout (Computational Biology, University of Pittsburgh,)
Flicker Hallucinations: Faraday waves in the brain
3.30 p.m., Tuesday, 6 March
MC 5158.
Surface Structure
Surface is not simple
- surface irregularities, such as pores and hairs
- these may be aligned
- for example, aligned hairs can give the appearance of finely spun
cloth
- surface polishes, such as wax
Organized surface structure effects
- Jagged surface
- captures all light falling on the surface
- but doesn't put iut anywhere in particular
- how to create the blackest black
- returns no light at all
- stack of razor blades
- Parabolic surface facets pp. 147-150
- focusses all incoming light to a single point,
- but only if the incoming light is perpendicular to the
surface
- Put concentrated pigment at the focus
- Why would you do this?
- plants move their pigment around
- focus deep inside the leaf to where the chlorophyll is
- Light that is not absorbed
- leaves in a single direction perpendicular to the leaf
- on a small scale this looks like glowing points
- on a large scale it looks like a glossy highlight without a
source of light
- that is, the highlight doesn't move with the eye position
- off the perpendicular the leaf is simply dull green
- at glancing angles it's yellow
Body Structure
Cell walls made of cellulose
- index of refraction is 1.40,
- compared to water, which is 1.30.
- more complex internal light paths
Leaves that change colour by pigment motion
- green in the dark
- the chlorophyll is spread thinly accross the bottom of a cell
- transparent after short exposure to light
- the chlorophyll moves to the vertical walls of the cell
Leaves with more than one pigment
- chlorophyll and anthocyanin
- correctly balanced these absorb almost all light
Iridescence
Property of thin transmitting structures
Law of Refraction
- n1 sin(th1) = n2 sin(th2)
Constructive interference in an iridescent blue leaf
- 2 n2 d cos(th2) = (2m+1) lmd / 2
- refractive index of plant cells: n2 = 1.4
- thickness of iridescent film: d = 80 nm
- wavelength for constructive interference: 4 * 1.4 * 80 = 440 nm
It looks as though this is a side-effect of acquiring more red light
Constructive interference in a CD
- 2 d cos(th2) = m lmd
- pitch of CD: d = 1600 nm
- you are seeing m ~ 5
- speed 1.2 m/sec
- bits/sample 16
- samples per sec: 44,000
- bits per sec: 700,000
- bits per m: 700,000 / 1.2 = 600,000
- microns per bit 1600 nm
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