The
Contents 1 Advantages 2 Differentiation 3 CIELAB 3.1 Perceptual differences 3.2 RGB and CMYK conversions 3.3 Range of coordinates 4 CIELAB-CIEXYZ conversions 4.1 Forward transformation 4.2 Reverse transformation 5 Hunter Lab 5.1 Approximate formulas for Ka and Kb 5.2 As an Adams chromatic valence space 6 Cylindrical representation: CIELCh or CIEHLC 7 See also 8 References 9 External links Advantages[edit] An example of color enhancement using LAB color mode in Photoshop. The left side of the photo is enhanced, while the right side is normal. Unlike the RGB and CMYK color models, Lab color is designed to
approximate human vision. It aspires to perceptual uniformity, and its
L component closely matches human perception of lightness, although it
does not take the
In Adobe Photoshop, image editing using "Lab mode" is CIELAB
D50.[6][7]
In Affinity Photo, Lab editing is achieved by changing the document's
Colour Format to "Lab (16 bit)"
In ICC profiles, the "Lab color space" used as a profile connection
space is CIELAB D50.[5]
In
CIELAB[edit] Play media Play media The sRGB gamut (left) and visible gamut under D65 illumination (right) plotted within the CIELAB color space. a and b are the horizontal axes; L is the vertical axis. CIE L*a*b* (CIELAB) is a color space specified by the International
Commission on Illumination (French Commission internationale de
l'éclairage, hence its CIE initialism). It describes all the colors
visible to the human eye and was created to serve as a
device-independent model to be used as a reference.
The three coordinates of CIELAB represent the lightness of the color
(L* = 0 yields black and L* = 100 indicates diffuse white; specular
white may be higher), its position between red/magenta and green (a*,
negative values indicate green while positive values indicate magenta)
and its position between yellow and blue (b*, negative values indicate
blue and positive values indicate yellow). The asterisk (*) after L, a
and b are pronounced star and are part of the full name, since they
represent L*, a* and b*, to distinguish them from Hunter's L, a, and
b, described below.
Since the L*a*b* model is a three-dimensional model, it can be
represented properly only in a three-dimensional space.[11]
Two-dimensional depictions include chromaticity diagrams: sections of
the color solid with a fixed lightness. It is crucial to realize that
the visual representations of the full gamut of colors in this model
are never accurate; they are there just to help in understanding the
concept.
Because the red-green and yellow-blue opponent channels are computed
as differences of lightness transformations of (putative) cone
responses, CIELAB is a chromatic value color space.
A related color space, the CIE 1976 (L*, u*, v*) color space (a.k.a.
CIELUV), preserves the same L* as L*a*b* but has a different
representation of the chromaticity components. CIELAB and
L ⋆ = 116 f ( Y Y n ) − 16 a ⋆ = 500 ( f ( X X n ) − f ( Y Y n ) ) b ⋆ = 200 ( f ( Y Y n ) − f ( Z Z n ) ) displaystyle begin aligned L^ star &=116 f!left( frac Y Y_ mathrm n right)-16\a^ star &=500left(f!left( frac X X_ mathrm n right)-f!left( frac Y Y_ mathrm n right)right)\b^ star &=200left(f!left( frac Y Y_ mathrm n right)-f!left( frac Z Z_ mathrm n right)right)end aligned where f ( t ) = t 3 if t > δ 3 t 3 δ 2 + 4 29 otherwise δ = 6 29 displaystyle begin aligned f(t)&= begin cases sqrt[ 3 ] t & text if t>delta ^ 3 \ frac t 3delta ^ 2 + frac 4 29 & text otherwise end cases \delta &= frac 6 29 end aligned Here, Xn, Yn and Zn are the
X n = 95.047 , Y n = 100.000 , Z n = 108.883 displaystyle begin aligned X_ mathrm n &=95.047,\Y_ mathrm n &=100.000,\Z_ mathrm n &=108.883end aligned Values for illuminant D50 are X n = 96.6797 , Y n = 100.000 , Z n = 82.5188 displaystyle begin aligned X_ mathrm n &=96.6797,\Y_ mathrm n &=100.000,\Z_ mathrm n &=82.5188end aligned The division of the domain of the f function into two parts was done to prevent an infinite slope at t = 0. The function f was assumed to be linear below some t = t0, and was assumed to match the t1/3 part of the function at t0 in both value and slope. In other words: t 0 1 / 3 = m t 0 + c (match in value) 1 3 t 0 − 2 / 3 = m (match in slope) displaystyle begin aligned t_ 0 ^ 1/3 &=mt_ 0 +c& text (match in value) \ frac 1 3 t_ 0 ^ -2/3 &=m& text (match in slope) end aligned The intercept f(0) = c was chosen so that L* would be 0 for Y = 0: c = 16/116 = 4/29. The above two equations can be solved for m and t0: m = 1 3 δ − 2 = 7.787037 … t 0 = δ 3 = 0.008856 … displaystyle begin aligned m&= frac 1 3 delta ^ -2 &=7.787037ldots \t_ 0 &=delta ^ 3 &=0.008856ldots end aligned where δ = 6/29.[15] Reverse transformation[edit] The reverse transformation is most easily expressed using the inverse of the function f above: X = X n f − 1 ( L ⋆ + 16 116 + a ⋆ 500 ) Y = Y n f − 1 ( L ⋆ + 16 116 ) Z = Z n f − 1 ( L ⋆ + 16 116 − b ⋆ 200 ) displaystyle begin aligned X&=X_ mathrm n f^ -1 left( frac L^ star +16 116 + frac a^ star 500 right)\Y&=Y_ mathrm n f^ -1 left( frac L^ star +16 116 right)\Z&=Z_ mathrm n f^ -1 left( frac L^ star +16 116 - frac b^ star 200 right)\end aligned where f − 1 ( t ) = t 3 if t > δ 3 δ 2 ( t − 4 29 ) otherwise displaystyle f^ -1 (t)= begin cases t^ 3 & text if t>delta \3delta ^ 2 left(t- frac 4 29 right)& text otherwise end cases and where δ = 6/29. Hunter Lab[edit] L is a correlate of lightness, and is computed from the Y tristimulus value using Priest's approximation to Munsell value: L = 100 Y / Y n displaystyle L=100 sqrt Y/Y_ mathrm n where Yn is the Y tristimulus value of a specified white object. For surface-color applications, the specified white object is usually (though not always) a hypothetical material with unit reflectance that follows Lambert's law. The resulting L will be scaled between 0 (black) and 100 (white); roughly ten times the Munsell value. Note that a medium lightness of 50 is produced by a luminance of 25, since 100 25 / 100 = 100 ⋅ 1 / 2 displaystyle 100 sqrt 25/100 =100cdot 1/2 a and b are termed opponent color axes. a represents, roughly, Redness (positive) versus Greenness (negative). It is computed as: a = K a ( X / X n − Y / Y n Y / Y n ) displaystyle a=K_ mathrm a left( frac X/X_ mathrm n -Y/Y_ mathrm n sqrt Y/Y_ mathrm n right) where Ka is a coefficient that depends upon the illuminant (for D65, Ka is 172.30; see approximate formula below) and Xn is the X tristimulus value of the specified white object. The other opponent color axis, b, is positive for yellow colors and negative for blue colors. It is computed as: b = K b ( Y / Y n − Z / Z n Y / Y n ) displaystyle b=K_ mathrm b left( frac Y/Y_ mathrm n -Z/Z_ mathrm n sqrt Y/Y_ mathrm n right) where Kb is a coefficient that depends upon the illuminant (for D65, Kb is 67.20; see approximate formula below) and Zn is the Z tristimulus value of the specified white object.[16] Both a and b will be zero for objects that have the same chromaticity coordinates as the specified white objects (i.e., achromatic, grey, objects). The name for the system is an attribution to Richard S. Hunter. Approximate formulas for Ka and Kb[edit] In the previous version of the Hunter Lab color space, Ka was 175 and Kb was 70. Hunter Associates Lab discovered[citation needed] that better agreement could be obtained with other color difference metrics, such as CIELAB (see above) by allowing these coefficients to depend upon the illuminants. Approximate formulae are: K a ≈ 175 198.04 ( X n + Y n ) displaystyle K_ mathrm a approx frac 175 198.04 (X_ mathrm n +Y_ mathrm n ) K b ≈ 70 218.11 ( Y n + Z n ) displaystyle K_ mathrm b approx frac 70 218.11 (Y_ mathrm n +Z_ mathrm n ) which result in the original values for Illuminant C, the original
illuminant with which the
L = 100 Y / Y n displaystyle L=100 sqrt Y/Y_ mathrm n and, as the uniform chromaticity coordinates: c a = X / X n Y / Y n − 1 = X / X n − Y / Y n Y / Y n displaystyle c_ mathrm a = frac X/X_ mathrm n Y/Y_ mathrm n -1= frac X/X_ mathrm n -Y/Y_ mathrm n Y/Y_ mathrm n c b = k e ( 1 − Z / Z n Y / Y n ) = k e Y / Y n − Z / Z n Y / Y n displaystyle c_ mathrm b =k_ mathrm e left(1- frac Z/Z_ mathrm n Y/Y_ mathrm n right)=k_ mathrm e frac Y/Y_ mathrm n -Z/Z_ mathrm n Y/Y_ mathrm n where ke is a tuning coefficient, we obtain the two chromatic axes: a = K ⋅ L ⋅ c a = K ⋅ 100 X / X n − Y / Y n Y / Y n displaystyle a=Kcdot Lcdot c_ mathrm a =Kcdot 100 frac X/X_ mathrm n -Y/Y_ mathrm n sqrt Y/Y_ mathrm n and b = K ⋅ L ⋅ c b = K ⋅ 100 k e Y / Y n − Z / Z n Y / Y n displaystyle b=Kcdot Lcdot c_ mathrm b =Kcdot 100k_ mathrm e frac Y/Y_ mathrm n -Z/Z_ mathrm n sqrt Y/Y_ mathrm n which is identical to the Hunter Lab formulas given above if we select
K = Ka/100 and ke = Kb/Ka. Therefore, the Hunter
Play media Play media The sRGB gamut (left) and visible gamut under D65 illumination (right) plotted within the CIELCHab color space. L is the vertical axis; C is the cylinder radius; h is the angle around the circumference. The CIELCh color space is a CIELab cube color space, where instead of
C ⋆ = a ⋆ 2 + b ⋆ 2 , h ∘ = arctan ( b ⋆ a ⋆ ) displaystyle C^ star = sqrt a^ star ,2 +b^ star ,2 ,qquad h^ circ =arctan left( frac b^ star a^ star right) Conversely, given the polar coordinates, conversion to Cartesian coordinates is achieved with: a ⋆ = C ⋆ cos ( h ∘ ) , b ⋆ = C ⋆ sin ( h ∘ ) displaystyle a^ star =C^ star cos(h^ circ ),qquad b^ star =C^ star sin(h^ circ ) The LCh color space is not the same as the HSV, HSL or HSB color
models, although their values can also be interpreted as a base color,
saturation and lightness of a color. The HSL values are a polar
coordinate transformation of what is technically defined RGB cube
color space. LCh is still perceptually uniform.
Further, H and h are not identical, because HSL space uses as primary
colors the three additive primary colors red, green, blue (H = 0, 120,
240°). Instead, the LCh system uses the four colors yellow, green,
blue and red (h = 90, 180, 270, 360°). Regardless the angle h, C = 0
means the achromatic colors, that is, the gray axis.
The simplified spellings LCh, LCH and HLC are common, but the latter
presents a different order.
Color theory HSL and HSV RGB color model CMYK color model CIECAM02 HCL color space References[edit] ^ Hunter, Richard Sewall (July 1948). "Photoelectric Color-Difference Meter". JOSA. 38 (7): 661. (Proceedings of the Winter Meeting of the Optical Society of America) ^ Hunter, Richard Sewall (December 1948). "Accuracy, Precision, and Stability of New Photo-electric Color-Difference Meter". JOSA. 38 (12): 1094. (Proceedings of the Thirty-Third Annual Meeting of the Optical Society of America) ^ A discussion and proposed improvement, Bruce Lindbloom ^ Explanation of this history, Bruce MacEvoy ^ a b International Color Consortium, Specification ICC.1:2004-10 (Profile version 4.2.0.0) Image technology colour management — Architecture, profile format, and data structure, (2006). ^ Margulis, Dan (2006). Photoshop Lab Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace. Berkeley, Calif. : London: Peachpit ; Pearson Education. ISBN 0-321-35678-0. ^ The Lab Color Mode in Photoshop, Adobe TechNote 310838 ^ TIFF: Revision 6.0 Archived 2007-07-01 at the Wayback Machine. Adobe Developers Association, 1992 ^ Color Consistency and Adobe Creative Suite Archived 2008-07-25 at the Wayback Machine. ^ Adobe Acrobat Reader 4.0 User Guide "The color model Acrobat Reader uses is called CIELAB…" ^ 3D representations of the L*a*b* gamut, Bruce Lindbloom. ^ CIE-L*C*h Color Scale ^ Fairchild, Mark D. (2005). "Color and Image Appearance Models". Color Appearance Models. John Wiley and Sons. p. 340. ISBN 0-470-01216-1. ^ Jain, Anil K. (1989). Fundamentals of Digital Image Processing. New Jersey, United States of America: Prentice Hall. pp. 68, 71, 73. ISBN 0-13-336165-9. ^ János Schanda (2007). Colorimetry. Wiley-Interscience. p. 61. ISBN 978-0-470-04904-4. ^ Hunter Labs (1996). "Hunter Lab Color Scale". Insight on Color 8 9 (August 1–15, 1996). Reston, VA, USA: Hunter Associates Laboratories. ^ Adams, E.Q. (1942). "X-Z planes in the 1931 I.C.I. system of colorimetry". JOSA. 32 (3): 168–173. doi:10.1364/JOSA.32.000168. External links[edit] Demonstrative color conversion applet CIELAB Color Space by Gernot Hoffmann, includes explanations of L*a*b* conversion formulae, graphical depictions of various gamuts plotted in L*a*b* space, and PostScript code for performing the color transformations. Color Differences LAB Color Spaces with MATLAB Convert Rgb to Lab v t e Color space List of color spaces Color models CAM CIECAM02 iCAM CIE CIEXYZ CIELAB CIECAM02 CIELUV Yuv CIEUVW CIE RGB RGB RGB color space sRGB rg chromaticity Adobe Wide-gamut ProPhoto scRGB DCI-P3 Rec. 709 Rec. 2020 Rec. 2100 YUV YUV PAL YDbDr SECAM PAL-N YIQ NTSC YCbCr Rec. 601 Rec. 709 Rec. 2020 Rec. 2100 ICtCp YPbPr xvYCC YCoCg Other CcMmYK CMYK Coloroid LMS Hexachrome HSL, HSV HCL Imaginary color OSA-UCS PCCS RG RYB Color systems and standards ACES ANPA Colour Index International CI list of dyes DIC Federal Standard 595 HKS ICC profile ISCC–NBS Munsell NCS Ostwald Pantone RAL list For the vision capacities of organisms or machines, see & |