rgb2lab

Convert RGB to CIE 1976 L*a*b*

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Syntax

lab = rgb2lab(RGB)
lab = rgb2lab(RGB,Name,Value)

Description

example

lab = rgb2lab(RGB) converts sRGB values to CIE 1976 L*a*b* values.

example

lab = rgb2lab(RGB,Name,Value) specifies additional conversion options, such as the color space of the RGB image, using one or more name-value pair arguments.

Examples

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Open Live Script

Use rgb2lab to convert the RGB white value to L*a*b.

rgb2lab([1 1 1])
ans = 1×3

  100.0000         0    0.0000
Open Live Script

Convert an Adobe RGB (1998) color value to L*a*b* using the ColorSpace parameter.

rgb2lab([.2 .3 .4],'ColorSpace','adobe-rgb-1998')
ans = 1×3

   30.1783   -5.6902  -20.8223
Open Live Script

Use rgb2lab to convert an RGB color to L*a*b using the D50 reference white.

rgb2lab([.2 .3 .4],'WhitePoint','d50')
ans = 1×3

   31.3294   -4.0732  -18.1750
Open Live Script

Read RGB image into the workspace.

rgb = imread('peppers.png');

Convert the RGB image to the L*a*b* color space.

lab = rgb2lab(rgb);

Display the L* component of the L*a*b* image.

imshow(lab(:,:,1),[0 100])

Input Arguments

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RGB color values to convert, specified as a numeric array in one of these formats.

  • c-by-3 colormap. Each row specifies one RGB color value.

  • m-by-n-by-3 image

  • m-by-n-by-3-by-p stack of images

Data Types: single | double | uint8 | uint16

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: rgb2lab([0.25 0.40 0.10],'WhitePoint','d50')

Color space of the input RGB values, specified as the comma-separated pair consisting of 'ColorSpace' and one of 'srgb', 'adobe-rgb-1998', or 'linear-rgb'. If you specify 'linear-rgb', then rgb2lab assumes the input RGB values are linearized sRGB values.

Data Types: char

Reference white point, specified as the comma-separated pair consisting of 'WhitePoint' and a 1-by-3 vector or one of the CIE standard illuminants listed in the following table.

ValueWhite Point
'a'

CIE standard illuminant A, [1.0985, 1.0000, 0.3558]. Simulates typical, domestic, tungsten-filament lighting with correlated color temperature of 2856 K. 

'c'CIE standard illuminant C, [0.9807, 1.0000, 1.1822]. Simulates average or north sky daylight with correlated color temperature of 6774 K. Deprecated by CIE.
'e'Equal-energy radiator, [1.000, 1.000, 1.000]. Useful as a theoretical reference.
'd50'CIE standard illuminant D50, [0.9642, 1.0000, 0.8251]. Simulates warm daylight at sunrise or sunset with correlated color temperature of 5003 K. Also known as horizon light.

'd55'

CIE standard illuminant D55, [0.9568, 1.0000, 0.9214]. Simulates mid-morning or mid-afternoon daylight with correlated color temperature of 5500 K.  

'd65'CIE standard illuminant D65, [0.9504, 1.0000, 1.0888]. Simulates noon daylight with correlated color temperature of 6504 K. 
'icc'Profile Connection Space (PCS) illuminant used in ICC profiles. Approximation of [0.9642, 1.000, 0.8249] using fixed-point, signed, 32-bit numbers with 16 fractional bits. Actual value: [31595,32768, 27030]/32768.

Data Types: single | double | char

Output Arguments

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Converted L*a*b* color values, returned as a numeric array of the same size as the input. The output type is double unless the input type is single, in which case the output type is also single.

AttributeDescription
L*Luminance or brightness of the image. Values are in the range [0, 100], where 0 specifies black and 100 specifies white. As L* increases, colors become brighter.
a*Amount of red or green tones in the image. A large positive a* value corresponds to red/magenta. A large negative a* value corresponds to green. Although there is no single range for a*, values commonly fall in the range [-100, 100] or [-128, 127).
b*Amount of yellow or blue tones in the image. A large positive b* value corresponds to yellow. A large negative b* value corresponds to blue. Although there is no single range for b*, values commonly fall in the range [-100, 100] or [-128, 127).

Data Types: double | single

Tips

  • If you specify the input RGB color space as 'linear-rgb', then rgb2lab assumes the input values are linearized sRGB values. If instead you want the input color space to be linearized Adobe RGB (1998), then you can use the lin2rgb function.

    For example, to convert linearized Adobe RGB (1998) image RGBlinadobe to the CIE 1976 L*a*b* color space, perform the conversion in two steps:

    RGBadobe = lin2rgb(RGBlinadobe,'ColorSpace','adobe-rgb-1998');
LAB = rgb2lab(RGBadobe,'ColorSpace','adobe-rgb-1998');

Extended Capabilities

See Also

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Topics

Introduced in R2014b

Image Processing Toolbox Documentation

Support