Logistics About the readings: Intro to Light Undergrads: Due - - PDF document

1 Intro to Light

Logistics

 About the readings:

 Undergrads: Due Friday by midnight  Grads: Due Mon/Wed by midnight

 If the dropbox is closed…please e-mail.

Logistics

 Reminder

 Project Proposals due Monday  Raytracer Checkpoint 1 due Wednesday.

 Hey, that’s today!

Plan for today

 Intro to light  Ray tracing basics  Checkpoint #2 – Raytracing through a

camera model

Computer Graphics as Virtual Photography

camera (captures light) synthetic image camera model (focuses simulated lighting)

processing

photo processing tone reproduction real scene 3D models Photography: Computer Graphics: Photographic print

Photography and Light

pho•tog•ra•phy, n., the process or art of producing images of objects by the action of light on a sensitized surface, e.g., a film in a camera.

Photography = writing with light

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Light

 Why important? (photorealistic

images!)

 What it is  How it is measured

 Radiometric  Photometric

 How it behaves

Light -- What it is

 Electromagnetic radiation

power induction heating radio waves infrared ultra violet x-rays gamma rays 1016 1014 1010 108 1012 106 102 1 10-2 10-4 10-6 10-8 Wavelength (nm) 104 visible light secondary cosmic rays

Red

• range

yellow green blue violet 700 nm 650 nm 600 nm 550 nm 450 nm 400 nm

Light -- How it is measured Radiometric Units

 Light is radiant energy  Measure in Joules (Q)  One joule is the equivalent of one watt

• f power radiated or dissipated for one

second.

 CG uses particle model of light

 Light travels in localized particles or wave

packets.

Light – Radiant Flux

 Radiant Flux (Φ -Radiant Power/Watt)

 Amount of energy / unit time  Joules (Q) per second

dt dQ = Φ

Light – Radiant Flux Density

 Radiant Flux Density

(Irradiance/Radiant Exitance)

 Amount of flux per unit area arriving at or leaving

from a point on the surface

 Measured in Watts / m2  (Remember a Watt is Joules/sec.)

Light -- Irradiance

 Irradiance ( E )– radiant flux density coming in

dA d E Φ =

dA

3

Light – Radiant Exitance

 Radiant exitance (M)- radiant flux leaving

the surface

dA

dA d M

• =

Light -- Radiance

 Radiance (L)

 Flux arriving at or leaving from a given point

• r surface in a given direction.

 Measured in Watts / m2 / steradian

dA(dω cos θ) d 2Φ L=

steradian

Light -- How it is measured

 Steradian (sr) - Standard International unit of

solid angular measure. There are 4 pi steradians in a complete sphere – (See

http://whatis.techtarget.com/definition/0,289893,sid9_g ci528813,00.html)

dω is the measurement of the cone size

Light – Radiant Intensity

 Radiant Intensity (I) – point source

 Amount of radiant flux in a given direction  Watts / steradian  Point light sources

ω d d I Φ =

Light – Measurement Summary

 Radiant Flux - energy / time - (Joules/sec)  Radiant Flux Density - total flux entering

(irradiance) or leaving (radiant excitance) a point

• r surface - (Watts/m2)

 Radiance - total flux entering or leaving a point or

surface in a given direction - (Watts/m2/ steradian)

 Radiant intensity - flux in a given direction for

point light sources - (Watts/steradian)

 All measures can vary with wavelength!!!

Light -- How it is measured Photometric Units

 Photometry measures visible light according

to the sensitivity of human eye:

 Cones: blue – short, green – medium, red – long  Rods: low illumination  Eye sensitivity varies with wavelength, e.g.., green

light appears brighter than red/blue of same intensity!

 So, photometric units are radiometric units

scaled by the luminosity function

 Same concepts -> different units

4 Light – CIE Luminous Efficiency Curve

 Created using perception matching brightness of

monochromatic light at different wavelengths

 Provides weighting curve/function used to convert from

radiometric to photometric measurements

20 40 60 80 100 120 3 7 5 4 4 2 5 4 5 4 7 5 5 5 2 5 5 5 5 7 5 6 6 2 5 6 5 6 7 5 7 7 2 5 7 5 Wavelength % Efficiency

Light – Photometric Units

 Luminous Flux - energy / time - (lumen)  Luminous Flux Density - total flux entering or

leaving a point or surface - (lux = lumen/m2)

 Luminance - total flux entering or leaving a point or

surface in a given direction - (nit = lumen/m2/steradian)

 Luminance intensity - flux in a given direction for

point light sources - (candela = lumen / steridian)

 All scaled by CIE Luminous Efficiency Curve

Light -- How it is measured

 Example

 The luminance at a surface due to a blue

light of a given intensity would be less than the luminance at the same surface due to a yellow light of the same intensity.

 Why? Humans perceive yellow light to be

brighter than blue light

Light -- How it behaves

 Reflection  Absorption  Refraction  Scattering  Diffraction / Interference  All can be wavelength dependent!!!

Light - Reflection

 Angle of incidence = Angle of reflectance  Perfect mirror surface

i

• r
• Light - Absorption

 What is not reflected, can be absorbed

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Light - Refraction

 Bending of light as it travels through different

media

i

• t

η

i

• t

θ

d t n Where and are the indices of refraction. (http://www.physics.northwestern.edu/ugrad/vpl/optics/snell. html)

i

• t

η t t i i

θ η θ η sin sin =

Light interactions

 …at the wavelength level

 Scattering  Diffraction  Interference

Light - Scattering

 Light is scattered by small particles in its path (e.g.

haze, smoke, etc.)

 Given by fraction of light with respect to direction

from particle light impact.

 Size of particles are on the order of wavelengths of

light.

• r

Light -- Scattering

 r << λ total absorption (no scattering)  r < λ Rayleigh Scattering  r ≈ λ Mie scattering  r >> λ Geometric optics

• r

Light – Raleigh Scattering

 Raleigh scattering (smoke / dust ), the

probability that the light will scatter in direction α.

) cos 1 ( 4 3 ) (

2

• +

= P

Light – Mie Scattering

 Mie Scattering (haze / fog)

8

2 cos 1 9 1 ) (

• +

+ =

• P

Sparse / hazy

32

2 cos 1 50 1 ) (

• +

+ =

• P

Dense / murky

6

Atmospheric Light Scattering

 Rayleigh scattering

 preferentially scatters light at the wavelength of

blue light...

 this is what makes Earth skies appear blue

 Mie scatter

 caused primarily by water vapor, fumes, smoke,

and dust

 results in red/orange appearance of evening skies

Mie Scattering

www.acclaimimages.com/_gallery/

Light -- How it behaves

 Diffraction

 Bending of light around objects  Contributes to soft shadows, color bleeding

 Interference

 Superimposition of two waves  Accounts for colors in thin films, bubbles,

• il slicks, peacock feathers

Light -- How it behaves

 Diffraction  Interference

http://www.physik.tu-muenchen.de/ http://sol.sci.uop.edu/

Light -- How it behaves

 And of course…

 All can be wavelength dependent!!!  Power spectrum of light determines

color.

 Will talk more about color later in the

course.

Light – How it behaves

 Now that we know how light travels,

can we simulate this with the goal of image synthesis…

 Enter…

 Ray Tracing!!!!