Paper Summaries • Any takers? Color Assignments Projects • Checkpoint 4 • Approx 17 projects – Grading complete • Listing of projects now on Web • Checkpoint 5 • Presentation schedule – Due today – Presentations (20 min max) • Checkpoint 6 – Last 3 classes (week 10 + finals week) – Given today – Sign up • Renderman • Email me with 1 st , 2 nd , 3 rd choices – Due Nov 4 th • First come first served. – Getting distributions on mycourses • Mid-quarter report due last week – Server name: see news item on mycourses Finals date Photography and Light pho•tog•ra•phy, n ., the process or art of • For last day of presentations producing images of objects by the action of light on a sensitized surface, e.g., a film in a • Friday, November 18 th camera. • 12:30pm – 2:30pm • 70-1445 Photography = writing with light • Times on SCHEDULE 1
Light -- What it is Computer Graphics as Virtual Photography • Electromagnetic radiation real camera photo Photographic Photography: scene (captures processing print light) induction radio ultra gamma secondary power infrared x-rays heating waves violet rays cosmic rays processing 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 Wavelength (nm) visible light Red 700 nm orange 650 nm camera Computer 3D synthetic yellow 600 nm tone green 550 nm model Graphics: models image blue 450 nm reproduction violet 400 nm (focuses simulated lighting) Light and Color Light - Color • “Indeed rays, properly expressed, are • Color is the perceptual response to not colored” light of wavelengths 400 - 700 nm hitting the retina. -- Sir Isaac Newton • Spectral power distributions exist in the physical world but color exists only • I.e., light rays are not colored; we in the eye and brain, e.g., there is no perceive them as colored! real white light! Light – Spectral Density Functions (SDF) Light - Color • AKA spectral power distributions • Describes the distribution of the strengths of � Black Body Radiators light at given wavelengths emitted from a � Spectrum resulting from heating a standard source. “body” to a given temperature � Plank’s formula: c λ = 1 M ( , T ) λ λ − 5 c / T ( e 2 1 ) = × − = × − 2 16 c 1 . 4388 10 c 3 . 7418 10 2 1 2
Light - Color Light -- Color • Black Body Radiators and other light sources • Black Body Radiators and daylight – Daylight from the sun & total sky (5000K - 7000K) – D 65 - Average daylight (6504K) – Daylight w/occluded sun (> 7000K) – Daylight from sun alone (< 5000K) Light -- Color Light - Color � Light Filters • Not all lighting sources have smooth SDFs � Absorbs light at given wavelengths � Allows light at other wavelengths through � Using filters � Actual SDF is determined SDF for a filter by multiply SDF of light by SDF of filter wavelength by wavelength. Light and color Light and color • The “color” of an object we see is a function of: – Spectral qualities of the material being viewed: • Absorption • Reflection • Diffraction • Etc. • Absorption – Spectral qualities of the illuminating light. – Material can absorb light on a wavelength by wavelength basis – Responsible for object color 3
Light and color Light and Color • Color appearance applets • Color is the perceptual response to light of wavelengths 400 - 700 nm hitting the retina. http://www.cs.rit.edu/~ncs/color/a_spectr.html • When rendering, spectrum must be http://www.cs.brown.edu/exploratories/freeSoftware sampled. /repository/edu/brown/cs/exploratories/applets/spe ctrum/reflection_guide.html • Color vision is inherently trichromatic. Light and Color Light and Color • CIE Experiments – used X,Y,Z values to quantify • Color matching applet chromatic characteristics of color stimuli http://www.cs.rit.edu/~ncs/color/a_game.html • There are lots of color spaces and most of the time we can convert between them, but not always. Light and Color Light and Color • CIE RGB curves • CIE XYZ color matching curves 40 30 200 20 150 R y 10 G 100 x B 0 z 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 50 7 0 2 5 7 0 2 5 7 0 2 5 7 0 2 5 -10 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 0 -20 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 Wavelength 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 Wavelength 4
Light and Color Chromaticity Coordinates • Chromaticity coordinates (X,Y,Z have no • often given in xyY perceptual correlates, although Y is luminance, • xy give the and x and z provide hue information) chromaticity y • Y gives brightness X Y Z = = = x y z + + + + + + X Y Z X Y Z X Y Z + + = x y z 1 x Light and Color Light and Color • sRGB • RGB (or any primary set) can be – Standard proposed by Microsoft and HP determined from XYZ – Based on ITU-R 709.BT – Need chromaticies of primaries and white – It is a lighting model for “many” CRTs point. = − − • Primaries generally determined by device . R 3 . 2410 X 1 . 5374 Y 0 . 4986 Z = − + + • RGB values are incomplete without G 0 . 9692 X 1 . 8760 Y 0 . 0416 Z specification of primaries & white point. = − + B 0 . 0556 X 0 . 2040 Y 1 . 0570 Z Light and Color Light and Color • sRGB • Other color spaces – HSV (hue-saturation-value) – CMYK (printing) – CIELAB / CIELUV (perceptual) • Why does CG use RGB? – Convenience http://www.cs.rit.edu/~ncs/color/a_chroma.html 5
Light -- Color Light - Color • Full spectral renderers are hard to find • Converting from SDF to RGB. – Expensive in time and memory 40 – Most renderers specify color using RGB 30 triplet (red, green, blue) 20 – For accuracy, must convert from SDF to R 10 G RGB B 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 -10 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 • Full spectral rendering going on at RIT -20 Munsell Color Lab (Mark Fairchild) Wavelength Light - Color Light - Color • Converting from SDF to RGB • Converting from SDF (S) to RGB ∫ = λ λ λ R r ( ) S ( ) d = R * λ ∫ = λ λ λ G g ( ) S ( ) d = G * λ ∫ = λ λ λ B b ( ) S ( ) d * = B λ Based on how “average” eye works Light - Color Light - Color • Converting SDF to XYZ • Problems with direct conversion to RGB – Negative values 200 – Which RGB? (may not match RGB of monitor) 150 y • Solution: Use XYZ space 100 x z 50 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 Wavelength 6
Light - Color Light - Color • Converting SDF to XYZ • Converting from SDF to XYZ ∫ = λ λ λ X x ( ) S ( ) d = X * λ ∫ = λ λ λ Y y ( ) S ( ) d = Y λ * ∫ = λ λ λ Z z ( ) S ( ) d λ = Z * Light - Color Light - Color • Problems with using XYZ • Converting XYZ -> RGB – Non-intuitive – need definition of your primaries (R, G, B) in terms of XYZ coordinates – Not an abundance of XYZ renderers • Good if you are starting with SDFs = + + R r X r Y r Z X Y Z • Good as an interchange space = + + G g X g Y g Z X Y Z = + + B b X b Y b Z X Y Z Light - Color Light - Color • Converting XYZ -> RGB • Converting from XYZ->RGB – Construct the following matrix: ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ r r R r X r r r Y Z X ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ Z X Y ⎢ ⎥ = = g g g G ⎢ ⎥ Y ⎢ ⎥ ⎢ ⎥ M g g g X Y Z ⎢ ⎥ ⎢ ⎥ X Y Z ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ B b b Z b ⎢ ⎥ ⎣ ⎦ Y Z X b b b Z Y X 7
Light - Color Light - Color • Converting from RBG -> XYZ • White Point – Invert matrix – Chromaticity of point (1, 1, 1) – For any color (R, G, B) we can calculate (X,Y,Z) ⎡ ⎤ -1 ⎡ ⎤ -1 ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ r r r r X r R X r 1 Y Z Y Z X X ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ = = g g g g g g Y ⎢ ⎥ G Y ⎢ ⎥ 1 ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ X Y Z X Y Z ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ Z b b B Z b b 1 b b Y Z Y Z X X White Point Light -- Color • A SDF will result in a single RGB triplet. • However, an RGB triplet can be the result of many SDFs. • Metamer -- Separate SDFs that produce the same sensation of color. • Interestingly though, reflectance and transmission reactions are not necessarily the same, nor need the response be the same under different light sources! Wikipedia Light - Color Light - Color • Example of Metamers (perceived the same) • Metamers applet http://www.cs.brown.edu/exploratories/freeSoftware /repository/edu/brown/cs/exploratories/applets/spe ctrum/metamers_guide.html 8
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