Color Representation. CIEXYZ Color Coordinate System. Trichromatic Color Theory. Lecture 3. Calculating the CIEXYZ Color Coordinate System

Transcription

1 Lecture 3 Color Representation CIE Color Space CIE Chromaticity Space HSL,HSV,LUV,CIELab CIE 93 The Commission International de l Eclairage (CIE) Defined a standard system for color representation. The CIE-. In this system, the Tristimulus values can describe any visible color. The system is based on the color matching experiments Trichromatic Color Theory Every color can be represented by 3 values tri =three chroma =color e e 2 e 3 Space of visible colors is 3 Dimensional. Primary Intensity Calculating the CIE 3 2 b(λ) CIE-RGB g(λ) r(λ) David Wright , & John Guild 93 7 observers responses to Monochromatic lights between 4-7nm using viewing field of 2 deg angular subtense. Primaries are monochromatic : nm 2 deg field. These were defined as CIE-RGB primaries and CMF. are a linear transformation away from the observed data.

2 CIE Luminous-Efficiency function of the human eye CIE Criteria for choosing Primaries,, and Color Matching Functions x,y,z. ) CMFs are non-negative over visible wavelengths. (i.e. any color is represented by 3 positive values). 2) Equal amounts of the Primaries produce white. (i.e. == for stimulus of equal luminance at each wavelength). Luminous Efficiency 3) The y color matching function is defined to match the luminous-efficiency function of the human eye ) Primaries are as tight as possible around the set of possible colors (Maxwell triangle Projects to equilateral in space). CIE CIE-RGB to CIE- CIE-RGB Chromaticity space (rg). * Cr, Cg, Cb must enclose the Gamut. * Line Cb-Cr is defined by being Luminance Function. (the Alychne = line of zero luminance). * Line Cr-Cg is tangent at 65+ (z is zero beyond 65). * Thus Cr is defined. * Equal Energy (x=y=z=/3) puts constraint on Cb-Cg * Tight around Gamut -> line Cb-Cg is close to green. * Cb and Cg are defined.

3 CIE-RGB to CIE- CIE Color Standard - 93 CIE RGB space to space. Map Cb Cg Cr to x=(,) y=(,) z=(,) Tristimulus values.8.4 z(λ) y(λ) x(λ) y is predefined. Non negative over the visible wavelengths. (, Several Hundreds,..). The 3 primaries associated with x y z color matching functions are unrealizable (negative power in some of the wavelengths). Integral over the CMF gives equal values. CMF are linear transformation away from CIE-RGB and from LMS. CIE Color Standard Colorimeters Stiles and Birch data (959): Color Matching Experiment with: Deg view Primaries: CIE-

4 Tristimulus values Color matching functions vs LMS - cone photoreceptor responses.8.4 Tristimulus System x(λ) y(λ) z(λ) Relative sensitivity Cone Spectral Sensitivity L M S CIE RGB Primaries are monochromatic : nm = R G B The cone responses form a 3D linear system. Cone responses are equivalent for metamers. thus The cone spectral sensitivities and the color matching functions are related by a 3 x 3 linear transformation. = L M S CIE x y z Color Matching Functions CIE.8 Tristimulus values.4 z(λ) y(λ) x(λ)

5 y CIE Chromaticity Diagram = x ++ = y ++ = z x+y+z = CIE Chromaticity Diagram (ax,ay,az) x. A common representative of color signal: [x,y,] Color Naming CIE-RGB Primaries y green cyan yellowgreen yellow 59 orange 6 white 6 pink red blue 48 purple magenta x.5.

6 Blackbody Radiators and CIE standard Illuminants Blackbody Radiators.8 CIE Standard Illuminants: A - tungsten light B - Sunset C - blue sky D65 - Average daylight E - Equal energy white (x=y=z=/3) Relative energy 3K 35K y C B A E D65 9K.4.8 x Television Primaries and Gamut Signal Lights R G B - Primaries used for PAL R 2G 2B 2- Primaries used for NTSC D65 - reference white for PAL C - reference white for NTSC.8 G 2 NTSC PAL G y.4 D65 E C R R 2 B B2.4.8 x CIE Chromaticity + Gamut applet :

7 Color Space Hue vs Saturation Chromaticity in Polar Coordinates Given a reference white. Dominant Wavelength wavelength of the spectral color which added to the reference white, produces the given color Reference white Chromaticity in Polar Coordinates Given a reference white. Complementary Wavelength wavelength of the spectral color which added to the given color, produces the reference white. Chromaticity in Polar Coordinates Given a reference white. Purity the ratio of the lengths between the given color and reference white and between the dominant wavelength and reference white. Ranges between Reference white.4 Reference white

8 EAMPLE: Reference white is CIE standard illuminant - C. Chromaticity in Polar Coordinates Dominant Wavelength of color S is D of color S 2 is D 2. Complementary Wavelength of color S is D. S2 does not have a complimentary wavelength. Excitation Purity of S is the ratio CS /CD of S 2 is the ratio CS 2 /CD 2 of S 3 is the ratio CS 3 /CD purity Dominant/complimentary Wavelength.8 D2 reference white.4 S2 S D.4.8 D C S3 S D3.4.8 Color Description Hue (red, green, yelow, blue...) Saturation (pink,bright red,...) Lightness (black, grey, white...) (Value) White Munsell Color System (95) Equal perceptual steps in Hue Saturation Value. Hue: R, R,, G, G, BG, B, PB, P, RP (each subdivided into ) Chroma:... 2 (neutral... saturated) Value: i... (dark... pure white) / Value 5/ Saturation G B Hue R RP 5RP P 5P 5R R 5R R / 5 5G /2 /4 /6 /8 / Black Brightness PB 5PB B Example: /2 /4 G /6 5G /8 / G 5R 8/4 5B GB 5GB

9 Munsell Book of Colors Color Polytopes Atlas of thr Munsell Color System (95) Applets: MayuraDraw PowerPoint

10 Photoshop Color Picker Color Space Summary Spectral Power Distribution (SPD) High Dimensional 3 Dimensional Spaces: LMS - Human Cone responses. Given by the cone sensitivity curves. CIE RGB - Based on color Matching Experiments by Wright+Guild. Defined by Primaries R G B (monochromatic nm) and cmf r g b. CIE- - Standard Color space. Linear transformation of above that confirms to set of constraints. Defined by Primaries (unrealizable) and cmf x y z. Munsell Color Space Perceptually equally spaced samples in 3 dimensions: Hue, Chroma, Value.