Paper Summaries • Any takers? Light and Color Plan for today Computer Graphics as Virtual Photography • Light and Color real camera photo Photographic Photography: scene (captures processing print • Project Possibilities light) – Andy Phelps (IT) processing – Paul Craig (Chemistry) camera Computer 3D tone synthetic model Graphics: models image reproduction (focuses simulated lighting) Photography and Light Light pho•tog•ra•phy, n ., the process or art of • Why important? (photorealistic images!) producing images of objects by the action of • What it is light on a sensitized surface, e.g., a film in a • How it is measured camera. – Radiometric – Photometric Photography = writing with light • How it behaves • How it is specified • Light and color 1
Light -- How it is measured Light -- What it is Radiometric Units • Electromagnetic radiation • Light is radiant energy • Measure in Joules ( Q ) induction radio ultra gamma secondary power infrared x-rays heating waves violet rays cosmic rays • One joule is the equivalent of one watt of 10 10 10 8 10 16 10 14 10 12 10 6 10 4 10 2 1 10 -2 10 -4 10 -6 10 -8 power radiated or dissipated for one second. Wavelength (nm) visible light Red 700 nm • CG uses particle model of light orange 650 nm yellow 600 nm green 550 nm – Light travels in localized particles or wave blue 450 nm violet 400 nm packets . Light – Radiant Flux Density Light – Radiant Flux • Radiant Flux Density (Irradiance/Radiant • Radiant Flux ( Φ -Radiant Power/Watt) Exitance) – Amount of energy / unit time – Amount of flux per unit area arriving at or leaving from a point on the surface – Joules ( Q ) per second – Measured in Watts / m 2 – (Remember a Watt is Joules/sec.) dQ Φ = dt Light -- Irradiance Light – Radiant Exitance • Irradiance ( E )– radiant flux density coming in • Radiant exitance ( M ) - radiant flux leaving the surface Φ Φ d = d = E M dA dA dA dA 2
Light -- Radiance Light -- How it is measured • Radiance ( L ) • Steradian (sr) - Standard International unit of solid angular measure. There are 4 pi – Flux arriving at or leaving from a given point or surface in a given direction . steradians in a complete sphere – (See http://whatis.techtarget.com/definition/0,289893,sid9 – Measured in Watts / m 2 / steradian d 2 Φ _gci528813,00.html) L = d ω is the dA(d ω cos θ ) measurement of the cone size steradian Light – Radiant Intensity Process of Creating Images light • Radiant Intensity ( I ) – point source Direct light – Amount of radiant flux in a given direction – Watts / steradian – Point light sources Reflected light Φ d Viewing = scene window I ω d Images are a view of the world from a given perspective Light -- How it is measured Light – Measurement Summary Photometric Units • Radiant Flux - energy / time - ( Joules/sec ) • Photometry measures visible light according to the • Radiant Flux Density - total flux entering sensitivity of human eye: ( irradiance ) or leaving ( radiant excitance ) a – Cones: blue – short, green – medium, red – long point or surface - ( Watts/m 2 ) – Rods: low illumination • Radiance - total flux entering or leaving a point or – Eye sensitivity varies with wavelength, e.g.., green light surface in a given direction - ( Watts/m 2 / steradian ) appears brighter than red/blue of same intensity! • Radiant intensity - flux in a given direction for • So, photometric units are radiometric units scaled point light sources - ( Watts/steradian ) by the luminosity function • All measures can vary with wavelength!!! • Same concepts -> different units 3
Light – CIE Luminous Efficiency Curve Light – Photometric Units 120 • Luminous Flux - energy / time - ( lumen ) % Efficiency 100 • Luminous Flux Density - total flux entering or leaving a 80 60 point or surface - ( lux = lumen/m 2 ) 40 • Luminance - total flux entering or leaving a point or 20 surface in a given direction - ( nit = lumen/m 2 /steradian ) 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 7 0 2 5 7 0 2 5 7 0 2 5 7 0 2 5 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 • Luminance intensity - flux in a given direction for point Wavelength light sources - ( candela = lumen / steridian ) • Created using perception matching brightness of • All scaled by CIE Luminous Efficiency Curve monochromatic light at different wavelengths • Provides weighting curve/function used to convert from radiometric to photometric measurements Light -- How it is measured Light -- How it behaves • Example • Reflection – The luminance at a surface due to a blue light • Absorption of a given intensity would be less than the • Refraction luminance at the same surface due to a yellow • Scattering light of the same intensity. – Why? Humans perceive yellow light to be • Diffraction / Interference brighter than blue light • All can be wavelength dependent!!! Light - Absorption Light - Reflection • What is not reflected, can be absorbed • Angle of incidence = Angle of reflectance • Perfect mirror surface θ θ r i 4
Light - Refraction Light - Scattering • Bending of light as it travels through different media r α η θ = η θ sin sin θ d n i i t t i η i η t θ • Light is scattered by small particles in its path (e.g. haze, t t smoke, etc.) η η • Given by fraction of light with respect to direction from Where and are the indices of refraction. i t particle light impact. ( http://www.physics.nwu.edu/ugrad/vpl/optics/snell.html ) • Size of particles are on the order of wavelengths of light. Light – Raleigh Scattering Light -- Scattering • Raleigh scattering (smoke / dust ), the probability that the light will scatter in r α direction α . 3 α = + 2 α P ( ) ( 1 cos ) • r << λ total absorption (no scattering) 4 • r < λ Rayleigh Scattering • r ≈ λ Mie scattering • r >> λ Geometric optics Light – Mie Scattering Light -- How it behaves • Mie Scattering (haze / fog) • Diffraction – Bending of light around objects ⎛ + α 8 ⎞ 1 cos α = + ⎜ ⎟ Sparse / hazy P ( ) 1 9 – Contributes to soft shadows, color bleeding ⎝ ⎠ 2 • Interference ⎛ + α ⎞ 32 – Superimposition of two waves 1 cos Dense / murky α = + ⎜ ⎟ P ( ) 1 50 ⎝ ⎠ – Accounts for colors in thin films, bubbles, oil 2 slicks, peacock feathers 5
Light -- How it behaves Light – How we specify • When placing lights in a virtual scene, you • And of course… must specify the following: – All can be wavelength dependent!!! – Position / Direction – Intensity – Distribution – And of course…color Light - Intensity Light – Distribution • Describes the quantity of light emitted. • The strength of light emitted in different • Most APIs use 0 – 1…This can be a problem, directions we’ll see this later with tone reproduction – Simple Point Light Source • Assuming physical units: – Directional Light – Radiometric – Spotlight • Point light source - radiant intensity • Area light source - radiant exitance / radiance • Beam Shape – Photometric • Beam Falloff • Point light source - luminous intensity • Intensity Falloff • Area light source - luminance exitance / luminance Light - Directional Light Light - Simple Point Light Source • Light distributed equally in all directions • Light distributed equally from a given direction • Point light source at infinity is an estimation for sunlight 6
Light - Spotlight Light - Stage Lighting • Basic CG Spotlight • Basic Shape -- cone A [Advanced Renderman] Light - Stage Lighting Light - Stage Lighting • Shape – Gobos (use go between to shape beam) • Shape – Barn doors (beam shape) Light - Stage Lighting Light - Stage Lighting • Beam Falloff • Beam Falloff (at edge) 7
Light -- Stage Lighting Light - Distribution • Intensity Falloff • Area Light Sources – sampled point sources within area Light - Distribution Light and Color • “Indeed rays, properly expressed, are • The Five-Times Rule not colored” – Light can be considered a point light source if the distance from the lumiere to the surface is at -- Sir Isaac Newton least 5 times the largest dimension of the lumiere, i.e.., the largest dimension of the light. • I.e., light rays are not colored; we – Break perceive them as colored! Light - Color Light – Spectral Density Functions (SDF) • Color is the perceptual response to • AKA spectral power distributions light of wavelengths 400 - 700 nm • Describes the distribution of the strengths of hitting the retina. light at given wavelengths emitted from a • Spectral power distributions exist in source. the physical world but color exists only in the eye and brain, e.g., there is no real white light! 8
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