Light CGT 511 CGT 511 Light Normal human eye senses electromagnetic radiation between 380 nm and 750 nm Bedřich Beneš, Ph.D. dři h š h Purdue University Department of Computer Graphics © Bruce MacEvoy © Bedrich Benes Light © Bruce MacEvoy Light Sources • Characterized by its emission spectrum (spectral characteristic, spectral radiance) intensity [W] 400 500 600 700 wavelength [nm] • this is the daylight emission spectrum • it can be measured it can be measured © Bedrich Benes © Bedrich Benes Light Sources Light Sources • important special cases of light sources • achromatic is perceived as white or gray • increasing/decreasing the intensity of the achromatic light makes brighter/darker gray intensity [W] intensity [[W]] 400 500 600 700 wavelength g [nm] • achromatic light • lat. chroma = color © Bedrich Benes 400 500 600 700 wavelength [nm] monochromatic light • monochromatic (includes one chroma) is usually as a pure color is usually as a pure color • the only intensity is called dominant wavelength or dominant frequency dominant wavelength or dominant frequency © Bedrich Benes Light Sources Metamers • point light source • Different lights perceived as the same color • directional light source • Two lights with different spectra • area light source • Similarly, different color images can result in the same gray scale image the same gray‐scale image • spot light (reflector) spot light (reflector) © Bedrich Benes © Bedrich Benes Color Matching Experiment Color Matching Experiment R G B • let the three lights be called R, G, and B • They are called Th ll d tristimulus values • they are called primaries the are alled primaries • user can change intensity • goal is to match li h the unknown color • let R= a() = 440nm G= b() = 645nm B= c() = 526nm • 0a,b,c1 0a b c1 are the intensities are the intensities • the color is then a linear combination E=a*A + b*B + c*C (additive colors) © Bedrich Benes © Bedrich Benes Metamers Metamers – Why? Three types of cones – different sensitivity intensity [W] intensityy [W] Long (green) l() Medium (red) m() Medium (red) m() Short (blue) s() Perceived as the same color 400 500 600 700 wavelength [nm] 400 500 600 700 wavelength l th [nm] © Bruce MacEvoy © Bedrich Benes © Bedrich Benes Metamers – Why? Metamers ‐ conclusion • Response of the cones (or of any sensor) to a spectral radiance (convolution) L = s() l()d M = s() m()d S = s() s()d • L,M,S “three numbers” • If the numbers are equal, we see the same color • We need three monochromatic colors to represent “any colour you like” © Bedrich Benes • The most commonly used are RGB y • Problems Problems – purity, price, different response purity price different response of different eye, etc. © Bedrich Benes Object Color Light Reflection • Object is characterized by its ability to reflect material • This interacts with the light • Reflection is the process by which electromagnetic flux (power) leaves that surface or medium from the incident side without change in frequency • Reflectance is the fraction of the incident flux that is reflected (it is a normalized value) © Bedrich Benes © Bedrich Benes BRDF Light Reflection • The ability of a material to reflect light is characterized by • reflectance p() depends on the wavelength • reflectance is combined with incoming light I() = p() t() • where: ( g ) I is irradiance (reflected light) t is the incoming light • the process is subtractive the process is subtractive Bi‐directional reflectance distribution function • that that says how much of light Lf (light field) says how much of light Lf (light field) arriving to a point x from a given direction w is reflected to another direction w’ is reflected to another direction w © Bedrich Benes © Bedrich Benes Light Reflection Color Perception intensity [W] Color perception Light source properties irradiance Tristimulus value irradiance intensity [W] 400 500 600 700 wavelength [ ] [nm] 400 reflectance 400 400 400 © Bedrich Benes 500 600 700 500 600 700 500 600 wavelength [nm] 700 reflectance wavelength [nm] 400 © Bedrich Benes 500 600 700 wavelength [nm] Reflected light wavelength [nm] Material properties 500 600 700 wavelength [nm] Cone sensitivity Summary Readings • • • • • • • • • Andrew Glassner, Principles of Digital Image Synthesis Light light sources emission spectra metamers and color matching experiment and color matching experiment cones response to light three primaries light reflection, reflectance putting it all together © Bedrich Benes Readings • F.S.Hill, Computer Graphics, Prentice Hall 1990, pages 564‐> • Shirley, P, et al, Fundamentals of Computer y, , , f p nd Graphics, 2 edition, pages 451‐476 • Cohen, Wallace, Radiosity and Realistic Image Synthesis and Realistic Image Synthesis © Bedrich Benes • Donald Hearn, M.Pauline , Baker. Computer p Graphics, 2nd edition, Prentice Hall 1994 © Bedrich Benes
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