Fall 2015 Dr. Michael J. Reale

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1 CS 490: Computer Vision Color Theory: Color Models Fall 2015 Dr. Michael J. Reale

2 Color Models Different ways to model color: XYZ CIE standard RB Additive Primaries Monitors, video cameras, etc. CMY/CMYK Subtractive primaries Printers, copiers, etc. HSI (HSV, HSL Hue, saturation, and intensity Better for human vision interpretation YIQ and YUV Intensity + chromatic components TV video signal L*a*b* also from CIE; device-independent

3 XYZ Model (CIE Standard Tristimulus values Red, green, and blue values needed to represent color APPROXIMATELY, Red = X, reen = Y, Blue = Z X, Y, Z more or less refer to long, medium, and short cone stimulation, respectively reen perceived as brighter than red or blue, so Y = luminance in CIE standard Color specified by trichromatic coefficients: X x X Y Z Y y X Y Z Z z X Y Z Note that: x y z 1 So usually compute z from x and y

4 XYZ Model (CIE Standard Trichromatic coefficients Normalized values x how much red is involved y how much green is involved X x X Y Z Y y X Y Z Z z X Y Z z not used directly, but calculated from the others Y luminance Color can be described with xyy

5 XYZ Model (CIE Standard

6 XYZ Model (CIE Standard Center x = y = z white light On border pure color

7 XYZ Model (CIE Standard

8 XYZ Model (CIE Standard All visible colors in horseshoe shaped cone in XYZ space Curve boundary = pure colors White around position (1/3, 1/3 in x,y Can determine range of colors possible between: Two colors line Three colors triangle

9 RB Model Each color red, green, blue coordinates Unit cube # of bits # of colors 24-bit color 8 bits per channel (2 8 3 colors = 16,777,216 colors

10 RB and Safe Colors A lot of systems limited to 256 colors So, often use Safe RB colors Also called: All-systems-safe colors In web apps safe Web colors or safe browser colors Set of 216 colors Each value of R,, or B can only be 0, 51, 102, 153, 204, or 255

11 RB Safe Colors

12 CMY and CMYK Models CMY and CMYK Models To convert from RB: Often add K (black Four color printing B R Y M C 1 1 1

13 Problems with RB and CMY(K RB, CMY(K Ideal for hardware implementations Not well suited for describing colors (to humans We often describe color in terms of: Hue Saturation Brightness

14 HSI Model H = hue S = saturation I = intensity Decouples color from intensity

15 HSI Model Intensity = height Angle = hue Saturation = distance from center line

16 Converting to HSI from RB Converting to HSI from RB Hue H: Saturation S: Intensity I: B if 360 B if H 2 1/ 2 1 ( ( ( ( ( 2 1 cos B B R R B R R,, min( ( 3 1 B R B R S ( 3 1 B R I Note: all these assume normalized R,,B values (in range [0,1]

17 Converting from HSI to RB Converting from HSI to RB Depends on which sector you are in (based on hue H R sector (0 <= H < 120: B sector (120 <= H < 240: BR sector (240 <= H <= 360: ( 3 cos(60 cos 1 (1 B R I H H S I R S I B ( 3 cos(60 cos 1 (1 R I B H H S I S I R H H 120 First ( 3 cos(60 cos 1 (1 B I R H H S I B S I H H 240 First

18 HSL Model Similar to HSI, but L sort of refers to perceived light

19 HSV Model Hue-Saturation-Value

20 HSV Model Slice

21 YIQ Model Y = luminance Same as CIE model s Y I = red-orange axis Q = roughly orthogonal to I Eye is most sensitive to Y, followed by I, and finally Q thumb/8/82/yiq_iq_plane.svg/1024px- YIQ_IQ_plane.svg.png By Tonyle (Own work [FDL ( CC-BY- SA-3.0 ( or FAL], via Wikimedia Commons Used in NTSC standard Needed to separate color (chromaticity from B&W (luminance information backwards compatible with B&W TV s

22 Converting to YIQ from RB Y = 0.299R B I = 0.596R B Q = 0.212R B

23 YUV Model Y = luminance U = blue V = red Used in PAL standard File:YUV_UV_plane.svg#mediavie wer/file:yuv_uv_plane.svg Basis for two color encodings: YPbPr analog YCbCr digital In both cases, color components can be compressed, sampled, etc.

24 YUV Model Example Original U Y V

25 CIE L*a*b* Model L = luminance a = range from green to red b = range from blue to yellow Covers ALL visible colors Covers entire gamut of colors Including some imaginary colors we can t see Device independent So is CIE XYZ, since both are based on a mathematical model RB, CMY(K, etc. all display differently on different monitors and printers

26 Comparison of Color amuts

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